The Mechanism Of Serrated Flow Behavior In Mg-3Nd-1Zn Alloy

Magnesium alloys are praised as the “green structural materials” in 21 st century.It has an important application value in aerospace,military field due to many advantages such as high efficient,energy conservation,environmental protection.Unfortunately,Mg alloys usually have a poor ductility and formability at room temperature,which limits their extensive applications in industry.Moreover,an inhomogeneous deformation i.e.serrated flow has been found in many magnesium alloys during high temperature deformation,which has a bad influence on their ductility.In this dissertation,the Mg-3Nd-1Zn(NZ31)alloy has been tensile tested at different temperature and strain rate,and the microstructure evolution was characterized by using the OM,SEM and TEM.The influence of temperature,strain rate and precipitates on serration types and critical strain of serrated flow were investigated.The main work and achievements are summarized as follows.The influence of temperature and strain rate on serration type transition in NZ31 alloy were investigated.The serrated flow appeared at temperature from 150 oC to 300 oC,with the strain rate ranging from 1?10-4s-1 to 1?10-2s-1,and the serration type will transforms from type A to type B and then evolves into type C with the increase in temperature and decrease in strain rate.During this process,temperature controls the diffusion of solute atoms and the strain rate dominates the moving rate of dislocations.Temperature increase or strain rate decrease will improve the pinning ability of solute atoms,thus the serration type trainsition from type A to B.While excess temperature or too low strain rate will restrain the unpinning process.Accordingly the serration type trainsiton from type B to C.The influence of temperature and strain rate on the critical strain in NZ31 alloy were investigated.Results showed that normal critical strain appeared with type A and B serrated flow at temperature from 150 oC to 250 oC,and inverse critical strain appeared with type C at temperature from 275 oC to 300 oC.The production of serration required improvement in diffusion for solute atoms for pinning process at low temperature,and enhance the moving ability of dislocations for unpinning process at high temperature,which need the assistance of the strain and stress respectively.The critical strain for pinning and the critical stress for unpinning processes were defined,which give a better explanation to the variation tendency of two definitions in accordance with temperature.Furthermore,this relationship results in the critical strain for onset of serrated flow changing from normal to inverse and corresponding different serrations.Different heat treatments were used to regulate and control precipitates in NZ31 alloy.When ageing at high temperature(225~275?)for a short time(15~30min),the ? phase produced and distributed in line had a composition of Mg12 Nd and a granular shape,and mainly formed along the 0001 basal plane leading to a large strengthening effect.The effect of ?'' phase on serrated flow mainly express in two factors in NZ31 alloy: one factor is that the precipitates can promote the process which solute atoms gather at dislocation,as the same time it also restrain the dislocation getting rid of solutes.So the serration type will changes from the sharp shape to smooth shape.Another factor is that the formation of precipitates led to decrease the concentration of solute atoms,which has a result in weaken the serrated flow behavior.