Fabrication And Mechanical Properties Of Large-sized Bulk Metallic Glass Composites Ductilized By Transformation-induced-plasticity Effect

Due to their long-range disordered atomic packing,bulk metallic glasses(BMGs)lack crystalline defects like dislocations and grain interfaces and thus possesses a unique combination of mechanical,chemical and physical properties,which makes them attractive as engineering materials.Nevertheless,strain softening behavior and room-temperature brittleness have been the Achilles' heel for their real structural applications.Recently,the concept of transformation-induced plasticity(TRIP)has been successfully introduced into BMGs,which provides a new avenue for developing advanced BMG composites with high yield strength,strong work hardening capability and good tensile ductility.However,all currently developed TRIP-ductilized BMG composites have a limited attainable size,which seriously limits their practical uses.Moreover,the microstructures of developed TRIP-ductilized BMG composites are sensitive to alloying element and fabrication conditions,and crystalline phase is unstable and easily to grow up and connected together.At last,the transformation mechanism and microstructure evolution during loading are still unclear.Therefore,large-sized TRIP-ductilized BMG composites with prominent work hardening capability and good tensile ductility are desperately needed,and their transformation mechanism are urgently to be exploredBased on the scientific and engineering problems mentioned above,the effect of alloying element A1 and Ag on glass forming ability of the matrix and stability of metastable austenitic crystalline phase B2-CuZr were systematically studied firstly.It was found that addition of Al and Ag both improve the glass forming ability,and therefrom,the relationship among alloying content,cooling rates and phase formation was established,which lay the foundation for fabricating large-sized BMG composites with single crystalline phase of B2-CuZr.Second,the heterogeneous nucleation theory was introduced to BMG alloys.It was also found that minor addition of Sn can react with the main element Zr to form highly stable intermetallics Zr5Sn3.The planar disregistry between Zr5Sn3 and the metastable austenitic crystalline phase B2-CuZr is very low,and thus the Zr5Sn3 compound can act as an effective nucleant for B2-CuZr,to promote nucleation and inhibit B2-CuZr grow up.Consequently,BMG composites with homogeneous distribution of B2(the average size is less than 100 ?m)were developed(e.g.,Cu46.25Zr48Al4Ag1Sn0.75).Moreover,addition of Sn was found to be not only improve the glass forming ability of the matrix,but also has negligible influences on the martensite transfomation capability of B2-CuZr.Finally,the microstructure evolution and deformation mechanism of TRIP-ductilized BMG composites with a diameter of nearly 1 centimeter were studied in detail.The B2-CuZr phase has two sizes,i.e.the micro-size and nano-size.During the transformation process,the micro-size B2-CuZr transforms to martensitic plates with multiple high-density dislocations by shearing,whereas the nano-size B2-CuZr transforms to B19'-CuZr via twinning.Through the microstructure characterization of load-unload tensile specimens,it was revealed that the crystalline phase is composed of bright and dark sub-grains in large-sized TRIP-ductilized BMG composites.The bright phase is B2-CuZr,while the dark phase is B2-CuZr with a superlattice structure.The primary martensite nucleated in the sub-grain boundary preferentially,and expanded to bright phase(B2-CuZr).With further tensile loading,the primary matensite extends and grows up until the BMG composites fail.During deformation,strain energy was successfully dissipated due to the strain induced martensitic transformation.Moreover,the austenitic phase impedes the propogation of shear band,and promotes shear band multiplication.Therefore,good tensile ductility and prominent work-hardening capability were obtained simultaneously.The current unique strategy and deformation mechanism are believed to be not just limited to CuZr-based BMGs,but also applicable to other TRIP-ductilized BMG composite systems.Our findings have important implications for developing advanced BMG composites simultaneously with a large size and enhanced properties,which certainly would broaden engineering application range for amorphous alloys.