Materials design by generalized stability

1.Introduction,Understanding and predicting microstructure and its relationship with the mechanical properties of metallic materials require detailed knowledge of phase transformations(PTs)and plastic deformations(PDs).These two processes,as central topics in material science,can be fundamentally described to a large extent by a framework of thermodynamic and kinetics(thermo-kinetics).The situation of PTs arises when the microstructure is losing phase stability,whereas the issue of PDs corresponds to the break of mechanical stability,all of which can be uniformly defined as the so-called loss of thermodynamic stability(TS).In this context,the higher TS will lead to the higher driving force G essentially required for the PT and/or PD forming and/or changing the microstructure.In the case of PTs,extensive examples showing the high TS can be found,such as martensite transformations(MTs)upon continuous cooling or strain[1,2],coherent precipitation[3,4],so-called Schwarz crystal[5],etc.

Thermo-kinetic connectivity by integrating thermo-kinetic correlation and generalized stability

Designing structured materials with optimized mechanical properties generally focuses on engineering microstructures,which are closely determined by the processing routes,such as phase transformations(PTs)and plastic deformations(PDs).Both PTs and PDs follow inherent trade-off relation between thermodynamic driving force ΔG and kinetic energy barrier Q,i.e.,so-called thermo-kinetic correlation.By analyzing nucleation and growth and proposing a conception of negative driving force integrating strain energy,interface energy and any kind of energy that equivalently inhibits the PT itself,ΔG^(S),unified expressions for the thermo-kinetic correlation and generalized stability(GS)were derived for three kinds of PTs,i.e.,diffusive PTs with simultaneously decreasedΔG and increased Q,diffusive PTs with simultaneously increasedΔG and decreased Q,and displacive PTs with simultaneously increased ΔG and decreased Q.This leads to so-called thermo-kinetic connectivity by integrating the thermo-kinetic correlation and the GS,where,by application in typical PTs,it was clearly shown,a criterion of high ΔG-high GS can be predicted by modulating chemical driving force,negative driving force and kinetic energy barrier for diffusion or nucleation.Following thermo-kinetic connectivity,analogous procedure for dislocation evolution upon PDs was performed,and materials design in terms of the highΔG-high GS criterion was discussed and prospected.

Transition of dynamic recrystallization mechanism during hot deformation of Incoloy 028 alloy

Dynamic recrystallization(DRX)plays significant roles in manipulating of microstructures during hot deformation and the result mechanical properties;however,the underling mechanism leading to multi scale-microstructures remains poorly understood.Here,the DRX mechanism under wide processing conditions(i.e.950-1200°C,0.001-10 s-1)in Incoloy 028 alloy was investigated,where the relationships among flow stress,Z parameter and grain size,as well as the evolution of characteristic microstructures(grain size,sub-grain boundaries,and high angle grain boundaries),are established.As the values of Z parameters decrease(corresponding to decreased flow stresses),three typical softening mechanisms successively occur,ranging from continuous DRX controlled by dislocation glide,discontinuous DRX dominated by dislocation motion(climb and cross/multiple slip)and grain boundary migration,to dynamic normal/abnormal grain growth resulting from grain boundary migration,with transition regions where two adjacent mechanisms occur simultaneously.Correspondingly,these above three softening mechanisms result in ultrafine,fine and coarse grains,respectively.The present findings demonstrate a comprehensive understanding of DRX mechanism over a wide range of processing conditions,and further provide a new guideline for preparing single crystals.