Metal Science and Heat Treatment

Vol. 44, Nos. 7 – 8, 2002

TITANIUM ALLOYS UDC 669.295.5:621.762

HIGH-STRENGTH GRANULATED TITANIUM ALLOYS WITH THE INTERMETALLIC TYPE OF HARDENING N. V. Sysoeva1 and V. N. Moiseev1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 38 – 42, July, 2002. Creation of titanium-base high-strength high-temperature alloys is of great scientific interest. Alloy VT22 additionally alloyed with carbon or boron, or simultaneously alloyed with carbon and boron is considered, with the aim of estimating the effect of the degree of dispersity, shape, and uniformity of distribution of intermetallic phases (titanium carbides and borides) on the combination of physical and mechanical properties and the structure of titanium alloys with the intermetallic type of hardening. The test rods were fabricated by traditional (hot deformation of the ingot) and granule (hot isostatic pressing of granules hardened from the melt) methods. The granule process is shown to provide substantially finer segregations of intermetallic phases and their more uniform distribution in the structure, which results in an increase in the high-temperature and conventional strengths and in the creep resistance of the alloy.

As a rule, modern deformable high-temperature titanium alloys contain a very low amount of chemical compounds in the a- or b-matrix. The primary alloying additives forming chemical compounds in titanium are aluminum (Ti3Al) and silicon (Ti5Si3 ). Multicomponent alloys may bear other chemical compounds. Unfortunately, we should admit that the kinetics of formation of chemical compounds has been studied insufficiently well even in commercial high-temperature alloys. Many chemical compounds that form in ingot melting are quite stable and cannot be controlled (dissolved) by subsequent thermal or thermomechanical treatment. The only exception is the Ti3Al compound (a2-phase) that easily dissolves when heated. At the present time, high-temperature titanium alloys are based on aluminum-saturated a-solid solution with a low amount of b-phase. In this connection, all commercial alloys belong to the class of pseudo-a-alloys or martensitic alloys bearing a certain amount of a-stabilizing elements. All of them are commonly used in an annealed state. High-temperature titanium alloys based on a- and (a + b)-solid solutions with additional hardening provided by a2-phase have quite high long-term strength and creep resistance at 500 – 600°C. In recent years Russian and foreign scientists have been working on the creation of alloys capable of hardening due to

INTRODUCTION The structural and high-temperature titanium alloys used at present in various branches of industry have a structure based on a-, (a + b)-, or b-phases represented by solid solutions of various alloying elements (Al, Mo, V, Nb, etc.) in titanium. The alloying sets for these alloys have been chosen based on the fact that their strength and heat resistance increase due to solid-solution reinforcement of the phases with substitutional elements. This is natural, because it has been assumed that titanium alloys with solid-solution hardening possess the best combination of strength, ductility, and other properties. The effect of precipitation hardening provided in (a + b)-titanium alloys by quenching and aging is determined by the degree of decomposition of the metastable b-phase in aging, the dispersity of the segregated particles, and other factors. It seems that the potentialities of titanium alloys with traditional solid-solution hardening have been considerably exhausted. In our opinion, further attempts to create commercial structural titanium alloys with this kind of hardening by using various combinations of alloying elements will not be very fruitful. 1

All-Russia Institute of Aircraft Materials (VIAM), Moscow, Russia.

304 0026-0673/02/0708-0304$27.00 © 2002 Plenum Publishing Corporation

High-Strength Granulated Titanium Alloys with the Intermetallic Type of Hardening

a third phase based on chemical compounds, i.e., alloys with intermetallic type of hardening. For strictness of the terminology we should define an “intermetallic compound as any phase that crystallizes forming a structure that differs from the structures of the constituent components” [1]. In this connection, we will apply the term “an intermetallic compound” or “intermetallic phase” to chemical compounds of titanium with various elements, for example, carbon, boron, or silicon. The present study is aimed at determining the criteria for the choice of the alloying elements forming chemical compounds with titanium, developing methods for controlling the morphology and the segregated phases based on chemical compounds, and estimating the effect of the degree of dispersity, shape, and uniformity of distribution of intermetallic phases on the combination of physical and mechanical properties of titanium alloys with intermetallic type of hardening. METHODS OF STUDY We studied alloy VT22 additionally alloyed with carbon (about 0.3%), boron (about 0.3%), or carbon and boron simultaneously (about 0.25% C + 0.2% B). The specimens were fabricated in two ways, namely, using (1 ) traditional (casting + deformation) and (2 ) granule (hot isostatic pressing (HIP) of rapidly quenched granules + deformation) technologies. The granules were obtained by centrifugal sputtering of a rotating electrode in a protective atmosphere with plasma heating of the end. The cooling rate of the melt was 103 – 5 ´ 104 K/sec. We studied the specimens under a JSM-35CF scanning electron microscope at a magnification of ´ 1000 – 10,000 and performed an x-ray diffraction phase analysis using a DRON-3 installation in copper Ka radiation. The mechanical properties of the studied alloys were determined by standard methods by testing the specimens for the tensile strength at various temperatures (20, 300, 450, and 500°C), for the impact toughness, long-term strength, and creep resistance at 450°C. RESULTS AND DISCUSSION The physical and mechanical properties of titanium alloys with intermetallic hardening chiefly depend on the properties of the solid-solution matrix and the properties, shape, degree of dispersity, and uniformity of distribution of the intermetallic phases. The strength and heat resistance of such a matrix depend on the alloying of the a- and b-solid solutions and are determined by the effect of solid-solution hardening of the phases by substitutional elements. Analyzing the phase diagrams of titanium with various elements, we determined a rather great number of elements

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that form intermetallic compounds with titanium, for example, C, B, Cr, Fe, Cu, and rare earth metals (REM). However, they should meet the following requirements in order to provide intermetallic hardening of titanium alloys: – have sufficient solubility in molten titanium and a low solubility (tenths of percent) in the solid state; – have a low diffusivity in the temperature range of process heating (300 – 1000°C); – be capable of forming thermodynamically stable compounds. B, C, Si, and rare earth metals (REM) satisfactorily satisfy these requirements. It is well known that the operating properties of titanium alloys with intermetallic hardening substantially depend on the degree of dispersity, shape, and uniformity of distribution of the intermetallic phases. For example, carbides and borides promote disintegration of the cast structure (are used as casting modifiers [2, 3]) and raise the temperature of recrystallization of titanium alloys [4]. In addition, carbon and boron additives to these alloys substantially increase the characteristics of their strength, heat resistance, and creep [5, 6]. However, in crystallization of the ingot, carbides and borides form coarse acicular segregations that cannot be disintegrated to the requisite degree by subsequent hot deformation and heat treatment. Coarse segregations of hard-todeform structural components of an acicular shape cause a substantial decrease in the ductility and increase the sensitivity of the alloys to crack nucleation and propagation under cyclic loading. The evolution of such alloys is closely connected with the problem of controlling of the intermetallic hardening. In addition to the correct choice of the composition, a very important problem in the creation of titanium alloys with intermetallic type of hardening is providing the required dispersity and uniformity of distribution of segregated intermetallic phases in the structure. Carbon and boron, which are poorly solvable in titanium and form independent segregations of carbides and borides, present interest as alloying additives. Therefore, it is necessary to find methods that would provide substantial disintegration and controllable uniformity of the distribution of the segregations of intermetallic phases (TiC, TiB, etc.) in the structure of the alloy. In our opinion, the granule metallurgy is a quite suitable method for controlling the kinetics of formation of chemical compounds as one of the most developed processes of rapid quenching from the melt. The extra rapid cooling of the alloy from the liquid state provides granules with a structure of supersaturated solid solutions that disintegrate in subsequent artificial aging and decompose forming disperse particles of chemical compounds. We studied the regular features of the decomposition of rapidly quenched granules of alloy VT22 additionally alloyed with carbon, boron, or carbon and boron simultaneously. The cooling rates attained in the granulation process

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à

b

c

d

Fig. 1. Microstructure of granules in alloy VT22 additionally alloyed with carbon and boron (´ 500): a) initial state (after granulation); b ) 0.25% C + 0.2% B, 6-h annualizing at 600°C; c) 0.3% C, 3-h annealing at 920°C; d ) 0.25% C + 0.2% B, 5-h annealing at 600°C + 3-h annealing at 920°C.

(vcool = 103 – 5 ´ 104 K/sec) are sufficient for intensification of the dissolution of elements in the a-solid solution, i.e., to 0.3% C, to 0.3% B, and to 0.25% C + 0.2% B. The absence of carbide and boride segregations in the structure of rapidly quenched particles is confirmed by the results of x-ray phase analysis (100% b-phase) and metallographic study (Fig. 1a ). Annealing of granules of the studied alloys causes decomposition of the b-phase. For example, 6-h annealing at 600°C leads to the appearance of titanium carbide (2.6% TiC) and boride (4% TiB) for VT22 alloyed with only carbon (to 0.3%) or only boron (to 0.3%). The structure of granules of VT22 alloyed with 0.25% C + 0.2% B and annealed for 6 h at 600°C in vacuum bears titanium carbide and boride segregations simultaneously. Their total amount is 4.5%. The carbides and borides are chiefly segregated over the boundaries of the initial dendrite cells (Fig. 1b ). Annealing of alloy VT22 with 0.3% C under the suggested temperature of compaction (3 h at 920°C) causes segregation of 5% TiC. The carbide has the form of fractured plates predominantly positioned over the boundaries of the formed b-grains (Fig. 1c ). Grain boundary segregation of carbides may cause brittleness of the alloy and is therefore undesirable. Double vacuum annealing conducted in two stages, i.e., a low-temperatures one (600°C) and a high-temperature one (920°C), suppresses the grain boundary segregation of carbides and borides virtually fully and fixes them inside the b-grains (Fig. 1d ), which prevents grain boundary brittleness. Thus, the application of double annealing to granules of the studied alloys makes it possible to control the size, morphology, uniformity of distribution, and prevailing places of segregation of carbides and borides in the structure. Study of the microstructure of granules of alloy VT22 with additives of carbon and boron allowed us to perform

pioneering compaction of granules in a gasostat according to the following scheme: heating to 600°C, 10-h hold, feeding of the working gaseous medium into the gasostat, heating to 920°C (with simultaneous increase in the pressure of the working gas in the gasostat to 200 MPa due to thermal expansion), 3-h hold, and cooling accompanied by a decrease in the working pressure of the gas in the gasostat. The annealing temperature of 600°C was chosen for the following reasons: it is sufficiently high for activating the diffusion processes of decomposition of the structure of rapidly quenched granules and at the same time sufficiently low for segregation of carbides and borides in a very disperse form. The compaction of granules in the gasostat is conducted with a technological hold at 500 – 600°C for providing plasticity of the capsule and feeding the gas into the gasostat. This makes it possible to combine the low-temperature stage of annealing of VT22 granules alloyed with carbon and boron with the process of compaction in the gasostat by extending the hold time of the already existing technological pause. Thus, fabrication of semiproducts from granules of VT22 alloy with additives of carbon and boron by the method of HIP makes it possible to control the dispersity, morphology, and uniformity of distribution of carbides and borides. For this purpose we should choose the temperature and duration of the hold in the heat treatment of the granules, which precedes or is conducted simultaneously with direct compaction of granules in the gasostat. We studied the macrostructure and mechanical properties of alloy VT22 alloyed with carbon and boron and fabricated by the conventional process of deforming the ingot and by hot isostatic pressing of granules with subsequent hot deformation of the compact. The microstructure of the rods fabricated from VT22 with 0.25% C by the two mentioned methods bears carbide segregations in the form of white particles. In the structure of the traditionally fabricated rods they are distributed less uniformly and have the form of crushed plates differing in shape and having a minimum size of 4 – 7 mm. In the rods fabricated by from the granule compact the titanium carbide segregations are distributed more uniformly and a have a round shape 1.5 – 2 mm in size. The microstructure of the traditionally fabricated rods of alloy VT22 with 0.3 B bears well-defined coarse acicular segregations of titanium boride crushed by the deformation. The thickness of the plates is 2 – 3 mm. In the structure of the rods obtained by deformation of the granule compact we observed uniformly distributed round segregations of titanium boride 0.2 – 0.5 mm in diameter. The structure of rods from alloy VT22 with 0.25% C + 0.2% B obtained by both methods contains segregations of titanium carbide and titanium boride (light and dark particles in Fig. 2, respectively). As in the previous cases the segregations are substantially disintegrated and more uniformly distributed in the rod rolled from the granule compact (Fig. 2).

High-Strength Granulated Titanium Alloys with the Intermetallic Type of Hardening

1 1

2 10 mm

à 2 1

10 mm

b

Fig. 2. Structure of rods from alloy VT22 with 0.25% C + 0.2% B obtained by traditional (a) and granule (b ) methods (´ 1000): 1 ) titanium borides; 2 ) titanium carbides.

Thus, the use of granule metallurgy makes it possible to reduce the size of titanium carbide particles by a factor of 4 – 5 and that of titanium boride particles by a factor of 10 and substantially increase the uniformity of their distribution in the structure of the semiproducts. The shape of carbide and boride segregations changes from acicular (traditional process) to round (granule metallurgy). We performed tensile tests of rods fabricated by traditional and granule methods (Tables 1 and 2). It can be seen from the tables that virtually any alloy obtained by the granule method has strength parameters 30 – 70 MPa higher than the alloys produced by the traditional process.

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It has been shown in [6] that the hardening effect in deformation and fracture of metallic materials bearing hard-todeform particles in their structure substantially depends on the shape and distribution of the particles. As a rule, if the segregated particles are round in shape and quite uniformly distributed in the plastic matrix, the strength of the material increases at a certain decrease in the ductility. The strength of materials bearing coarse enough particles up to 20 mm in size (in our case the traditionally fabricated rods) is primarily determined by the strength of the matrix. The Orowan hypothesis that dislocations overcome such particles by enveloping them was an important contribution to the theory of plastic flow of alloys possessing particles not deformable by shear. The level of the stresses is inversely proportional to the distance between the segregations. When a dislocation is arrested by point obstructions, it inflects in the gaps between the obstructions. Strong point obstructions hold the dislocation until it forms an Orowan loop. The mechanism suggested by Orowan is valid for small-size segregations (1 – 0.2 mm). Titanium carbides and borides in alloys obtained by the granule method have parameters very close to these values. When the test temperature is increased, the energy of breaking loose of the dislocation decreases as well as the stress at which the segregation is enveloped by the dislocations. It should be noted that this stress is inversely proportional to the distance between the segregated particles. Among the studied alloys the specimens obtained by the granule method followed by deformation have the smallestsize segregations and the shortest distance between them. Therefore, their strength is higher that that of traditional rods both at room and elevated temperatures, and the impact toughness is somewhat lower. We performed comparative tests of specimens of alloy VT22 alloyed with carbon and boron for long-term strength (Table 3) and creep resistance (Table 4). The alloys obtained by the granule method, other conditions being equal, had a higher endurance in the tests for long-term strength and a

TABLE 1. Mechanical Properties of Titanium Alloys at 20°C Alloy

VT22 + 0.2% C VT22 + 0.3% C VT22 + 0.2% B VT22 + 0.25% C + 0.2% B

sr , MPa

d, %

y, %

KCU, J/cm2

1290 1370 1330 1370 1300 1340 1320 1370

166 . 148 . 135 . 121 . 115 . 80 . 86 . 7.0

400 . 37.0 38.7 37.9 360 . 27.0 286 . 205 .

22 20 20 20 24 22 18 15

Notes. 1. The properties of the alloys are presented after a heat treatment involving 1-h hold at 840°C, 3-h air cooling with the furnace to 790°C, and 8-h aging at 580°C. 2. The numerators present the properties of alloys fabricated by the traditional method; the denominators present the properties of alloys obtained by HIP + deformation.

TABLE 2. Ultimate Rupture Strength of Titanium Alloys at Various Temperatures Alloy

VT22 + 0.2% C VT22 + 0.2% B VT22 + 0.25% C + 0.2% B VT22

Note. See the notes to Table 1.

sr , MPa, at a temperature of, °C 300

450

500

1045 1080 1070 1100 1090 1145 980 980

937 1018 945 1040 990 1070 880 890

844 940 850 928 887 960 760 780

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TABLE 3. Endurance of Titanium Alloys Obtained by Two Methods and Tested for Long-Term Strength Alloy

VT22 + 0.25% C

VT22 + 0.2% B

VT22 + 0.25% C + 0.2% B

s, MPa

550

570

590

TABLE 4. Residual Strain of Titanium Alloys eres after Creep Tests

t, h

67/123 77/125 78/180 81/192 56/125 64/167 78/173 80/209 64/123 48/141 71/150 82/179

Notes. 1. The tests were performed at t = 450°C and various stresses s. 2. The numerators present the time to failure of traditionally fabricated alloys; the denominators present the duration of the tests (the specimens did not fail) for alloys obtained by the granule method.

eres Alloy

VT22 + 0.25% C

VT22 + 0.2% B

VT22 + 0.25% C + 0.2% B

traditional method

HIP + deformation

0.40 0.38 0.38 0.60 0.54 0.52 0.34 0.33 0.32

0.20 0.19 0.18 0.21 0.20 0.18 0.18 0.15 0.13

Note. The creep tests were performed at t = 450°C, s = 200 MPa, and a hold of 100 h.

lower residual strain in the creep tests. These results are explainable from the standpoint of the theory of plastic flow of alloys bearing hard-to-deform particles [6]. As in the tests for short-term tensile strength, the levels of creep resistance and long-term strength were controllable by the degree of dispersity of the segregations of titanium carbides and borides and the uniformity of their distribution in the structure of the alloys.

3. The substantial disintegration and higher uniformity of distribution of titanium carbides and borides in the structure of semiproducts from alloy VT22 alloyed with C, B, and C + B and fabricated by the granule method provide higher characteristics of strength, heat resistance, and creep resistance than in alloys produced by the conventional process. 4. Granule metallurgy opens possibilities for creating new titanium alloys with controllable intermetallic type of hardening, which will possess enhanced characteristics of high-temperature strength, creep resistance, and strength.

CONCLUSIONS

REFERENCES

1. The cooling rates of the granules in the sputtering of rotating electrode from alloy VT22 are sufficient for preserving carbon (up to 0.3%), boron (up to 0.3%), and boron and carbon simultaneously (up to 0.25% C + 0.2% B) in the b-solid solution. 2. Double annealing at 600°C for 5 h and at 920°C for 3 h (the temperature of compaction) of rapidly quenched granules of alloy VT22 alloyed with C, B, and C + B virtually fully suppresses grain boundary segregation of titanium carbides and borides. By changing the temperature and time of the annealing, we can control the size, morphology, uniformity of distribution, and range of prevailing segregation of titanium carbides and borides in the structure.

1. R. W. Kahn and P. Haasen (eds.), Physical Metallurgy, Vol. 1. Atomic Structure of Metals and Alloys [Russian translation], Metallurgiya, Moscow (1987). 2. S. G. Glazunov and V. N. Moiseev, Structural Titanium Alloys [in Russian], Metallurgy, Moscow (1974). 3. Melting and Casting of Titanium Alloys, Ser. “Titanium Alloys” [in Russian], Metallurgiya, Moscow (1978). 4. I. S. Pol’kin, Yu. I. Zakharov, T. V. Ishun’kina, et al., “Effect of modifying additives on the structure and properties of alloy VT22,” Tekhnol. Legk. Splavov, No. 7, 26 – 27 (1986). 5. I. I. Kornilov, Titanium [in Russian], Nauka, Moscow (1975). 6. R. W. Kahn and I. Haasen, Physical Metallurgy, Vol. 3. Physicomechanical Properties of Metals and Alloys [Russian translation], Metallurgiya, Moscow (1985).