孪晶片层厚度对纳米孪晶Cu疲劳性能的影响

EFFECT OF TWIN LAMELLAR THICKNESS ON THE FATIGUE PROPERTIES OF NANO-TWINNED Cu

通过恒应力幅控制拉-拉疲劳实验,比较了脉冲电解沉积制备的不同孪晶片层厚度纯Cu样品的疲劳寿命和疲劳耐久极限.结果表明:在应力疲劳下,样品的疲劳寿命与疲劳耐久极限均随孪晶片层厚度的减小而提高.疲劳样品的宏观表面变形形貌(SEM观察)和微观结构(TEM观察)表明:当平均孪晶片层厚度为85 nm时,材料的塑性形变由位错滑移和剪切带共同承担,进而疲劳裂纹沿剪切带萌生;而当平均孪晶片层厚度为32 nm时,材料的塑性形变由位错孪晶界交互作用主导,从而导致疲劳裂纹沿孪晶界形成.

The fatigue property of metals is one of the most important concerns in industrial design. It is affected by various factors, such as the microstructure, mechanics and the environment. For the polycrystalline metallic materials, grain boundaries （CBs） usually play an important role and affect the fatigue behaviors significantly. GBs could strengthen materials by blocking the motion of dislocations; meanwhile, the stress concentration which is caused by the dislocations pile-up in the vicinity of GBs would result in the initial fatigue crack easily. As a special interface of low-energy, twin boundaries （TBs） can strengthen materials by blocking the motion of dislocations in a manner similar to that of GBs. Our studies have indicated that a high density of nano-scale twin lamellae can provide the high strength without significantly compromising ductility, which is fundamentally different from that of GB strengthening. So far, most studies of the TB-related fatigue and cracking behaviors are concentrated on the twins with a thickness of few or tens micrometers. The study of the TB-related fatigue behavior in nanometer scale is rare. In this work, high purity Cu specimens with high density of nano scale coherent TBs were synthesized by means of the pulsed electro-deposition （PED）. The twin lamellar thickness dependence of fatigue life and fatigue endurance limit of the nano-twinned Cu （nt-Cu） were studied by conducting tension tension fatigue tests under constant stress amplitude control at room temperature. It is found that both the fatigue life and fatigue endurance limit increase with the decrease of the twin lamellar thickness. Postmortem SEM observations suggest a transition in crack initiation site from shear bands （SBs） to TBs, when the twin lamellar thickness is reduced from 85 to 32 nm. For the nt-Cu samples with thick twin lamellae, the lattices accommodate the plastic strain, which results in the SB cracking. For the nt-Cu samples with thin twin lamellae, the abundant TBs accommodate the plastic strain. The stress concentration along TBs which is caused by the interactions of dislocation-TBs facilitates the fatigue cracking along TBs.