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Acta Metall Sin  2017, Vol. 53 Issue (9): 1055-1064    DOI: 10.11900/0412.1961.2016.00457
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Microstructures and High Temperature Tensile Properties of Ti-43Al-4Nb-1.5Mo Alloy in the Canned Forging andHeat Treatment Process
Tianrui LI1, Guohuai LIU1(), Mang XU1, Hongzhi NIU2, Tianliang FU1, Zhaodong WANG1, Guodong WANG1
1 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
2 School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China
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Abstract  

TiAl alloys are highly promising for high temperature structural application due to their excellent mechanical properties. However, the widespread applications of TiAl alloys have been limited for their low temperature brittleness and poor workability. The further thermo-mechanical treatments is applied for fine microstructures and improved ductility to promote the commercial applications, during which the investigations of hot deformation behavior and microstructural evolution are necessary for the improved microstructure and mechanical properties. The canned forging and subsequent heat treatments of Ti-43Al-4Nb-1.5Mo alloy have been conducted, during which the hot deformation behavior, flow softening mechanism, microstructure evolution and mechanical properties were investigated. The results show that the flow softening process of the canned forging TiAl alloy can be attributed to the soft β phase, α2/γ lamellae decomposition and the dynamic recrystallization induced by dislocation slipping and twinning in γ phase, and the final microstructure is composed of remnant α2/γ lamellae and equiaxed α2, γ and B2 phases. With the increasing heat treatment temperature, the microstructure changes from the multi-phase structure (remnant α2/γ lamellar, equiaxed α2, γ and B2 phases) at 1250 ℃ to the α2/γ lamellar and γ phase at 1285 ℃, and then the fully α2/γ lamellar structure at 1300 ℃, during which the B2 phase is gradually dissolved due to the solution diffusion, and the remnant α2/γ lamellae change to equiaxed α2/γ colonies according to the α2/γγ+α2+B2 transition, and the final fully α2/γ lamellar structure is promoted by γα transition at high temperature. Moreover, the tensile tests of the hot isostatic pressed (HIPed) samples, canned forged and heat treated samples at 800 ℃ are conducted, in which the fully lamellar structure shows the high properties with the ultimate strength of 663 MPa and the elongation of 26%. The deformation process of the fully α2/γ lamellar can be strengthened by the lamellae twisting, microvoid inhibition and wavy growth of the cracks, leading to the optimal high temperature performance. Moreover, the disordered bcc β phase can promote the deformation during the hot working process at the high temperature (≥1200 ℃), while the hard-brittle B2 phase severely deteriorates the service properties, which should be controlled accurately for the high mechanical properties during the thermo-mechanical processing.

Key words:  TiAl alloy      canned forging      heat treatment      microstructure      mechanical property     
Received:  17 October 2016     
ZTFLH:  TG146  
Fund: Supported by National Key Research and Development Program of China (Nos.2016YFB0301200 and 2016YFB- 0300603), National Natural Science Foundation of China (No.51504060), Fundamental Research Funds for the Central Universities (No.N140703003) and PhD Start-up Foundation of Science Project of Liaoning Province (No.201501150)

Cite this article: 

Tianrui LI, Guohuai LIU, Mang XU, Hongzhi NIU, Tianliang FU, Zhaodong WANG, Guodong WANG. Microstructures and High Temperature Tensile Properties of Ti-43Al-4Nb-1.5Mo Alloy in the Canned Forging andHeat Treatment Process. Acta Metall Sin, 2017, 53(9): 1055-1064.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00457     OR     https://www.ams.org.cn/EN/Y2017/V53/I9/1055

Fig.1  Microstructures and phase constituents of the hot isostatic pressed (HIPed) Ti-43Al-4Nb-1.5Mo (atomic fraction, %) alloy (a) microstructure (b) lamellar structure(c) γ phase and B2 phase at the lamellar colony boundary (Inset shows the SAED pattern of B2 phase)
Fig.2  Low (a1~c1) and high (a2~c2) magnified microstructures at the different regions of the canned forging Ti-43Al-4Nb-1.5Mo alloy(a1, a2) close to the edge of the pancake(b1, b2) the quarter of the pancake showing the decomposition of α2/γγ+α2+B2/β(c1, c2) the core of the pancake showing the dynamic recrystallization (DRX) grains and the remnaut lamellae
Fig.3  EBSD results showing the microstructures of canned forging Ti-43Al-4Nb-1.5Mo alloy (a) image quality (IQ) map (b) grain boundary misorientation angles and phases (c) grain distribution

(d) grain orientation spread (GOS) map (GOS values show the average difference in orientation between the average grain orientation and the bigger GOS value, the higher dislocation density, and vice versa)

Fig.4  Deformed microstructures and the local flow softening mechanism of the canned forging Ti-43Al-4Nb-1.5Mo alloy (a) shear band (b) elongated B2 phase (c~e) schematics for the local flow softening
Fig.5  TEM images of the canned forging Ti-43Al-4Nb-1.5Mo alloy (a) lamella decomposition (b) the equiaxed (γ+B2) and remnant α2/γ colonies(c) dislocations in γ grains (d) twins in γ grains
Fig.6  Microstructures of canned forging Ti-43Al-4Nb-1.5Mo alloy after heat treatments at the different temperatures for 30 min and by furnace cooling (Insets show the high magnified images) (a) 1250 ℃ (b) 1275 ℃ (c) 1285 ℃ (d) 1300 ℃
Fig.7  Evolutions of the volume fractions of different phases with the heat treatment temperatures of the canned forging Ti-43Al-4Nb-1.5Mo alloy
Specimen σ0.2 / MPa σb / MPa δ / %
HIP (1250 ℃, 120 MPa, 4 h, FC) 416 492 2.9
Canned forging 534 594 33.0
Canned forging +1250 ℃, 30 min, FC 506 576 36.0
Canned forging +1300 ℃, 30 min, FC 552 663 26.0
Table 1  High temperature tensile properties at 800 ℃ of the HIP, canned forging and heat treatment Ti-43Al-4Nb-1.5Mo alloy
Fig.8  Crack propagation of Ti-43Al-4Nb-1.5Mo alloy with HIP (a~c) and 1300 ℃, 30 min, FC treatment (d~f)(a) overall appearance of the crack propagation with HIP treatment (b, c) the corresponding crack propagation around B2 phase(d) overall appearance of the crack propagation with 1300 ℃, 30 min, FC treatment(e, f) the corresponding crack initiate and crack growth
Fig.9  Morphologies of the high temperature tensile fracture after HIP (a) and 1300 ℃, 30 min, FC (b) in Ti-43Al-4Nb-1.5Mo alloy at 800 ℃
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