| Ti-55531?Ti-5Al-5Mo-5V-3Cr-1Zr?is a new type of near?titanium alloy with high toughness,low transformation point,excellent deformation ability,high hardenability and excellent welding performance.It is widely used in the manufacturing of key load bearing components in the aviation field.Heat treatment process is one of the most effective ways to improve the mechanical properties of near?titanium alloy.Appropriate heat treatment process can regulate the microstructure of near?titanium alloy,so as to obtain the comprehensive properties of high strength,high toughness and high plasticity.This article takes Ti-55531 titanium alloy as the research object,introduces certain defects through different pretreatment processes,and studies the effect of defect concentration on the precipitation behavior and mechanical properties of?phase.In the present paper,Ti-55531 alloy is used as the research object.Firstly,the DIL805A/D phase transformation instrument combined with OM,PAL,XRD,SEM,TEM,EBSD and TEM are performed to study microstructure and strengthening mechanism of alloy after different pretreatment.The main conclusions are as follows:With the increase of solution temperature,the concentration of vacancy defects increases.As the cooling rate of alloy changes from 0.5?/s to 200?/s,the concentration of vacancy defects firstly decreases and then increases with the cooling rate and the concentration of vacancy defects is the lowest at 50?/s;The alloy is subjected to 4%and 9%cold rolling deformation after solid solution at 880?for50 min and the vacancy defect concentration increased with the amount of cold rolling deformation.The???phase transition occurs during high-temperature aging treatment and?phase presents three forms:triangle,cluster and dendrite.The vacancy defects produced by pretreatment are closely related to the shape and the volume distribution of?phase.The greater the concentration of vacancy defects,the greater the volume fraction of precipitated alloys at the initial aging treatment at 550?.The area of PFZs increased with the aging time.The larger the vacancy defects in Ti-55531 alloy after pre-treatment,the easier?phase precipitates from the small angle grain boundary during the later aging treatment as follows:?phase is easy to precipitate from small angle grain boundaries at 860?solution treatment,while?phase is easy to precipitate from large angle grain boundaries at 900?solution and aging treatment.After cooling to room temperature at 0.5?/s after solid solution at 900?.?phase is easy to precipitate at large angle grain boundary during the direct aging treatment at 550?The size and distribution of?WM and?WGB phases are related to aging temperature,when Ti-55531 is aged at aging treatment at 450?650?and solution treatment at 880?/50 min/WC.The larger the size of?phase and the higher the proportion of?WMphase at higher the aging temperature.Defects generated after cold rolling deformation play important role in the precipitation of the second phase.As the amount of cold-rolling deformation increases,the amount of?phase precipitation increases significantly and is densely distributed in the?matrix.The thickness of?phase decreases first and then increases,and an obvious variant selection effect appears locally.The defect concentration of Ti-55531 alloy after pretreatment has an important influence on the mechanical properties after aging treatment.The higher the defect concentration is,the faster the hardness changes in the early stage of aging and the higher the strength after aging treatment.As the aging time increases and the amount of cold rolling deformation increases,the Vickers hardness of the alloy increases;while the Vickers hardness increase rate of the alloy decreases with the aging time. |
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