Ni-P Nanostructured Metallic Glass Prepared By Pulsed Electrodeposition And Its Mechanical Properties

Metallic glasses have been characterized by an absence of long-range order in the atomic arrangement and have a short-range ordering over distances of a few atomic spacings.It has been proved that metallic glasses have outstanding physical,chemical and mechanical properties.And they have been gradually used in various fields,including sport equipment?military weapon?space flight and aviation and electron device.However,the study of the structure and property control is imcomplete,therefore,the large-scale applications in industry fields are hindered now.The concept of nanostructured metallic glasses broadens the knowledge of the amorphous alloy materials.Lots of amorphous state interfaces are introduced into the metallic glasses,so that,the structures and properties can be adjusted by the control of amorphous grains and interfaces.In this research,we try to find a new technology with high efficiency and low cost to prepare nanostructured metallic glasses,and study the microstructure and properties of the materials.The main results obtained are summarized as follows:1.Preparation of nanostructured metallic glass by multi-phase pulse electrodeposition:Ni-P nanostructured metallic glass is successfully prepared by a brand new multi-phase pulse electrodeposition method.It has been proved that there're obvious nano-scale amorphous grains and amorphous interfaces on the surface morphology and inside the material.Compared with conventional metallic glass ribbon,the nanostructured metallic glass obtained presents more remarkable structural inhomogeneity and atomic density difference.There're more initial free volume inside Ni-P nanostructured metallic glass,and it's easier for the formation and growth of the crystal nucleus in the nanostructured metallic glass.From a mechanical point of view,the Ni-P nanostructured metallic glass presents a higher elastic modulus and a lower hardness.2.Study on plastic deformation behavior of Ni₇₈P₂₂nanostructured metallic glass:The plastic deformation behavior of Ni₇₈P₂₂metallic glasses is closely related to free-volume softening due to the shear-induced dilatation and thermal softening due to the adiabatic heating in the shear bands.During nanoindentation process,the free-volume softening plays a dominant role in the low loading rate range.And the thermal softening gradually becomes obvious with the increasing of loading rate.Consequently,as the loading rate increases,the hardness increases firstly in the low loading rate range and then decreases in the large loading rate range.More loosely stacked structure with higher free volume density in the interface of Ni₇₈P₂₂nanostructured metallic glass is beneficial to viscous flow and promotes to free-volume softening and thermal softening.Thus,the hardness of Ni₇₈P₂₂nanostructured metallic glass increases much faster than Ni₇₈P₂₂metallic glass ribbon in the low loading rate range,and decreases more significantly in the large loading rate range.The plastic deformation of Ni₇₈P₂₂nanostructured metallic glass is much more homogeneous in macroscopic sense.3.Study on creep behavior of Ni₇₈P₂₂nanostructured metallic glass at room temperature:Within the small indentation depth regime(<100 nm),it's easy for the creep curves of Ni₇₈P₂₂metallic glasses to be punctuated by some discrete bursts of rapid displacement at nearly constant load.This phenomenon is analogous to the discrete“pop-in”events during the loading segment.The discrete bursts appear infrequently at large indentation depths(>100 nm)and the displacement in each burst is small.It can be ascribed to the indenter tip-sample interface diffusion mechanism at small indentation depths.The interface diffusion process fades away with the indentation depth and intrinsic deformation mechanism of Ni₇₈P₂₂metallic glasses dominates the creep deformation at larger depths.According to the strain gradient plasticity theory,the creep strain rate sensitivity coefficient gradually decreases with the indentation depth.As the loading rate increases,the viscoplastic deformation is enhanced,the creep strain rate sensitivity coefficient gradually increases and the shear transformation zone volume decreases due to the increase of excessive free volume concentration during the loading segment.The accumulated excessive free volume of nanostructured metallic glass at the end of the loading segment is much more than metallic glass ribbon.During the creep deformation,the higher free volume content in the interface regions of nanostructured metallic glass can promote the nucleation and multiplication of shear transformation zones,additionally,the looser atomic structure in the interface regions is beneficial to viscous flow.Consequently,the Ni₇₈P₂₂nanostructured metallic glass shows more significant and homogeneous creep deformation behavior than the metallic glass ribbon.4.The effect of structure relaxation on the mechanical property of Ni₇₈P₂₂nanostructured metallic glass:Defect annihilation and densification of atomic structure suppress the nucleation and propagation of large size scale shear bands in Ni₇₈P₂₂metallic glasses during the nanoindentation process.The elastic modulus and hardness increase after annealing treatment.In addition,free-volume softening and thermal softening which govern the shear-banding instability are weakened due to the defect annihilation and densification of atomic structure.The plastic deformation of Ni₇₈P₂₂metallic glasses becomes more locally inhomogeneous.The effect of structure relaxation is more significant for Ni₇₈P₂₂nanostructured metallic glass.It suggests that the unique microstructure in the nanostructured metallic glass profits the thermal activation process of atoms and the defect annihilation is much more obvious than the conventional metallic glass ribbon.