Study of Concurrence of α/γ Phase Transformation and Grain Growth in Nanostructured Iron-based Alloy

Author:Huang Lin Ke

Supervisor:liu feng


Degree Year:2018





Fabrication of metallic materials with high strength and good ductility is one of the research focuses in the material science.Nanostructured materials(NSMs)exhibit generally superior strength,but poor thermal stability and low ductility are roadblocks to their practical applications.Solid-state phase transformation and grain growth,as two typical solid-state reactions in thermal processing,are two kinds of microstructure changes when NSMs suffer thermal instability,but can be used to introduce heterogenous structures exhibiting high strength and good ductility.Therefore,investigating these two solid-state reactions is significant for manipulating nanostructure and designing the NSMs.Recently,extensive endeavors have been devoted to the grain growth in NSMs;by contrast,the investigations on phase transformation,however,are relatively limited,where most of these related studies are primarily concentrated on the thermodynamics,and kinetic aspect concerning the transformation pathway is still in its infancy.It is found that phase transformations are usually concomitant with grain growth in NSMs,however,the corresponding microstructure evolution and the mechanistic understanding behind the physical phenomenon has not been elucidated.Reverse austenite transformation(RAT)plays critical roles in fabrication of advanced high-strength steels(AHSS).The investigation of RAT kinetics in the NS Fe alloys,therefore,is highly important and required,to offer optimal ways for thermal treatment and nanostructure manipulation,and the development of AHSS.Herein,this thesis will provide thorough investigations on the typical body-centered cubic(bcc,α)to face-centered cubic(fcc,γ)phase transformation upon heating in the model Fe91Ni8Zr1 alloy,including characterizing the concurrent kinetics of phase transformation and grain growth,elucidating the mechanism of concurrence,tailoring the nanostructure by the concurrence,exploring the austenite growth kinetics,revealing the grain boundary(GB)-constrained transformation mechanism,as well as modelling theα/γtransformation kinetics in NCMs.The following conclusions are obtained.(1)The kinetic evidence for the concurrence of phase transformation and grain growth in NS Fe-based alloy is provided,where grain growth takes place simultaneously with,but is inhibited prior to the end of phase transformation.This macroscopic phenomenon corresponds to the GB and phase boundary(PB)migrations on the micro-scale.On this basis,an interaction between GB and PB migrations is uncovered, the PB moves toward and interacts with the GB,the velocity of PB migration will be blocked and the associated direction will be changed as well due to the structure and orientation effects;accordingly,the GB migration is also hindered by Zener drag exerting from the new phase formed at the GB.(2)The concurrence of grain growth and phase transformation is ubiquitously observed in NS alloys,whose physical origin lies in that the driving force of grain growth will increase at least one order of magnitude and then becomes comparable to that of phase transformation,and the activation energy of GB migration is oftentimes lower than or comparable to that of PB migration.An interaction between phase transformation and grain growth will occur upon the concurrence and the underlying mechanism concerning the interaction is related to the type of phase transformation.On this basis,a new kind of heterogeneous nanostructure is obtained in the Fe91Ni8Zr1alloy,i.e.NS ferrite embedded with ultrafine austenite,which is defined as dual-phase bimodal nanostructure(DPBN).DPBN is expected to hold substantial promise in balancing the strength-ductility tradeoff.(3)α/γtransformation in the NS Fe91Ni8Zr1 alloy exhibits rather broad temperature interval,exhibiting sluggish transformation kinetics.The ultrafine austenite with globular-shaped morphology is observed to nucleate on the high-angle GB of NS ferrite.The formed austenite is characterized by a relatively slower growth velocity and an appreciable solute partitioning,suggesting a diffusion-controlled mechanism ofα/γtransformation.The sufficiently high thermal stability in a broad temperature interval is a result of combined effect of grain size and alloying.The dominant reason for the sluggishα/γtransformation kinetics is the presence of substantial GBs resulting from the nanoscale grain size.The GBs play a double-edged role in controlling the new phase growth,that is,enhancing the diffusivity(enhanced effect)and facilitating the formation of constrained diffusion field(constrained effect).We refer to this constrained effect as the GB-constrained mechanism,which leads to an easy occurrence of so-called soft-impingement,as the origin of sluggish transformation velocity.(4)A kinetic model to describe the diffusion-controlledα/γtransformation kinetics in NCMs is proposed by combination of the Cahn model and the modified austenite growth model considering the double-edge role of GBs and the grain growth effect.This model satisfactorily reproduces the experimentally determinedα/γtransformation kinetics in the NS Fe91Ni8Zr1 alloy,including the austenite growth,the partitioning behavior,as well as the evolution of austenite fraction and ferrite grain size.Besides,based on the Cahn model,a kinetic model to describe the GB-controlled phase transformation kinetics in NSMs is developed,where the interplay between phase transformation and grain growth is concerned.Further,the proposed model is tested against the experimental result in the NS Fe98Ti2 alloy,with good agreement obtained.