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Acta Metall Sin  2016, Vol. 52 Issue (9): 1079-1088    DOI: 10.11900/0412.1961.2015.00555
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DENSIFICATION MECHANISM OF TiAl PRE-ALLOY POWDERS CONSOLIDATED BY HOT ISOSTATIC PRESSING AND EFFECTS OF HEAT TREATMENTON THE MICROSTRUCTURE OF TiAl POWDER COMPACTS
Gang WANG1,Lei XU2,Yuyou CUI2(),Rui YANG2
1 Yingkou Institute of Technology, Yingkou 115003, China
2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

Owing to the low density, high strength, good creep properties at elevated temperatures, TiAl alloy is considered for high temperature applications in aerospace industries. However, a major issue to the industrial applications is the alloy's intrinsic brittleness at room temperature. Therefore, extensive efforts have been made to overcome this defect by near net shape fabrication techniques. An alternative for fabricating TiAl alloy is the powder metallurgy processing of pre-alloyed powders, and by this technique TiAl alloy with fine and homogenous microstructure can be obtained. In this work, TiAl pre-alloyed powders with nominal composition Ti-47Al-2Cr-2Nb-0.2W-0.15B (atomic fraction, %) and Ti-45Al-8Nb-0.2Si-0.3B are produced by electrode induction melting gas atomization (EIGA). The pre-alloyed powders are consolidated by hot isostatic pressing (HIP). The effects of heat treatment on the microstructure of TiAl compacts and the influence of cooling rate on the solid-state transformations which occurs during continuous cooling of the TiAl compacts have been studied. It is found that the cooling rate of the pre-alloyed powders is between 105~106 K/s. As the cooling rate increases, the martensitic transformation, i.e., βα' occurs in some fine pre-alloyed powders. The heating DSC curves indicate that the transformation from α2 phase to γ phase takes place between 700~800 ℃. During the HIP processing, the pre-alloyed powders particles randomly accumulate, and the relative density of HIP compact is increased by the particles moving, rotating and rearranging at the initial stage. As the temperature increases, α2 phase transforms into γ phase. With further temperature increasing, significant plastic deformation and the following formation of sintering necks occur in the powder particles. With the annealing time increasing, the pores between the particles are closed by means of surface diffusion, volume diffusion and diffusion creep. The microstructure of Ti-47Al-2Cr-2Nb-0.2W-0.15B powder compacts consists of fine γ and a small number of α2 and β; and the microstructure of Ti-45Al-8Nb-0.2Si-0.3B powder compacts consists of fine γ phase, a small number of α2 phase and dispersed ξ-Nb5Si3 phase. The area fractions of γ phase, α2 phase and α2/γ lamellar structures vary with the annealing temperatures, depending on the Gibbs free energies of the phases. The cooling rate has a significant effect on the continuous cooling transformation of both TiAl powder compacts. For Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy, the microstructure is composed of predominant equiaxed α2 phase after water cooling, but of lamellar structures after air, oil or furnace cooling. For Ti-45Al-8Nb-0.2Si-0.3B alloy, the microstructure is composed of γm phase and large α2 phase after water cooling; after oil and air cooling the alloy consists of a mix of feathery like structures, Widmanst?tten laths and lamellar structures; while furnace cooling leads to fully lamellar structures. Comparing the continuous cooling transformation curves, the increase of Nb can effectively extend the continuous cooling transformation to the diffusionless area.

Key words:  TiAl alloy      heat treatment      microstructure      phase transformation     
Received:  30 October 2015     
Fund: Supported by Science and Technology Funds of Liaoning Education Department (No.L2014598)

Cite this article: 

Gang WANG,Lei XU,Yuyou CUI,Rui YANG. DENSIFICATION MECHANISM OF TiAl PRE-ALLOY POWDERS CONSOLIDATED BY HOT ISOSTATIC PRESSING AND EFFECTS OF HEAT TREATMENTON THE MICROSTRUCTURE OF TiAl POWDER COMPACTS. Acta Metall Sin, 2016, 52(9): 1079-1088.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00555     OR     https://www.ams.org.cn/EN/Y2016/V52/I9/1079

Fig.1  XRD spectrum of Ti-47Al-2Cr-2Nb-0.2W-0.15B pre-alloyed powders
Fig.2  Low (a) and high (b) magnification SEM-BSE and cross-section OM (c) images of Ti-47Al-2Cr-2Nb-0.2W-0.15B pre-alloyed powders with diameter of 400 μm, and low (d) and high (e) magnification SEM images of Ti-47Al-2Cr-2Nb-0.2W-0.15B pre-alloyed powders with diameter of 60 μm
Fig.3  Relationship between powder diameter and cooling rate of Ti-47Al-2Cr-2Nb-0.2W-0.15B
Fig.4  DSC curves of TiAl pre-alloyed powders at heating rate of 10 ℃/min
Fig.5  OM (a), low (b) and high magnification SEM-BSE (c) and bright field TEM (d) images of Ti-47Al-2Cr-2Nb-0.2W-0.15B pre-alloyed powders after hot isostatic pressing (HIP) at 1260 ℃ for 4 h
Area Ti Al Cr Nb W
A 51.20 44.53 1.74 2.30 0.23
B 57.35 37.12 3.03 2.23 0.28
C 50.90 37.07 8.58 2.09 1.35
Table 1  EDS analysis of areas marked in Fig.5c (atomic fraction / %)
Fig.6  OM (a), low (b) and high magnification SEM-BSE (c) and bright field TEM (d) images of Ti-45Al-8Nb-0.2Si-0.3B pre-alloyed powders after HIP at 1260 ℃ for 4 h (Arrows in Figs.6c and d indicate ξ-Nb5Si3 phases)
Fig.7  SEM-BSE images of Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy annealed at 1220 ℃ (a), 1250 ℃ (b), 1280 ℃ (c), 1310 ℃ (d) and 1340 ℃ (e) for 2 h
Fig.8  Phase evolution with annealing temperatures for Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy after annealing for 2 h
Fig.9  SEM-BSE images of Ti-45Al-8Nb-0.2Si-0.3B alloy annealed at 1220 ℃ (a), 1250 ℃ (b), 1280 ℃ (c), 1310 ℃ (d) and 1340 ℃ (e) for 2 h
Fig.10  OM images of Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy after water cooling (a), oil cooling (b), air cooling (c) and furnace cooling (d), respectively
Fig.11  OM images of Ti-45Al-8Nb-0.2Si-0.3B alloy after water cooling (a), oil cooling (b), air cooling (c) and furnace cooling (d), respectively
Fig.12  Schematic of densification behavior of TiAl pre-alloyed powders in the initial stage (a), middle stage (b) and latter stage (c) of densification
Fig.13  OM image of partly densitified Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy
Fig.14  Schematic CCT curves of Ti-47Al-2Cr-2Nb-0.2W-0.15B (a) and Ti-45Al-8Nb-0.2Si-0.3B (b) alloys (WQ—water quenching, OQ—oil quenching, AC—air cooling, FC—furnace cooling, Tαα phase transus temperature, F—feathery structure, W—Widmanst?tten structure, L—lamellar structure)
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