Substitutional Alloying and Deformation Modes in High Chromium Ferritic Alloys I.M. WOLFF and A. BALL The effect of microalloying additions of between 0.05 and 2 wt pct Ni, Ru, Nb, and Ti on the plastic deformation of alloys based on Fe-40 wt pct Cr has been studied. The dislocation substructures in the deformed and recovered conditions have been characterized for a series of isothermal annealing cycles. Unalloyed Fe-40Cr deforms at room temperature by mixed twinning and slip. Solute additions modify the operative dislocation configuration. The addition of Ni enhances the propensity for mechanical twinning and raises the cleavage resistance in accordance with the solid-solution softening phenomenon. Ruthenium advances a greater degree of cellularity and promotes restoration of the wrought alloy by recovery. Niobium and titanium act primarily as stabilizing agents for the residual interstitial elements. The formation of a recovered substructure favors the long-range tensile ductility. Enhanced toughness is, however, associated with a reduced degree of cellularity and restricted cross slip.
I.
INTRODUCTION
A. Substitutional Alloying in Body-Centered Cubic Metals
SOLID-solution softening is a frequently observed phenomenon in body-centered cubic (bcc) alloys which is characterized by an increase in strength at nominal strain rates and an enhanced capacity for local stress accommodation at high strain rates and/or low temperatures, tu Solution softening by substitutional elements appears to be correlated with those elements which also induce solution hardening under nominal strain rates and higher temperatures. True softening has been defined to exist if the curves of yield stress against temperature for the alloy and pure solvent intersect at some temperature below which the alloy has the lower yield stress. However, it has been pointed out that some systems exhibit two intersections, implying that softening is only achieved within certain ranges, t21 Solution softening has been variably argued to be the product of (1) an increase in the mobile dislocation density, (2) substitutional-interstitial interactions, or (3) the intrinsic modification of the lattice flow stress and an increased mobility of screw dislocations. Little evidence has been found to substantiate the first of these, and an effect over and above that of the interstitial interactions has been rationalized in recent studies. Systematic analyses on high-purity metals favor a mechanism of enhanced double-kink nucleation on screw dislocation segments, t3-61 It is generally agreed that Ni and Ru act principally through alloy softening, while the strong interaction of Ti and Nb in small amounts with the interstitial elements causes a predominant scavenging effect. [1,7] Whereas solution softening is the product of relatively I.M. WOLFF, formerly with the Department of Materials Engineering, University of Cape Town, is with the Physical Metallurgy Division, MINTEK, Randburg 2125, Republic of South Africa. A. BALL, Department Head, is with the Department of Materials Engineering, University of Cape Town, Rondebosch 7700, Republic of South Africa. Manuscript submitted June 10, 1991. METALLURGICAL TRANSACTIONS A
small alloying additions, early experiments on Fe, Cr, Cr-Re, and Cr-Fe alloys t8-11] showed that large alloying additions also changed the slip character, producing a uniform distribution of dislocations rather than the celllike structure observed in pure iron and chromium. Roomtemperature deformation studies of Cr-47 pct Fe, for example, t12] indicated a higher tendency to twinning and straightening of the screw dislocations. The reduced mobility of the screw components was attributed to a lower stacking fault energy (SFE) which reduced climb and transverse slip of screw components. Little is known of the influence of the minor element chemistry and the solute-induced changes in the deformation processes in binary ferritic alloys. B. Alloys Based on Fe-40 Wt Pct Cr
A family of alloys based on a composition of Fe-40 wt pct Cr has been the subject of extensive research, aspects of which have been reported previously, t13-~91The alloys exhibit superior corrosion resistance, which is further enhanced by cathodic modification with small alloying additions of platinum group metals. [2~ The fully ferritic structures of the alloys exhibit a ductile-brittle transition (DBTT) in common with other bcc systems. Alloying additions made to modify the mechanical and/ or corrosion properties can influence this transition by changing the operative deformation mode. In addition to their effect on the deformation processes, solutes exert an influence on other factors which affect the IOBTT. These include the recrystallization kinetics, the grain size, the carbide size, and the introduction of spurious secondary phases. Previous work has shown that the cleavage transition is a strong function of the microstructural development. Improved toughness can be achieved intrinsically by, among other things: t~3] (1) minimizing the interstitial C + N content; (2) controlling the formation of secondary phases associated with brittle crack-initiation events; and (3) avoiding the formation of a stable recovered dislocation su~tructure. VOLUME 23A, FEBRUARY 1992--627
The fracture mode is the outcome of the competition between the tensile and shear forces at the crack tip. Plastic deformation is favored by increasing the effective dislocation mobility within the crack zone. Extrinsic means for achieving enhanced stress accommodation have included:[~41 (1) increasing the mobile dislocation density by prestraining; (2) dispersion toughening; and (3) alloysoftening, notably by additions of small quantities of n~kel. (By extrinsic we imply a mechanism introduced into the system. The mechanism of alloy softening is, in turn, believed to be intrinsic to certain binary bcc alloy systems.) Each of these mechanisms shields the crack tip from the external stress by changing the emission conditions and the ability of dislocations to interact with the crack tip. The requirements for adequate ductility and corrosion resistance can bring the microstructural parameters into conflict. For example, overstabilization with getters to prevent sensitization may cause embrittlement, t22,23j Alternatively, solute additions can change the recrystallization kinetics, leaving a recovered dislocation substructure.tl7] In an extension of the work reported earlier, the present study examines the influence, on the room-temperature (RT) deformation of Fe-40 wt pct Cr, of four microalloying additions, specifically, nickel for enhanced protection against brittle cleavage, ruthenium for cathodic modification, and titanium and niobium for stabilizing the residual interstitial C + N. II.
EXPERIMENTAL PROCEDURE
Experimental alloys were produced by vacuum induction melting of commercial electrolytic charge iron and chromium. The ingots were hot rolled from an initial thickness of 50 m m to plates 8 m m in section over the temperature range of 1050 ~ to 650 ~ and quenched after the final pass. The chemical compositions are given in Table I. Care was taken to maintain the interstitial C + N content below approximately 170 ppm. Niobium was added to one alloy in roughly 2.2 times the stoichiometric equivalent, shown to be necessary to ensure protection against sensitization. ~22) We have also examined the deformation structures of specimens of an alloy with a 0.05 pet Ti addition in our study. This was found to exhibit excellent toughness and ductility t23j and recorded an upper shelf toughness in excess of 330 J for 10-ram plate at RT. In order to take account of the microstructural parameters and isolate their effects on the DBTT, six standard
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Fig. 1 --Schematic diagram of isothermal heat-treatment schedules.
heat treatments were established. The experimental procedure is detailed elsewhere, t~3] The isothermal schedules, shown schematically in Figure 1, were based on solution heat treatments (SHT) at 1050 ~ and 1250 ~ in conjunction with aging cycles at 850 ~ prior to or subsequent to SHT. In each case, the heat treatment terminated in an iced-water quench. The main results of these treatments can be summarized as follows: (1) SHT at 1250 ~ rather than 1050 ~ is required to ensure complete dissolution of the precipitated carbonitride phases. At the same time, SHT at the higher temperature results in an order of magnitude increase in the grain size. (2) Prior aging at 850 ~ is essentially a recovery anneal which introduces a stable subgrain structure. This was shown to increase the kinetics of intragranular precipitation on subsequent quenching from the SHT temperature. (3) Aging at 850 ~ subsequent to S H T results in severe coarsening of the precipitated phases, particularly at grain boundaries. Although this improves long-range dislocation migration and tensile ductility, the impact resistance is severely compromised. Charpy-format specimens were sectioned in the transverse-longitudinal direction, while metric Hounsefield tensile specimens (gage length = 25 mm) were taken parallel to the rolling direction. All specimens were tested to failure at an initial strain rate of 1 0 - 4 / S (crosshead speed = 0.15 m m / m i n ) at RT. Controlled straining of specimens was performed by rolling at RT at the rate of 1.3 pct per pass. The surfaces were polished for deformation studies. ae ~
Table I.
Alloying Addition Base 2.0 pct 0.2 pct 0.2 pct 0.05 pct
Ni Ru Nb Ti
Chemical Analyses of Experimental Melts
Amount Recovered
C
N
O (Wt Pct)
S
P
-1.95 0.20 0.20 <0.02
0.010 0.011 0.010 0.010 0.006
0.002 0.002 0.002 0.002 0.002
0.10 0.10 0.10 0.06 0.04
0.010 0.010 0.010 0.010 <0.01
0.01 0.01 0.01 0.01 <0.01
628--VOLUME 23A. FEBRUARY 1992
METALLURGICAL TRANSACTIONS A
Samples taken for metallographic examination were prepared for Nomarski interferometry by electropolishing with a chromic-acetic solution. Specimens for electron microscopy were prepared using standard techniques. III.
RESULTS
A. Mechanical Properties
The tensile behavior of the Fe-40Cr alloys modified with 2.0~pct Ni, 0.2 pct Ru, and 0.2 pct Nb, respectively, is shown in Figure 2 for the two SHT temperatures. The properties are summarized in Table I1 relative to those of the base alloy. Those of the Ni alloy were described in earlier work tl4J and are included for completeness. The results also reflect the RT impact toughness and hardness values of the alloys. Some general observations pertain to all of the alloys studied. The tensile curves largely correspond to those of the unalloyed Fe-40Cr system. The alloys all displayed high tensile ductility, and the fracture mode, as determined from the fracture surfaces, was ductile microvoid coalescence. Solution heat treatment above 1050 ~ increases the strength, with a slight concomitant falloff in the strain to fracture. In all cases, aging subsequent to SHT reduces the yield stress and increases the strain to fracture. This is accompanied by a corresponding reduction in the flow stress as reflected by the hardness values. This has been explained in terms of the removal of obstacles to long-range dislocation migration, specifically by coarsening of the precipitates. Aging prior to SHT has only a marginal influence on the tensile ductility in most cases, and its effects are almost completely overridden by SHT at the higher temperature 1250 ~ which ensures complete recrystallization. The niobium alloy, however, displays maximum ductility when aged prior to 1050 ~ This can be attributed to reaction of the Nb with the interstitial C + N to form relatively stable precipitates. It is salient to note that the Nb alloy treated at 1050 ~ is the sole exhibitor of a distinct yield point, which may indicate pinning by fine Nb precipitates. This effect is nullified by the prior-aging treatment, which evidently coarsens the precipitates. Solution treat treatment at 1250 ~ is effective in ensuring complete solutioning of the Nb. Overstabilization has no clear trend in strengthening. The addition of 2 pct Ni increases the strength and flow stress and favorably shifts the DBTT for all the heattreatment schedules relative to the base alloy. The marked solid-solution strengthening and impact resistance of the alloy are characteristic of solid-solution softening. The tensile curves of the nickel were unique in exhibiting serrated yielding at RT, correlated with load drops and audible clicks during straining. This is the result of mechanical twinning in the form of Neumann bands, t141 The tensile curves of the Ni alloy also reflect an increased strain-hardening rate. Alloys solution-annealed at 1250 ~ showed a greater propensity for mechanical twinning, which may be accounted for by (1) the higher flow stress and (2) the larger grain size, both factors favoring plastic deformation by twinning. The small addition of ruthenium gives rise to substanMETALLURGICAL TRANSACTIONS A
tial strengthening of the alloy, although its solution softening effect is not clarified at the levels studied. The highest strain to fracture is recorded by the ruthenium alloy in the post-aged condition following SHT at 1050 ~ B. Grain Growth
The effect of the alloying additions on the grain size is shown in Table III. The grain size is generally heterogeneous and varies from the surface (fine, equiaxed grains) to the plate center (coarse grain structure) depending on the variability in the rolling parameters. We, therefore, confine ourselves to noting trends. The grain morphology of the Fe-40Cr alloys is shown in Figures 3 through 5. The relatively large volume of inclusions that are visible were identified as oxidesJ 19"241 The nickel alloy comprises large polygonal grains, similar to those of the base alloyJ TM At the lower SHT temperature, the Ni addition apparently retards grain growth, while SHT at 1250 ~ clearly increases the rate of grain growth. The addition of small quantities of niobium and ruthenium has a pronounced effect on the restoration processes during annealing of the warm-worked alloys. In the case of the Nb alloy, both the recovery and the recrystallization kinetics are retarded by the ostensible precipitation of Nb from solid solution. Solution heat treatment at 1050 ~ causes an order of magnitude refinement in the grain size, but solutioning at 1250 ~ leaves the grain size unaffected relative to the base alloy. The niobium results in constraint of the grain growth in the through-thickness direction. Veining denoting a subgrain structure is evident within the cube-on-face pancake grains of the alloy annealed at 1050 ~ Normal grain growth at 1250 ~ gives a polygonal ferrite morphology. Evidence of pinning of the boundary walls by precipitates in both the Nb and the Ti alloys was found in the transmission electron microscope (TEM). It was previously reported t17~ that the Ru modifies the recovery process, which can favor a microstructure susceptible to localized corrosion. The rampant secondary grain growth accompanying SHT of the Ru alloy at 1050 ~ is distinguished by a wavy grain-boundary morphology, indicative of recovery coalescence of low-angle boundaries (Figure 5(a)). In contrast, the 1250 ~ treatment yielded a fully recrystallized morphology comparable with the base alloy, attained by the migration and impingement of high-angle boundaries. C. Recovered Substructure
Restoratirn of all of the warm-worked Fe-40Cr alloys occurs by classic polygonization processes. Below 850 ~ the cell structure of the wrought base alloy is resolved by rearrangement of the dislocation network into low-angle boundaries, leaving regions of strain-free metal. The high stability of this subgrain structure was noted in earlier work. t131The dislocation arrays persist even after prolonged soaking at elevated temperature. Pinning by second-phase particles can stabilize the subgrains up to temperatures as high as 1250 ~ Recrystallization of the base alloy;is detectable after soaking for 1 hour at VOLUME 23A, FEBRUARY 1 9 9 2 - - 6 2 9
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(c) Fig. 2--Tensile deformation of Fe-40Cr alloys at RT: (a) Fe-4OCr-2Ni, (b) Fe-40Cr-0.2~u, and (c) Fe-40Cr-0.2Nb. 630--VOLUME 23A, FEBRUARY 1992
METALLURGICALTRANSACTIONSA
Table II. Ultimate Tensile Strength, (MPa)
Heat Cycle 1 2 3 4 5 6
Treatment 1 h 1050 ~ 1 h 1250 ~ 1 h 850 ~ 1 h 1050 ~ 1 h 850 ~ 1 h 1250 ~ 1 h 1050 ~ 1 h~50 ~ 1 h 1250 ~ 1 h 850 ~
Mechanical Properties of Fe-40Cr Alloys 0.2 Pct PS (MPa)
Pct Elongation
C V N (RT) ( J / c m 2)
HVs0
Base
Ni
Nb
Ru
Base
Ni
Nb
Ru
Base
Ni
Nb
Ru
Base
Ni
Nb
Ru
Base
Ni
Nb Ru
564 569 554
630 673 619
521 523 520
553 607 552
448 494 448
576 581 560
448 451 407
448 540 458
23 19 22
20 18 21
17 20 31
18 20 24
206 224 212
230 242 229
199 212 201
214 229 222
8 35 13
43 87 59
9 19 12
8 21 12
560
659
561
611
474
550
486
535
15
17
19
20
228
259
220
238
79
93
11
17
491
570
486
502
321
504
392
384
26
26
22
34
187
214
189
200
17
51
9
9
460
532
494
476
367
469
418
387
21
19
27
20
190
208
195
197
35
76
13
17
Note: PS = proof stress; HVs0 = Vickers hardness, 50 kg load; and C V N = Charpy V-notch energy.
850 ~ and at higher annealing temperatures, recovery and recrystallization occur as a merging process. Growth of the recrystallized grains is by migration of high-angle boundaries, and 1 hour at 1050 ~ is sufficient to ensure full recrystallization. Examination of the Nb alloy treated at 1050 ~ however, revealed extensive recovered subarrays responsible for the veining observed in Figure 4 (shown in Figure 6(a)). The high stability of the substructure can be explained by an increased flow stress attributed to pinning by second-phase particles. No evidence of a substructure was found in the Ti alloy, although a large number of decorated dislocations occurred within the matrix. The origin of the anomalous ribbonlike contrast visible in Figure 6(a) has not yet been resolved. This is shown at a higher magnification in Figure 6(b). We are tempted to call these stacking faults, bounded by partial dislocations. This is unlike a deformation twin, but the positive identification of extended dislocation configurations in bcc metals remains problematic. It is clear from the curves in Figure 2 that recovery annealing prior to SHT in no way debilitates the tensile ductility. To demonstrate this, specimens of the wrought base alloy were annealed at 850 ~ without any SHT. Restoration by recovery yields excellent ductility (but poor toughness), which persists virtually unaltered with increased soaking times (Figure 7). However, the substructures are known to act as sinks for mobile dislocations, t141 and prestraining of the alloys containing a substructure leads to dense tangles after strains of only 5 pct. This is accompanied by work hardening and a loss of impact resistance.
from wavy to planar slip, with an increase in the component of mechanical twinning. The excellent ductility of the alloys can be accounted for by their capacity to accommodate local stresses by both slip and twinning. This is illustrated in Figure 9, which shows a twin source at a terminating twin.
D. Deformation Modes Figure 8 shows the polished surface of the Fe-40Cr base alloy after deformation in tension. Deformation is by wavy slip. Under high strain rates, deformation changes Table l I l .
Solution Heat Treatment 1 h 1050 ~ 1 h 1250 ~
Effect of Alloying Additions on Grain Size
Base
Grain Size (/x) Ni Nb
Ru
181 306
80 410
291 304
METALLURGICAL TRANSACTIONS A
68 323
Fig. 3 - - G r a i n morphology revealed by polarized light: (a) Fe-40Cr2Ni (1050 ~ ~ d (b) Fe-40Cr-2Ni (1250 ~ VOLUME 23A, FEBRUARY 1992--631
Fig. 4 - - G r a i n morphology of Fe-40Cr-0.2Nb revealed by polarized light (850 ~ ~ Veining reveals partial recovery within the grains.
Fig. 6 - - ( a ) Stable recovered subarrays in Fe-40Cr-0.2Nb annealed at 1050 ~ and (b) enlarged view of anomalous ribbonlike contrast, suggestive of stacking faults.
30 5OO
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Fig. 5--Optical micrograph showing recovery and recrystallization of Fe-40Cr-0.2Ru (oxalic etch 5 pct, 5 V): (a) 1 h, 1050 ~ and (b) 1 h, 1250 ~ 632--VOLUME
23A, F E B R U A R Y 1992
-
10
Elongation
--
0
0.2~
I 15
20
(h)
Fig. 7 - - E f f e c t of annealing at 850 ~ on the tensile properties of Fe-40Cr. .. METALLURGICAL TRANSACTIONS A
Fig. 8 - - SEM image of wavy slip on the polished surface of Fe-40Cr deformed in tension.
The polished deformation surface of an impact specimen of Fe-40Cr-2Ni is shown in Figure 10(a). Under all heat-treatment conditions, the deformation mode is mixed twinning and planar slip. Figure 10(b) shows the plastic deformation on the surface of an impact specimen of the ductile Ti alloy. Although the twinning component exists to an increasing extent with decreasing temperature, the primary deformation mode at RT is slip. The polished surfaces adjacent to the impact fracture surfaces in the Ru alloy showed a complete absence of twinning or slip in the brittle material treated at 1050 ~ The material treated at 1250 ~ however, showed a profusion of mechanical twinning. The dislocation substructures of the alloys may be compared in the TEM sections in Figures 11 through 13. Figure 11 shows the dislocation configurations of the Fe-40Cr, Fe-40Cr-0.2Nb and Fe-40Cr-0.2Ru alloys prestrained by rolling at RT. The base alloy shows a uniform density of mobile dislocations, with high strain gradients at inclusions and in the grain-boundary re-
Fig. 9--Accommodation by twinning at a terminating twin adjacent to impact fracture. METALLURGICAL TRANSACTIONS A
Fig. 10--Deformation of polished surfaces adjacent to impact fracture, shown under polarized light: (a) Fe-40Cr-2Ni, showing mixed twinning and slip, and (b) Fe-4OCr-0.OSTi, showing predominant slip.
gions, t141 The occurrence of tangled bands indicates the beginning of cell formation. The large amount of dislocation debris and the straight appearance of the dislocations suggest that they are predominantly screw in character. Comparable dislocation structures are shown for the Ru and the Nb alloys prestrained by 5 pct. The Ru alloy shows a reduced degree of alignment of the dislocations and a greate,f tendency for tangling. The relatively long dislocations show numerous jogs along their length and the pinching off of dipoles, indicating movement out of the slip plane. The entanglement of dislocations following straining of the partially recovered Nb alloy was previously rep o r t e d . [14] Forests of dislocations in dense cellular arrays occur after only 5 pct prestrain. The dislocations in the fully recrystallized Nb alloy, however, exist in uniformly aligned arrays, despite the relatively higher degree of prestrain, namely 7 pct. VOLUME 23A, FEBRUARY 1992--633
Fig. 11 - - D i s l o c a t i o n substructure of Fe-4OCr alloys prestrained by rolling at RT: (a) Fe-4OCr annealed 1050 ~ strained 5 pct; (b) Fe-4OCr0.2Ru annealed 1050 ~ strained 5 pct; (c) Fe-40Cr-O.2Nb annealed 1050 ~ strained 5 pct; and (d) Fe-4OCr-O.2Nb annealed 1250 ~ strained 7 pct.
Figures 12 and 13 are representative of sections taken adjacent to the impact fracture surfaces of the Ni and Ti specimens, which exhibited good impact toughness and showed considerable plastic deformation. The most notable features can be summarized as follows. (1) The Ni alloy treated at 1050 ~ prior to testing exhibits slip predominantly on one system. The dislocations are relatively long and appear to lie within slipbands, although this may be the result of the specimen being sectioned further from the crack region. The spheroidal inclusions are sulfide species of the type CrS, shown to be unstable above 1050 ~ These may play a role in nucleating the dislocations during impact. [15] (2) The Ni alloy treated at 1250 ~ shows a uniform mobile dislocation structure after impact. (3) The tough Ti alloy similarly shows strong alignment of dislocations on the slip planes. (4) No evidence of cell structures was found in these impacted specimens. 6 3 4 - - VOLUME 23A, FEBRUARY 1992
Figures 11 through 13 show a progression from cross slip on several slip systems at nominal strain rates to well-defined planar slip at high strain rates. Plastic deformation of the Fe-40Cr alloy in the very early stages is confined to one set of slip planes. Corresponding amounts of deformation in the Ru alloy show an advanced extent of cross slip and dislocation interactions. The presence of a recovered dislocation network ensures severe entrapment of mobile dislocations, leading to a cellular substructure. The slip character then is a function of the flow stress, as determined by the strain rate, the alloy composition, and the dislocation substructure. IV.
DISCUSSION
A. Phenomenology Of the alloys studied, the addition of 2 pct Ni has the most pronounced effect on the plastic deformation, although the smaller additions of Ru, Nb, and Ti were all METALLURGICAL TRANSACTIONS A
observed to strongly modify the mobility of the dislocation arrays. Qualitatively, the effect of alloying in each case is equivalent to changing the flow stress and, hence, the slip character. Good tensile ductility has been shown to be compatible with (1) low strain rates; (2) an enhanced propensity for cross slip; (3) an increased degree ~f cellularity; (4) an absence of obstacles to long-range dislocation migration; and (5) the presence of low-angle dislocation subboundaries. Toughness, on the other hand, has been linked to a strong alignment of the dislocations and minimum dislocation-dislocation interactions. An increase in the flow stress at high strain rates effectively decreases the number of active slip systems and leads to the disappearance of a cell structure, in agreement with the observation of other authors, t8,1~J The plastic behavior of the Fe-40Cr alloys at high strain rates invites description of three deformation characteristics, concordant with the deformation behavior of bcc metals and alloys in general. First, there exists a regime where deformation proceeds from the slipping of long straight screws, on primary planes, to haphazard, illdefined systems and consequent dislocation interactions. Second, the occurrence of {112}011) twinning suggests that, under certain conditions, dissociation of the screw dislocations favors the {112} planes rather than the primary {110} planes. Finally, it must be recognized that, in addition to substitutional solutes, the presence of dispersed interstitial elements in small quantities can account for enhanced double-kink nucleation which governs the mobility of the screws, probably by lowering the nucleation e n e r g-y .[4. 5] We will consider these three aspects in greater detail.
Fig. 1 2 - - D i s l o c a t i o n substructures in sections taken adjacent to the impact fracture of Fe-40Cr-2Ni: (a) SHT at 1050 ~ and (b) SHT at 1250 ~
Fig. 1 3 - - D i s l o c a t i o n substructure in section taken adjacent to impact fracture of Fe-40Cr-0.05Ti showing strong alignment on the primary slip planes. METALLURGICAL TRANSACTIONS A
B. Dislocation Interactions
The results of these dislocation studies support earlier observations, namely that: (1) toughness is improved by a high density of mobile dislocations within the crack zone; (2) the ability of the dislocations to move and multiply to interact with the local crack field depends on the flow stress; and (3) the dislocation mobility is impaired by dislocationdislocation interactions, specifically with the low-angle boundary arrays, but also with one another, which superimposes on the lattice resistance. This leads to the apparent anomaly that enhanced cross slip reduces the ir ductility. Further to the observation that crack-tip shielding becomes operative provided that there is a sufficient density of mobile dislocations within the crack zone, it seems that the mobility of the dislocations is qualified by their ability to be confined to their slip planes. Thus, conditions of restricted slip (low temperatures, high strain rates) can actually improve the ductility, in accordance with the alloy softening mechanism. It is necessary to qualify our supposition that dislocation interactions debilitate the primary slip mode. The VOLUME 23A, FEBRUARY 1992--635
change in the dislocation structure with temperature has been the subject of a number of studies, and there is substantial evidence to link dislocation interactions to a change in the slip mode. For example, Nagakawa and Meshii ~41and Botta et al. TM note the occurrence of anomalous slip, as opposed to primary slip, in niobium alloys, caused by coplanar motion of primary and conjugate screw dislocations which exert mutual forces on one another. A softening effect was observed in specimens which deformed by primary slip, alloyed with zirconium to suppress the anomalous slip. However, Chomel and Cottu TM have argfaKi that the internal stress required to initiate kink pair nucleation is derived from the interaction between the mobile dislocations and all of the other dislocations in the crystal. They advance the theory that the effect of the solute is to change the distance between dislocations in the glide plane to this end. Clearly, it is necessary to distinguish between the types of dislocation interactions which may occur. Our observations lead us to conclude that recovered dislocation arrays and an increased propensity for cross slip lead to rapid strain hardening because of deflection and entrapment of the mobile dislocations, which increases the flow stress and is detrimental to the dislocation mobility. However, the introduction of uniform aligned dislocation arrays by small degrees of prestraining may provide such local stresses as envisaged by Chomel and Cottu to assist double-kink nucleation. C. Core Structure Implicit in our observations is the competition between slip on the primary glide and twinning {1 12} planes. In order to understand this, it is necessary to briefly consider the role of the dislocation core structure, which is thought to be the controlling factor in determining the mobility of screw dislocations. The cores of screw dislocations are relatively narrow, facilitating cross slip, but their structure is nonplanar and must undergo a transformation from a sessile configuration to a glissile one before slip can occur. This transformation can occur by the formation of (1) a multilayer fault on {112} planes or (2) an extended single-layer fault on {1 10} planes, t25j Slip should occur on the system with the lowest formation energy for double kinks. In the absence of interstitial solutes, the energies are approximately the same for these two systems, while a saturated core atmosphere favors a lower kink formation energy on {110} than {1 12}. [251 The well-established asymmetry of slip in bcc systems further accounts for preferential shear between the two systems. Slip is inclined to the {110} plane when the nearest {112} plane is sheared in the so-called antitwinning sense (high flow stress) and close to the maximum resolved shear stress plane when the corresponding {112} plane is sheared in the twinning sense (low flow s t r e s s ) . [261 Under conditions of increased flow stress, mixed twinning and slip therefore appear to be favored at the expense of primary {1 10} slip, but the reasons for this are not clear. Although the effect of the Ni addition may be inferred to be a fundamental change of the core structure in favor of {1 12} twinning, this is not borne out by the results of Chomel and Cottu, I31 who detected no sig6 3 6 - - V O L U M E 23A, FEBRUARY 1992
nificant variation in the flow stress at 0 K and the elastic constants on the addition of the substitutional element to Fe. D. The Importance o f Twinning The observed conditions of restricted slip that favor toughness, namely, high strain rates and substitutional alloying with Ni, also favor twinning. Since mechanical twinning is widely thought to occur by means of the dissociation of screw dislocations (as described, for example, by Mahajant27~), it is not improbable that the twinning and slip modes are coupled in the alloy softening phenomenon. The resistance to brittle fracture in the nickel alloy has been linked to a high capacity for stress accommodation by both slip and mechanical twinning. An increased incidence of twinning in super-ferritic alloys containing Ni has been reported by several investigators, t28 311 Magnin et al.t32J have shown that the effect of the Ni is to raise the temperature To at which the mobility of edge and screw dislocations become similar. Deformation is therefore controlled by screw dislocations up to much higher temperatures (or lower strain rates), and these, accordingly, give rise to mechanical twins over an extended range of test conditions. The operative mechanism whereby twinning can assist in crack blunting has been discussed at some length, and a synchronized response between the modified lattice friction in the solution-softened alloys and microtwinning was previously postulated, t14~ Several reasons have been advanced for an enhanced twin-related plasticity in bcc alloys. As has already been noted, the twinning plane {112} is a possible slip plane, and the twinning shear is parallel to the slip direction [ 1 1 1]. Also, the slip planes inside the twin are frequently realigned more favorably with respect to the applied stress, t331 This can result in highly localized deformation within the twin, as reported. Indeed, all of the Fe-40Cr alloys have been shown to be capable of twinning, and a higher density of twinning is associated with a greater degree of plastic deformation and toughness. There is no evidence to link twinning to a greater incidence of brittle cleavage initiation in the present system. In view of the above comments, the near absence of any twinning in the tough Ti alloy is a surprising result and leads to the conclusion that slip in this case is an independent response. The competition between tensile, shear, and twinning forces at the head of a local stress in this instance favors shear, and insufficient stress concentrations exist to operate the twin sources. This indicates a predisposition to dissociation of the screws on the {1 10} system, which may be a function of solute pinning at the dislocation cores.t25~ The operative mechanisms whereby the Ni and Ti enhance the cleavage resistance are qualitatively different and may be distinguished by the effect of the additions on the upper shelf toughness. While the Ni alloys exhibit virtually similar shelf energies to those of the base alloy, [14,231 the Ti addition markedly increases the shelf energy. The heterogeneous initiation of microvoids in the upper-shelf regime is a thermally activated process which may be modified by parameters such as r
METALLURGICAL TRANSACTIONS A
the particle shape, size, distribution, and interfacial strength, t~4,351 The Ti therefore appears to be active in changing the local strain parameter which governs the stress required to nucleate a void, apparently by modifying the second-phase particles, as expected. We infer that Ni, on the other hand, is active in modifying the emission conditions of the incipient crack.
E. Superposition of Microstructural Variables Rationalization of the microstructural parameters on the basis "~f the thermal history of the alloys allows us to summarize as follows. Our observations support the premise that cell formation is suppressed by an effective increase in the flow stress at high strain rates. Mobile dislocations entrapped by interaction with cellular arrays become impotent at the high strain rates. At low strain rates and high temperatures, these interactions can apparently be overcome by a thermally activated process, ostensibly by an unpinning mechanism under the influence of the applied stress, as envisaged by M c Q u e e n J TM At high strain rates, an increase in the lattice friction minimizes cross slip and induces slip primarily of planar screw dislocations. The effect of Ni is to increase the flow stress to this end, with a consequent increase in the mechanical twinning nucleated by the screws. Alloying additions of Ru advance the drive for restoration by recovery. This is indicative of a greater ease of cross slip and also a tendency to form a cellular deformation substructure. This corresponds to an increase in the stacking fault energy. Although Ru does give rise to substantial strengthening at RT, the alloying amounts studied in the present investigation do not indicate the solution softening effect reported by other investigators, although this cannot be stated categorically without comprehensive DBTT tests. It was noted that Ru induced lower levels of twinning in the brittle recovered alloys under impact conditions. This may indicate an ability of the low-angle boundaries to accommodate twinning stresses and an inability to nucleate twins. This is in agreement with a reported absence of twinning in Fe-40Cr alloys heat-treated within the recovery range, t231 The effect of grain size is subordinate to the concurrent effect of particle size. Refinement of the secondphase particles overrides the grain growth, which is a product of the higher SHT temperature. Overstabilization with Nb does not lead to general strengthening of the ferrite matrix, and our results do not support reports of solid-solution embrittlement by solutioning of the Nb. t37~ Niobium therefore appears to have a weak solution hardening effect and a correspondingly weak solution softening effect at the levels studied in the present system. The recrystallized Nb alloy is, however, amenable to prestraining, and strong alignment of the dislocations is conducive to good toughness, as discussed. Although the recovered substructure can account for the reduced toughness at the lower SHT temperature, it is not clear why stabilization of the interstitials does not improve the toughness at the higher temperature. A possible explanation is that the kinetics of reaction with the interstitials require longer aging times following soMETALLURGICAL TRANSACTIONS A
lutioning. However, an alternative explanation for embrittlement by overstabilization may be found by consideration of the effect of the interstitials on the slip character. As noted, small quantities of the interstitial elements can enhance double-kink nucleation. If this is the case, reduction of the free interstitials to below a critical level may impair the motion of screw dislocations by lowering the kink nucleation rate. Loss of toughness by overstabilization might therefore be attributable, in part, to the removal of these point defects. This is, in fact, borne out by the observation that since only a substoichiometric amount of Ti is recovered in the melt, it may not be necessary to fix all of the C + N for optimum toughness, t231 The considerably smaller addition of Ti appears to have a high reaction energy with the residual C + N. We conclude that the Ti is effective in neutralizing embrittling second phases, and the consequent improvement in toughness is attributed to elimination of brittle crack initiation sites. This supports our earlier conclusion that the toughness of the alloys is a measure of their resistance to crack initiation. V.
CONCLUSIONS
Microalloying additions of Ni, Ru, Nb, and Ti qualitatively modify the operative RT deformation modes of wrought Fe-40Cr base alloys. A study of the substructural development following heat treatment and deformation has shown the following: 1. Deformation is by mixed twinning and slip. 2. The addition of Ni enhances stress accommodation at high strain rates, in accordance with the alloy softening mechanism. Plastic deformation in the Ni alloy is associated with a coupling of twinning and slip modes. 3. Ruthenium advances a greater degree of cellularity and promotes restoration by recovery at the levels studied. 4. Niobium and titanium act primarily to modify the precipitation effects. 5. The formation of a recovered substructure is compatible with good tensile ductility. 6. Toughness is associated with a reduced degree of cellularity and a strongly aligned dislocation configuration. ACKNOWLEDGMENTS The authors wish to thank Mrs. E.A. DeMarsh for the provision of test samples of the titanium-stabilized alloy for microstructural analysis. It is a pleasure to acknowledge the assistance of Mssrs. N. Dreze, G. Newin~; and J. Maskrey in fabrication and specimen preparation and Mr. B. Greeves and Mrs. E. Lombaard for photographic processing. The authors are grateful to the Council for Mineral Technology (MINTEK), Randburg, South Africa, for financial assistance.
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