Ti-43Al-4Nb-1.5Mo合金包套鍛造與熱處理過程的微觀組織及高溫拉伸性能

Ti-43Al-4Nb-1.5Mo合金包套鍛造與熱處理過程的微觀組織及高溫拉伸性能Microstructure and High Temperature Tensile Properties during the Canned Forging and Heat Treatment of Ti-43Al-4Nb-1.5Mo Alloy

對Ti-43Al-4Nb-1.5Mo合金進(jìn)行包套鍛造和后續(xù)熱處理實(shí)驗(yàn)，考察了該過程TiAl合金的熱變形行為、流變軟化機(jī)制以及熱處理參數(shù)對微觀組織和力學(xué)性能的影響。結(jié)果表明，TiAl合金包套鍛造過程的高溫流變軟化以β相協(xié)調(diào)變形、片層相變分解、γ相內(nèi)位錯(cuò)滑移以及孿晶誘導(dǎo)的動態(tài)再結(jié)晶為主，最終組織為殘余α2/γ層片和等軸α2、γ、B2相的混合組織。隨熱處理溫度的增加，熱變形組織由殘余α2/γ層片和多相混合組織轉(zhuǎn)變?yōu)棣?/γ層片+γ相組織，在較高的溫度下(1300 ℃)轉(zhuǎn)變?yōu)槿珜悠M織。其中，B2相隨著溶質(zhì)擴(kuò)散程度的增加逐漸消失，殘余層片組織發(fā)生分解轉(zhuǎn)變?yōu)榈容Sα2/γ層片團(tuán)，同時(shí)發(fā)生γ→α轉(zhuǎn)變，形成全層片組織。將熱等靜壓、鍛態(tài)和熱處理試樣的高溫(800 ℃)拉伸性能進(jìn)行比較，經(jīng)熱處理后獲得的全片層組織具有最佳的綜合性能，強(qiáng)度為663 MPa，斷后延伸率達(dá)到26%。分析該過程的斷裂行為得出，由于存在層片扭曲拉長、微孔鈍化以及裂紋曲折延伸的斷裂機(jī)制，全層片組織具有良好強(qiáng)度-塑性的綜合力學(xué)性能。另外，熱變形過程中bcc結(jié)構(gòu)B2相能夠協(xié)調(diào)變形，而服役條件下硬脆的B2相容易引起裂紋萌生對力學(xué)性能極其不利。因此，TiAl合金在熱變形和服役過程中需要對組成相進(jìn)行嚴(yán)格控制進(jìn)而獲得良好的力學(xué)性能。

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 microstructure evolution is 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 phase. With the increasing heat-treatment temperature, the microstructure changes from the multi-phase structure (remnant α2/γ lamellar, equiaxed α2, γ and B2 phase) 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 disappeared due to the solution diffusion, and the remnant α2/γ lamellae change to equiaxed α2/γ colonies according to the L(α2/γ)→γ+α2+B2 transition, and the final fully α2/γ lamellar structure is promoted by γ→α transition at the high temperature. Additionally, the high-temperature tensile tests of the HIP, as-forged and heat-treatment samples at 800 ℃ are conducted, in which the fully lamellar structure shows the high properties with the tensile strength of 663 MPa and the elongation of 26%. The deformation process of the fully α2/γ lamellar can be strengthen 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 deteriorate the service-properties, which should be controlled accurately for the high mechanical properties during the thermo-mechanical processing.

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