| An available way to acquire basket-weave microstructure woven of lamellar ?,which is of excellent comprehensive performance,is provided by ? forging combined with heat treatment.Performance of Ti-alloy is highly depended on the volume,morphology and distribution of lamellar ?,which are vital to predict and control the property.However,the evolution of lamellar ?,which is sensitive to the processing parameters of the forging combined with heat treatment,is complicated during the process.Consequently,on the basis of understanding the processing procedure of acquiring basket-weave microstructure,the key to accomplish the controlling of the target of basket-weave microstructure is to figure out the evolution laws of lamellar a during the process of ? forging combined with heat treatment.In this thesis,the experiments of hot compression simulation and heat treatment,and the analysis of quantitative metallograph and single factor together with the technique of electron backscattered diffraction(EBSD)and X-ray diffration(XRD)were adopted to study the formation process of basket-weave microstructure and the evolution of lamellar ?.The main research contents and results are as follows:For Ti6242S Ti-alloy,basket-weave microstructure could be obtained by ? forging(water cooling)combined with two phase field heat treatment,and the formation process of basket-weave microstructure was found out:firstly,acicular martensite of high distortion energy,which was obtained through ? forging cooled by water,resolved during the heating process of two phase field heat treatment,and then the microstructure mainly consisted of high-temperature ? phase was formed during the holding process,lastly,large numbers of lamellae were precipitated and which were woven into basket-weave microstructure.The influence rules of the parameters of ? forging on the evolution of lamellar ? of Ti6242S Ti-alloy were revealed during ? forging combined with two phase field heat treatment.It was found out that the length of lamella was mainly influenced by the strain rate and deformation temperature:the lamellar length increased from 13.41 ?m to 14.17?m when the strain rate increased from 0.001s-1 to 0.1s-1,and it also increased from 12.10?m to 13.92?m.And there was much influence of deformation degree on lamellar thickness:the thickness became thicker from 1.61?m to 3.07?m as the deformation degree increased from 0 to 70%.The influence laws of the parameters of heat treatment on the evolution of lamellar ? in the Ti6242S Ti-alloy were assertained during the heat treatment after ? forging.The discoveries were as follows:firstly,the volume fraction of lamellar ? was much affected by the temperature,the holding time and the cooling rate of heat treatment:the volume fraction of lamellar ? decreased with the increase of heat treatment temperature from 920? to 980?,and it went up from 48.6%to 53.8%when the holding time increased from 20min to 60min,and also the fraction increased obviously from 39.3%to 62.2%;Secondly,the thickness was much influenced by the heating temperature and cooling rate of heat treatment:lamellar thickness increased from 1.17?m to 2.28?m with the increase of temperature from 920? to 980?,and it became thicker from 1.17?m to 2.28?m when the cooling rate reduced from water cooling to furnace cooling.Lastly,the length of lamella was highly inflluenced on the temperature of heat treatment,and the length increased from 10.53?m at 920? to 13.58?m at 980?.The characteristics of lamellar evolution were uncovered.The coarsening mechanism was dominant by the boundary splitting,and accompanied by termination migration and inter hole coalescence.Boundary splitting was consisted of two modes:sub-structure and localized shearing.Variants selection occurred during the precipitation of lamellar a:three kinds of variants of 60° misorientation had the priority to precipitate with the variants of 10° and 90°misorientation restrained.The degree of variants selection of 60° misorientation was reduced as the deformation degree increased from 30%to 50%,because the frequency of the precipitation of[11-20]/60° variants reduced accompanied with the decrease of the growth of the precipitation of[-10 5 5-3]/63.26° variants. |
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