Micro-scale Bending Fatigue Behavior Of Ultrafine-grained And Nano-twinned Copper

With the rapid development of modern industry,increasingly more metallic micro devices or components are widely used.The proporties of materials at the micro-scale is quite different from those at the macro-scale.However,at present,there are relatively few studies on the cyclic deformation and damage mechanisms of micro-scale metals,mainly due to the difficulty in preparing micro-scale samples and in accurately applying cyclic deformation at the micro-scale.In this work,we develop a test device for micro-cantilever bending fatigue and utilize it to investigate the low-cycle fatigue behavior of ultrafine-grained(UFG)and nano-twinned(NT)copper under a constant total strain amplitude.The specific findings are summarized as follows:(1)Based on digital image corrleation,the developed fatigue test instrument is demonstrated to be capable of accurately measuring the instantaneous deflection,which enables cyclic bending deformation controlled under constant total strain amplitude.(2)For the UFG copper,the maximum bending moment of each cycle increases with increasing the total strain amplitude,and the shape of the stress-strain hysterisis loop evidently differs for different total strain amplitude.With the increase of cycle number,the normalized bending moment decreases continuously,an indication of cyclic softening.The ultimate fatigue life decreases rapidly with the increment in the total strain amplitude.(3)With the increase of cycle number,deformation bands and fatigue cracks making an angle of about 45o with the load direction gradually develop on the arc transition region at the root of the UFG copper cantilever.Fatigue crack develops after obvious grain growth and mainly propagates along the grain boundaries.(4)For the NT copper,the stress-strain hysteresis loops are analogous to those of the UFG copper.With the increase of the total strain amplitude,the loop gradually changes from thin,sharp to fat and wide.The NT copper also softens continuously,and the larger the total strain amplitude,the faster the softening.The fatigue life of nano-twinned copper decreases rapidly with the increase of total strain amplitude.(5)With the increase of cyclic deformation of the NT copper,small fatigue cracks gradually nucleate on arc transition region of the micro-cantilever sample and propagate along the grain boundary of columnar grains.Significant plastic deformation can be identified near the cracks.