超声冲击强化对TC4钛合金拉压疲劳性能的影响

Effects of Ultrasonic Impact Modification on Tension-compression Fatigue Behavior of TC4

静荷载25 N、振幅30μm、冲击数36 000次/mm2下对TC4钛合金去应力退火及固溶时效两种状态进行超声表面冲击强化处理,研究其对TC4拉压疲劳性能的影响。对处理后的微观组织、硬度、强塑性变形(SPD)层、残余应力、疲劳性能和断裂特征进行分析。结果表明:处理后在TC4表面均获得约40μm深的强塑性变形层、表面硬度及残余压应力均被提高、表面粗糙度有较好的维持、两种状态的108周次的疲劳强度分别提升7.0%和10.7%;10~6周次前裂纹主要由表面萌生,10~6周次后裂纹源核心表现为SPD层与核心之间的变形α相,其形状狭长且平行于试件边缘。建立经典材料力学模型对其轴向应力进行分析,有助于理解内部裂纹源。

The effect of ultrasonic nanocrystal surface modification （UNSM） on the tension-compression fatigue behavior of Ti-6A1-4V （stress-relief annealed or solid solution-aging treated） was studied. UNSM is with a static load of 25 N, vibration amplitude of 30 μm and 36 000 strikes per unit. The microstructure, hardness, severe plastic deformation （SPD） layer, residual stress, fatigue property and fracture mechanism of the alloy after UNSM treatment were analized. The results show that UNSM produces about 40 Ixm SPD layers on both the two groups of specimens. UNSM increases the hardness and the compressive residual stress. UNSM helps to maintain a good surface roughness. The 10^8 cycles fatigue strength of the two group samples are improved by 7% and 11.7%, respectively. After UNSM, fatigue cracks mainly initiate from the surface of specimen before the fatigue life of 10^6 cycles, while they appear at the deformed a-phases at the zone between the SPD layer and the core after the fatigue life of 10^6 cycles. The shape of the deformed a-phases is long and narrow because of the ultrasonic impacts on the surface, and it is parallel to the edge of specimens. The analysis of the axial normal stress is discussed based on a classical model, which helps to understand the initiation of interior cracks.