Cao Xiaojian, Zhu Jiangpei, Gao Fei, Gao Zhu
School of Transportation & Civil Engineering, Nantong University, Nantong 226019, China.
Department of Technology, Shangdong Huawin Hauck-Energy Technology Co, Ltd., Jinan 250101, China.
Materials (Basel). 2020 Dec 31;14(1):171. doi: 10.3390/ma14010171.
By means of the ultrasonic surface impact (amplitude of 30 μm, strike number of 48,000 times/mm), nanograins have been achieved in the surfaces of both Ti6Al4V(TC4) and Ti3Zr2Sn3Mo25Nb(TLM) titanium alloys, mainly because of the dislocation motion. Many mechanical properties are improved, such as hardness, residual stress, and roughness. The rotating-bending fatigue limits of TC4 and TLM subjected to ultrasonic impact are improved by 13.1% and 23.7%, separately. Because of the bending fatigue behavior, which is sensitive to the surface condition, cracks usually initiate from the surface defects under high stress amplitude. By means of an ultrasonic impact tip with the size of 8 mm, most of the inner cracks present at the zone with a depth range of 100~250 μm in the high life region. The inner crack core to TC4 usually appears as a deformed long and narrow α-phase, while the cracks in TLM specimens prefer to initiate at the triple grain boundary junctions. This zone crosses the grain refined layer and the deformed coarse grain layer. With the gradient change of elastic parameters, the model shows an increase of normal stress at this zone. Combined with the loss of plasticity and toughness, it is easy to understand these fatigue behaviors.
通过超声表面冲击（振幅为30μm，冲击次数为48000次/mm），在Ti6Al4V（TC4）和Ti3Zr2Sn3Mo25Nb（TLM）钛合金表面均获得了纳米晶粒，这主要归因于位错运动。许多力学性能得到了改善，如硬度、残余应力和粗糙度。经超声冲击后的TC4和TLM的旋转弯曲疲劳极限分别提高了13.1%和23.7%。由于弯曲疲劳行为对表面状况敏感，裂纹通常在高应力幅值下从表面缺陷处萌生。借助尺寸为8mm的超声冲击头，在高寿命区域，大部分内部裂纹出现在深度范围为100~250μm的区域。TC4的内部裂纹核心通常表现为变形的狭长α相，而TLM试样中的裂纹更倾向于在三叉晶界处萌生。该区域穿过晶粒细化层和变形粗晶层。随着弹性参数的梯度变化，模型显示该区域的正应力增加。结合塑性和韧性的损失，这些疲劳行为就很容易理解了。
