Mechanical Properties And Deformation Mechanisms Of Zr-Based Metallic Glasses And Their Composites

Due to their unique amorphous structures,bulk metallic glasses(BMGs)have been considered to be potential candidates in the structural engineering fields,since most enduring attractions are their excellent mechanical properties.It is well known that BMGs exhibit high strengths and hardness,superior wear resistance,and good fracture toughness,etc.However,for actual structural engineering applications,many kinds of extreme conditions,compared to quasi-static and uniaxial loadings at ambient temperature,should be also appreciated.For instance,they can be subjected to dynamic and multiaxial loadings.In this paper,theoretical analysis,numerical calculation,and experiment verification are combined to reveal the inherent relationship between the mechanical properties and deformation mechanisms in BMGs and their composites.The strain rate effect on fracture strength and internal temperature rise of Zr-based BMGs has been uncovered by uniaxial tensile and compressive experiments upon quasi-static and dynamic loadings.Considering the structural softening,thermal softening,as well as strain rate hardening,the physical origin of rate-dependent fracture strength has been revealed based on the modified cooperative shear model.By employing the governing heat conduction equation,the functional relation between the shear band evolution time and maximum temperature within shear band was established,and abrupt fracture arises from structural instability generated by initiation of free volume and activation of shear transformation zone caused by rearrangement of atoms.In addition,the heat conduction plays a significant role during shear band evolution.The quasi-static and dynamic yield behaviors of BMGs and their composites under compression-shear loadings were studied through a new combinedcompression-shear technique based on split Hopkinson pressure bar.In order to acquire the yield surfaces upon quasi-static and dynamic loadings,the variation of yield strength of Zr-based BMGs and their composites along the different equivalent loading paths in the σ-τ space is obtained under different tilt angles and strain rates.Drucker-Prager criterion,coupled with the hydrostatic pressure and shear load,can well describe the initial yield behavior of BMGs and their composites under the compression-shear loadings.Based on the isotropic hardening model,the flow stress of in-situ metallic glass matrix composite under uniaxial compression was fitted by polynomial function to obtain the subsequent yield surface.Furthermore,the rate-dependent subsequent yield function of in-situ metallic glass matrix composite was established through the strain rate effect on the flow stress.The effect of dendrite arm thickness on mechanical properties of in-situ metallic glass matrix composite was investigated.Upon dynamic compressions,the plastic strain of the composite with fine dendrite promptly decreases,however,the composite with coarse one exhibits good plasticity.Upon quasi-static compressions,the improved plasticity of both composites is mainly attributed to the multiplication of shear bands within the glass matrix and pile-ups of dislocations within the dendrites.The mismatch between the toughness of the dendrite and glass matrix decreases with increasing the strain rate,which deteriorates the resistance to propagation of shear bands.The positive strain-rate sensitivity of the yielding strength prevails for the present composites,indicating that upon dynamic loading,compared to structural softening induced by the accumulation of free volume within glass matrix,the strengthening mechanism associated with dislocation movement within the crystalline dendrites assumes more dominance on strain-rate effects.Moreover,the constitutive model is employed to depict the strain-rate dependency of yielding strength.