The Sensitivity Of Strain Rate And Temperature On Shear Banding Behavior In Bulk Metallic Glasses

Bulk metallic glasses(BMGs),or known as amorphous alloys,are made by being quench-ing into glassy state from the liquid with a super high cooling rates.Owing to a glassy state and the absence of crystal structures inside,metallic glasses show quite different deformation behavior from traditional metals at ambient temperatures,which is termed as highly localized initiation and propagation of‘shear bands'.A full knowledge of various shear banding be-haviors under different loading conditions is crucial for understanding the mechanical responses and deformation mechanisms of BMGs.The initiation and propagation of shear bands are in-fluenced by coupling effects of strain rate,stress state and temperature during the deformation process,which makes it difficult to clarify their exact roles by a single experiment.To address this issue,we have designed a series of experiments,aiming at systematically investigating me-chanical responses,shear banding behaviors and fracture modes at various strain rates,stress states and temperatures.The mechanisms of strain-rate effect,temperature effect and stress-state effect are clarified by comparing these experiment results.We then attempt to summerize strain rate and temperature sensitivity based on theoretical models.These experiments include quasi-static and dynamic uniaxial compression,quasi-static and dynamic shearing,quasi-static and dynamic shearing at cryogenic temperatures and real-time measurement of temperature rise within shear bands under dynamic shearing.We have investigated the exact roles of strain rate,pressure and temperature in the deformation processes of BMGs through testing stress-strain curves,capturing real-time deformation processes and SEM observations.To further study whether different compositions with different plasticity would exhibit various rate depen-dence or temperature dependence,two compositions(Zr₆₅Cu₁₈Ni₇Al₁₀and Zr₅₀Cu₄₀Al₁₀,short for Zr65-BMG and Zr50-BMG)are chosen as testing materials.The works and important conclusions are summarized as below:1.Quasi-static and dynamic compression tests reveal that the ultimate strengths of BMGs drop dramatically from quasi-static loading to dynamic loading.Under quasi-static loading,Zr65-BMG exhibits considerable plasticity while Zr50-BMG exhibits limited plasticity.It is worthy to note that the two compositions show similar shear banding behaviors and cracking processes as well as obvious strain-rate sensitivity,even with different plasticity.Real-time imaging captures multiple shear banding under quasi-static compression,while single shear banding under dynamic compression;and sliding-dominated cracking process under quasi-static loading,while direct crack propagation under dynamic loading.The difference in plastic-ity under quasi-static loading is because Zr65-BMG undergoes progressive sliding and results in stable fracture,while Zr50-BMG occurs limited and rapid sliding and results in unstable frac-ture.The strain-rate effect on shear-band initiation is attributed to an increasing coalescence rate of available free volumes.The rate effect on shear banding patterns is originated from the rate effect on free-volume concentration level.In other words,free volumes are gradually concen-trated into a narrower area at high rates,where would be only enough for single shear banding beyond a critical strain rate.Furthermore,the rate effect on fracture modes is controlled by various shear banding patterns at different loading rates and their corresponding time scales.2.Quasi-static and dynamic shearing tests observe similar rate dependence.Comparing to compression tests,we find that normal stress delyas the cracking process,but does not change strain-rate sensitivity.Since normal-stress is ruled out in shear tests,how strain rate controls shear bands developing into cracks(shear-band-to-crack transition)could be investigated.The transition is originated from nano-scale voids,which then evolve into cavities along slip planes through off-axial shearing.At low strain rates,a large number of off-axial shearing occurs at different sites along a slip plane.Their displacements vertical to shear-band direction are relatively low,which leads to mirco-scale cavities.These cavities link with each other during sliding process and leads to final fracture.At high strain rates,a series of off-axial shearing are simultaneously triggered at a shear-band-to-crack transition site,resulting in considerable off-axial(tensile)stress component together with direct crack opening and propagation at this site.3.Quasi-static and dynamic shearing at cryogenic temperatures tests show the shear strength increases with decreasing temperature in Zr65-BMGs.Serrated flow disappears when the envi-ronment declines to a critical value under quasi-static loading,this is because off-axial shearing along shear bands and consequent cavity formation are inhibited below a critical temperature,which delays shear-band-to-crack transition.In addition,the temperature sensitivity is not ob-vious under dynamic loading.4.Dynamic shearing-based real-time measurements of temperature evolution within shear bands reveal that temperature rise under dynamic loading is also limited.The temperature rise reaches 194?207 K at the shear-band-to-crack transition moment,which is far below the tem-perature for causing thermal softening.It is worthy to note that the temperature rise is higher than that under quasi-static loading.The often observed melting droplets on fracture surfaces might be formed during the fracture process when the local temperature exceeds the glass tran-sition temperature.5.Strain-rate effect mainly affects shear banding behavior,while temperature sensitivity mainly affects shear-band-to-crack transition process.Rate-dependent fracture modes could be attributed to the difference in local heating at various loading rates.We then introduce cooperative shear of STZs model to characterize the rate-sensitive and temperature-sensitive mechanical responses.The rate-sensitive part and the temperature-sensitive part in the model are modified due to the rate-dependent shear banding patterns and tempera-ture evolution.To investigate the tendency of stable or unstable fracture in BMGs,we introduce the ratio of applied energy to critical dissipation energy by shear banding based on a concept of‘shear-band toughness'.In terms of strain-rate sensitivity,the ratio of Zr65-BMG under quasi-static loading remains below 1,which indicates applied energy is fully dissipated by shear banding and results in stabe fracture.The ratio of Zr50-BMG under quasi-static loading keeps beyond 1,which confirms an unstable fracture mode.Furthermore,the ratios of both the two compositions are much larger than 1,so they undergo unstable fracture under dynamic loading.In terms of temperature sensitivity,at low strain rate,the ratio confirms that Zr65-BMG keeps stable fracture and predicates Zr50-BMG would undergo a transition from unstable to stable fracture with decreasing temperature.In additions,the ratio confirms the difficulty in changing fracture mode under dynamic loading even by decreasing temperature.