摘要
对细晶Ti-2Al-2.5Zr合金进行了室温/低温(77 K)疲劳实验及微观组织观察.结果表明:室温低应变幅△εt/2(= 0.5%,1.0%)下,合金表现为循环软化;室温高应变幅(1.5%,2.0%)下,则表现为循环应力饱和;77 K时,不同应变幅下均表现为循环硬化,且随应变幅升高,循环硬化程度增强.疲劳寿命测试结果表明:低温疲劳寿命始终高于室温,断口SEM观察表明,室温和低温下,疲劳裂纹扩展区均有明显的疲劳条纹,疲劳裂纹以穿晶方式扩展,室温下伴随有大量二次裂纹,低温下的二次裂纹数量明显减少.TEM观察表明:低温下孪生是合金主要的变形方式,包括(10■1)和(11■1)型孪晶.疲劳变形位错组态为:室温较低应变幅(0.5%,1.0%)下,形成位错线和局部位错缠结;室温下应变幅提高到1.5%和2.0时,{10■0}柱面和{11■1}锥面滑移同时开动,位错组态演化为亚晶和明显的位错胞.77 K下,应变幅2.0%时形成沿柱面平行分布的位错带:77 K下应变幅升高到4.5%时,多滑移形成相互垂直的位错线.低温诱发形变孪晶是Ti-2Al-2.5Zr低温疲劳寿命升高的原因.
Ti 2Al-2.5Zr alloy with hcp structure is a kind of structural materials used under low temperature condition, e.g. pipeline system of liquid hydrogen and liquid oxygen in missile engine. It is usually serviced in condition of severe low temperature and dynamic loading. Deformation twinning is a common and important plastic deformation mode in the hexagonal close packed alloy, but will be severely restricted as the grain is refined from tens of microns to a few microns. On the other hand, twinning has a low sensitivity to temperature, consequently becomes a favorable deformation mode at low temperature in comparison with dislocation slip. The objective of this work is to study the coupling effect of grain refinement and testing temperature on twinning behavior and the low-cycle fatigue behavior of Ti-2Al 2.5Zr.
Symmetrical push-pull low-cycle fatigue grain size of about 5 μm at room temperature that the alloy exhibits the higher ductility and （LCF） tests were performed on Ti-2Al-2.5Zr with （RT） and low temperature （77 K）. The results show the longer low cycle fatigue life at 77 K than those at RT. The cyclic stress response curves show that the cyclic softening occurs at the low strain amplitudes of 0.5% and 1.0%. However, as strain amplitude increased to 1.5% and 2.0%, cyclic stress saturation appeared. When testing temperature declined to 77 K, the cyclic hardening was observed at all four strain amplitudes. The degree of cyclic hardening increases as the strain amplitude rises. The fractography analyses suggest that transgranular fracture with well-developed fatigue striations is the predominant failure mode. The amount of secondary cracks is much higher in the alloys deformed at RT than that at 77 K. TEM examination reveals that deformation twins become more active. The primary types of twinning are {1011} and {1121}. The typical deformation microstructures consist of individual dislocation lines together with the tangled dislocation at the low strain amplitudes of 0.5% and 1.0%. As the strain amplitude increased to 1.5% and 2.0%, {1010}prismatic slip and （1121} pyramidal slip were simultaneously activated, the subgrain and dislocation cells were formed. At 77 K and strain amplitude of 2.0%, the parallel dislocation bands distribute along prismatic plane. As the strain amplitude increased to 4.5%, mutual perpendicular dislocation lines appeared. The improvement of fatigue life of Ti-2Al-2.5Zr at 77 K is attributed to the constraint of inhomogeneous slip and the activation of deformation twinning.
出处
《金属学报》
SCIE
EI
CAS
CSCD
北大核心
2009年第4期434-441,共8页
Acta Metallurgica Sinica
基金
国家重点基础研究发腱计划项目2007CB386104和2004CB619303
国家自然科学基金项目50671077和50771080资助~~
