DOI 10.1007/s11041-016-0026-4 Metal Science and Heat Treatment, Vol. 58, Nos. 7 – 8, November, 2016 (Russian Original Nos. 7 – 8, July – August, 2016)
MAGNESIUM AND TITANIUM ALLOYS UDC 669.721.5:621.762.224
EFFECT OF UNIFORM COMPRESSION DEFORMATION ON THE STRUCTURE AND PROPERTIES OF HIGH-STRENGTH MAGNESIUM ALLOYS E. F. Volkova1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 32 – 38, July, 2016. The possibility of application of a nonstandard deformation process by the method of hydroextrusion (HE) to commercial high-strength magnesium alloys MA14 and MA22 is studied and proved. It is shown that specific modes of HE deformation raise the level of the strength properties of the alloys by 25 – 50% as compared to the standard deformation process.
Key words: magnesium alloys, hydroextrusion (HE), subgrain structure, intermetallic phases, mechanical properties.
nonequilibrium triaxial compression and extension. By the data of preliminary studies, the method of HE makes it possible to raise the strength characteristics of magnesium alloys [11]. The process of HE involves extrusion of a billet from a container through a die with the help of a liquid or quasi-liquid that prevents contact between the deformed material and the walls of the container and lowers the friction force in the source of plastic deformation. In HE, the deformed body undergoes uniform compression (hydrostatic pressure), and the plastic strain is localized in the region of the cone of the die. These conditions provide a maximum level of hydrostatic pressure, a favorable stress-strain condition of the material in the source of the deformation, and a minimum effect of external friction. As a result, the method of HE permits implementation of much higher forces and strains in solid bodies than in the conventional processes and makes it possible to use the pressure and the deformation for directed formation of structure and properties in the articles and creation of a virtually uniform distribution of the characteristics over their length and cross section [7]. The method seems to be quite promising for hard-to-deform magnesium alloys. The aim of the present work was to study the action of hydroextrusion deformation under uniform compression on the structure and main mechanical properties of high-strength magnesium alloys MA14 and MA22.
INTRODUCTION Lowering of the mass of articles in surface transport, aircraft and spacecraft engineering, as well as saving of fuel per unit effective load, remains a very important task of modern materials science with allowance for the rigid requirements of ecological safety [1 – 4]. At the same time, application of novel materials in advanced civil aircrafts is oriented today at a wide use of novel composite materials. However, this requires development of the materials themselves and creation of novel productions. The cost of such materials and articles from them should be much higher than the cost of metallic constructions [5, 6]. These considerations show that the problem of raising the competitiveness of magnesium alloys as light and inexpensive metal-base structural materials with advanced operating characteristics preserves its importance. This problem may largely be solved by alloying magnesium alloys with rare earth elements (REE) and by applying nonstandard processes, for example, deformation involving hydroextrusion (HE) [7 – 10]. Hydroextrusion makes it possible to overcome some difficulties with deformation of complexly alloyed highstrength magnesium alloys, which are commonly treated by 1
Federal State Unitary Enterprise “VIAM,” Moscow, Russia (e-mail:
[email protected]).
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METHODS OF STUDY The materials for the study were high-strength alloys MA14 (the Mg – Zn – Zr system) and MA22 (the Mg – Zn – Zr – REE system). The chemical composition of MA14 was determined by chemical and spectral methods and met the GOST 14957 Standard, i.e., Mg – 5.70 Zn – 0.48 Zr (in wt.%). The content of impurities matched the limits standardized by GOST 14957 (MA14) and certified for alloy MA22. We studied commercial ingots of alloys MA14 and MA22 with Æ 370 mm. After the mechanical treatment and homogenizing we pressed the billets with Æ 345 mm fabricated from the ingots in a horizontal hydraulic press for bars with Æ 45 mm. The bars with Æ 45 mm were cut into measured pieces, which were subjected to hydroextrusion in a horizontal 1600-tf hydraulic press equipped with a specially designed device. Alloy MA14 was heat treated by regimes prescribed by PI 1.2.655–2003 “Heat Treatment of Deformable Magnesium Alloys.” Alloy MA22 was not heat hardenable and therefore not heat treated. The mechanical properties of the pressed bars under uniaxial tension were determined after deformation and after a heat treatment matching GOST 1497–84 and GOST 25503–80. The microstructure of the alloys was studied under a “Leica” metallographic device. The images were obtained with the help of a VEC-335 (3 megapixels) digital camera and a JSM-840 scanning electron microscope with a “Link” attachment for microscopic x-ray spectrum analysis. The phases were identified by the method of physicochemical analysis with chemical phase isolation and subse-
Fig. 1. Microstructure of pressed bars with Æ 45 mm from magnesium alloys MA14 (a, b ) and MA22 (c, d ) in longitudinal direction: a, c) light microscope (LM); b, d ) scanning electron microscope (SEM).
quent x-ray diffraction and chemical analysis of the isolate by a powder method with the help of a D/Max-2500 “Rigaku” diffractometer.2 The images of the isolate and of the monolithic specimens were obtained in copper Ka monochromatic radiation. The results were interpreted using the Gade software. The phases were identified using a computer database with allowance for the chemical composition of the alloys. The shape of particles in individually separated phases was studied additionally under an electron microscope. The qualitative and quantitative microscopic x-ray spectrum analysis was performed using a Superprobe-733 device [10, 11]. RESULTS AND DISCUSSION Investigation of the microstructure of hot-pressed bars 45 mm in diameter from alloy MA14 under a light microscope does not make it possible to estimate the special features of the alloy. The visible grains are quite large in size (15 – 25 mm) and are stretched over the deformation axis. The results of the study of the structure of alloy AM14 under a scanning electron microscope allow us to distinguish a subgrain structure with an average grain size of about 4 – 6 mm. The boundaries of the subgrains are decorated with very fine precipitates of Zr3Zn2 and ZrZn2 (a Laves phase) intermetallics. Fragments of a eutectic Mg2Zn3 component in the form of extended dark inclusions (light microscope) or bright white inclusions (scanning electron microscope) are arranged along the pressing axis (Fig. 1a and b ). On the whole, the results of the study of the structure and phase 2
The study of the phase compositions was performed by G. I. Morozova.
Effect of Uniform Compression Deformation on the Structure and Properties of Magnesium Alloys
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TABLE 1. Mechanical Properties of Pressed Bars with Æ 45 mm from Alloys MA14 and MA22 in Different Conditions (Longitudinal Direction) Alloy
MA14
MA22
State
Not heat treated Aged (T1) Quenched + aged (T6) Not heat treated
sr , MPa
s0.2 , MPa
d, %
295 – 310 328 – 332 302 – 315 315 – 330
223 – 254 245 – 250 210 – 230 225 – 256
8.6 – 12.5 7.8 – 8.5 10.2 – 14.0 10 – 15
Note. The properties were averaged after testing 5 – 10 specimens.
TABLE 2. Effect of HE Treatment on the Mechanical Properties of Bars from Alloys MA14 and MA22 (Longitudinal Direction) Alloy
AM14
Treatment
Hot pressing + HE with e = 15%
Heat
sr , MPa
s0.2 , MPa
d, %
1
352 – 358
302 – 312
4.0 – 5.1
Hot pressing + HE with e = (20 + 15)%
380 – 383
343 – 348
6.0 – 7.0
Hot pressing + HE with e = (25 + 15)%
384 – 390
346 – 350
6.6 – 7.2
Hot pressing + HE with e = (30 + 15)% MA22
Hot pressing + HE with e = 15% ²
395 – 403
358 – 369
7.6 – 8.0
1
400 – 403
363 – 374
5.6 – 7.2
2
402 – 414
368 – 380
4.2 – 7.6
Note. The properties were averaged after testing 8 – 10 specimens.
composition of alloy MA14 match the data obtained earlier in [11, 12]. Alloy MA22 in the hot-pressed condition (bars with Æ 45 mm) is characterized by quite fine grains (4 – 8 mm) and complex phase composition. The phase composition includes not only the earlier mentioned zinc zirconides typical for alloy MA14 of the Mg – Zn – Zr system, but also variable-composition phases Zn2(Zr, REE) and (Mg, Zn)5REE (Fig. 1c ). We detected a certain content of e-ZrH2 and d-ZrH zirconium hydrides in both alloys. Investigation of the structure under a scanning electron microscope allows us to distinguish under a high magnification a fine net of grains and coarse crystals of white primary crystals, which obviously enter the eutectic, i.e., an a-solid solution + (Mg, Zn)5REE (Fig. 1d ). A characteristic feature confirmed in our work is the fact of stability of the primary phases in both alloys; they are preserved even after hot deformation, which agrees with the results of the earlier works [13 – 15]. Table 1 presents the properties of pressed bars with Æ 45 mm deformed by the standard process in the initial hot-pressed condition (MA22) and after the heat treatment recommended for MA14. Alloy MA14 possesses an optimum combination of strength and ductility properties in aged condition (Table 1), which is also confirmed by our earlier experimental results [14]. In accordance with the data obtained, the upper limit of
permissible strain (including the total one) in the case of the use of HE for alloy MA14 should be raised to 40 – 45%. For the high-strength and complexly alloyed AM22 we limited the upper boundary of admissible deformation to 15% with allowance for the results of preliminary tests. Despite the high values of the elongation (d = 10 – 15%) in the initial hot-pressed condition of the bars of alloy MA22 with Æ 45 mm, which exceeded the same parameters of alloy MA14, growth in the degree of deformation of alloy MA22 under HE above 20% caused fracture of the specimens by the mechanism of brittle cleavage. In our opinion, this was a result of the high microhardness of the matrix a-solid solution and of the considerably higher alloying of MA22 as compared to MA14 [11, 14]. Table 2 presents the mechanical properties of bars from the alloys studied after HE without heat treatment. Comparative analysis of the results of Tables 1 and 2 allows us to state that the effect of pressure treatment by the method of HE is the highest (in the absolute value) for alloy AM22. After the deformation with e = 15% the ultimate strength of MA22 increases by 25% on the average. For alloy MA14 the strength growth under the same conditions by 18% with respect to the initial value in the hot-pressed condition. The elongation of MA22 remains at a somewhat higher level (Table 2). However, in the case of two-stage HE (with 45% total deformation) alloy MA14 exhibits higher growth in the strength properties (in a relative measure) amounting to 32% with respect to the ultimate strength and to 53% with respect
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to the yield strength at an elongation ranging within 7.6 – 8.0%. Alloy MA22 exhibits higher absolute values of strength properties under the optimum treatment regime, whereas the relative effect of growth in the strength properties as a result of HE is higher for alloy MA14. Studying the microstructure of both alloys after hot pressing and HE with e = 15% we managed to establish some regular features. Alloy MA14 is characterized by scattered grain sizes and coarser grains (12 – 18 mm). However, fine subgrains (3 – 4 mm) start to appear in the structure of the alloy in this condition. The main mechanisms of straining in this case are sliding and twinning, traces of which are observable when the microstructure of alloy MA14 is studied under a light microscope (Fig. 2a ). Investigation under a scanning electron microscope shows segregations of fine hardening particles of Zr3Zr2 zinc zirconides and a ZrZn2 Laves phase (Fig. 2b and e). The main hardening intermetallics in combination with distortions and flaws accumulated in the crystal lattice of the alloy MA14 upon growth in the degree of HE deformation are responsible for the hardening. It should be noted that when the degree of the HE deformation increases, the number of twins in alloy MA14 grows, and features of cell formation appear; two-stage HE pro-
d
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Fig. 2. Microstructure of bars from magnesium alloys MA14 (a, b, d ) and MA22 (c, e) in longitudinal direction after hot pressing + HE with e = 15% (the arrows point at typical segregations of intermetallic particles): e) hot pressing + HE with e = (30 + 15)%; a, c) LM; d, e) SEM.
duces a fine subgrain structure and initiates the process of double twinning (Fig. 2e ). Alloy AM22 is characterized by the presence of fine grains (6 – 8 mm) and by a much higher density of fine particles of hardening intermetallics Zn2(Zr, REE) and (Mg, Zn)5REE (Fig. 2c and d ). We have mentioned that alloy MA22 possesses higher microhardness of the very a-solid solution due to its more complex alloying including REE. All these factors provide a higher hardening effect (in the absolute value) of MA22 as compared to AM14 under identical conditions of HE. Comparing and analyzing the structural changes in the two alloys under HE we should state that deformation by the method of HE creates conditions necessary for formation of dispersed nanosize hardening particles of intermetallics in both of them. In the zone of compression and subsequent tension of the metals the intragrain friction accompanying the process of HE increases local temperatures to the values of partial dissolution of the intermetallics, which are then segregated in the structure of the hydroextruded semiproducts in the form of nanosize particles. We witness a kind of phase recrystallization, which may be termed more accurately as dynamic dispersion of the phases, because the deformation process causes considerable refinement of the particles of the intermetallics.
Effect of Uniform Compression Deformation on the Structure and Properties of Magnesium Alloys sr ; s0.2 , ÌPà
y, %
350
50
sr
325
45
415
y
40 300
35
s0.2
d, %
30
275
24
d
25
16
250 20
à 0 No HE
20
25
e, %
8
b 30
15
No HE
20
25
e, %
30
Fig. 3. Dependence of the strength (a) and ductility (b ) characteristics of alloy AM14 on the degree of deformation under HE (e) and subsequent heat treatment by different regimes: )) aging (T1), regime 1; ^) aging (T2), regime 2; &) hardening + aging (T6), regime 3.
Since a hardening heat treatment is not applied to alloy MA22, we studied the joint action of HE and heat treatment (i.e., a thermomechanical treatment (TMT)) on the structure, phase composition and properties only for bars from alloy MA14. The modes of heat treatment were chosen for alloy MA14 in accordance with production instruction PI 1.2.655–2003 “Heat Treatment of Deformable Magnesium Alloys.” After the HE, the bars were divided into three groups each of which was heat treated by one of the following regimes: (1 ) aging T1, (2 ) aging T2, and (3 ) complete heat treatment T6 involving hardening + aging (T6). Analyzing the results obtained we established that the TMT of bars from alloy MA14 lowers the strength properties by 10 – 12% (to 330 – 340 MPa with respect to the ultimate strength and to 235 – 250 MPa with respect to the yield strength) as compared to the hydroextruded condition. Thus, the hardening effect produced by the HE is removed to a considerable measure after the TMT, and the alloy softens (Tables 1, 2 and Fig. 3a ). The values of the elongation and contraction increase by a factor of 1.5 – 2.0 simultaneously as compared to the results obtained after the heat treatment without HE (Table 1; Fig. 3b ). This is explainable by the fact that fine particles of zinc zirconides mostly consisting of a ZrZn2 Laves phase and some Zr3Zn2 form on twins and sliding planes typical for the HE deformation of alloy MA14 prior to and after the TMT (Figs. 2b and 4a). Artificial aging of the bars after the HE increases the linear sizes of the particles of zinc zirconides from 0.05 – 0.30 mm to about 0.40 – 0.55 mm (Figs. 2b and 4a). A complete heat treatment (hardening + aging) after the HE results in virtual disappearance of the twins. The particles of zinc zirconides continue to coagulate; their size grows to about
0.60 – 0.70 mm (Fig. 4b ). The mentioned lowering of the strength properties of alloy MA14 is a natural consequence of this. Thus, the results obtained show considerable possibilities of practical application of the nonstandard deformation process by the method of hydroextrusion to high-strength commercial magnesium alloys with the aim of raising the level of their strength properties at an enough ductility margin.
à
10 mm
b
10 mm
Fig. 4. Microstructure of specimens of alloy MA14 after TMT (SEM): a) HE with e = 30% + aging; b ) HE with e = 30% + complete heat treatment (hardening + aging).
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CONCLUSIONS 1. Application of a nonstandard process involving hydroextrusion (HE) to commercial high-strength magnesium alloys MA14 and MA22 makes it possible to raise the level of their strength properties by 25 – 50% (sr = 390 – 414 MPa; s0.2 = 340 – 380 MPa) at a good enough ductility level (d ³ 7%). 2. The degree of HE deformation of complexly alloyed high-strength MA22 should not exceed 15%; alloy MA14 may be treated by single-stage or double-stage HE deformation with maximum total degree 45% (30% + 15%). 3. Application of the tested modes of TMT to alloy MA14 is not effective, because it causes coagulation of nanosize particles of the hardening phases segregated in the process of HE and considerable softening. 4. The effect of HE in the studied alloys of the Mg – Zn – Zr and Mg – Zn – Zr – REE systems is produced due to growth of imperfections in the crystal lattices of the alloys including twins, development of the processes of cell formation, double twinning, formation of a disperse subgrain structure (dgr = 3 – 7 mm) and, partially, dynamic dispersion of the phases. REFERENCES 1. E. N. Kablov, Aerospace Materials Science in: All Materials. Encyclopedic Manual [in Russian], No. 3 (2008), pp. 2 – 14. 2. E. N. Kablov, “Innovative developments at FGUP “VIAM” GNTs RF on Implementation of the ‘Strategic Directions of Development of Materials and Treatment Processes until 2030’,” Aviats. Mater. Tekhnol., 1(34), 3 – 33 (2015). 3. I. S. Kornysheva, E. F. Volkova, E. S. Goncharenko, et al., “Prospects of application of magnesium and castable aluminum alloys,” Aviats. Mater. Tekhnol., No. S, 212 – 222 (2012). 4. A. Nicholas and S. Rol’nik, “Magnesium components in the aerospace industry,” Aerokosm. Kur’er, No. 1, 42 – 44 (2011). 5. V. V. Sadkov, Yu. L. Laponov, V. P. Ageev, and N. S. Korovina, “Prospects and conditions of application of magnesium alloys
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