高强韧Ti-3Al-5Mo-4Cr-2Zr-1Fe合金低周疲劳性能研究

通过室温下应变控制疲劳试验研究了高强韧Ti-3Al-5Mo-4Cr-2Zr-1Fe合金的低周疲劳性能。结果表明:在高应变幅值下(Δεt/2=1.0%,1.2%,1.4%,1.6%),合金的循环应力响应表现为初始循环软化,而后趋于循环稳定;在低应变幅值下(Δεt/2=0.6%,0.8%),合金的循环应力响应表现为循环饱和特征。断口形貌观察发现:应变幅值为0.6%时,疲劳裂纹源只有一处,在断口表面分布有大量细小的二次裂纹。当应变幅增加到1.6%时,组织中发现多处疲劳裂纹源,二次裂纹的数量明显减少,但长度和宽度明显增加。透射电镜结果表明:在低应变幅值下(Δεt/2=0.6%),在αp/β界面处出现大量的位错堆积,在此处易产生应力集中导致微裂纹形核。而在高应变幅值下(Δεt/2=1.6%),在αp相中有明显的变形不均匀性,在αp相内出现大量的位错缠结和位错碎片,并且在αs相中出现一些位错塞积,但在β基体中没有明显的位错堆积情况。由于长条αp相的存在,能够提升α相和β相变形的相容性,延缓疲劳裂纹形核和扩展,因此使Ti-35421合金有着优异的低周疲劳性能。

Study on Low Cycle Fatigue Behavior of Ti-3Al-5Mo-4Cr-2Zr-1Fe Alloy with High Strength and Toughness

Low cycle fatigue(LCF) behavior of Ti-3 Al-5 Mo-4 Cr-2 Zr-1 Fe(Ti-35421) alloy with bimodal microstructure consisting of lath α(αp) and βtrans was investigated by strain-controlled mode at room temperature. Results indicate that cyclic stress amplitudes of the Ti-35421 alloy with bimodal microstructure show cyclic softening at first, then reach to cyclic stability at high strain amplitude(Δεt/2=1.0%, 1.2%, 1.4%, 1.6%). However, the cyclic stress response characterizes cyclic saturation at low strain amplitudes(Δεt/2=0.6%, 0.8%). Only one fatigue crack source is found by fracture morphology observation when Δεt/2=0.6%, while a large number of small secondary cracks occur on the surface. On the contrary, multiple fatigue crack sources generate when the strain amplitude increases to 1.6%. The number of secondary cracks reduces, but the length and width of the secondary cracks increase significantly. TEM results indicate that a large number of dislocations generate at the αp/βtrans interface at the low strain amplitude(Δεt/2=0.6%), which leads to micro-crack nucleation due to the stress concentration. Meanwhile, at high strain amplitude(Δεt/2=1.6%), deformation inhomogeneity phenomena occur in the αp phase, a large number of dislocation tangles and dislocation debris form in the αp phase, and some dislocation pile-ups form in the αs phase instead of β matrix. Due to the elongated αp phase, it can improve the compatibility of alloy α phase and β phase deformation, and delay crack nucleation and propagation. Therefore, Ti-35421 alloy has excellent low cycle fatigue performance.