Study Of Microstructure Evolution Of Ti60 Titanium Alloy During Controlling Of Bimodal Structure

Due to high strength-to-weight ratio,near-?titanium alloys are utilized extensively for the production of compressor components of aero-engine gas turbine,combing great combination of physical and mechanical properties at elevated temperature.Ti60 alloy,developed by Institute of Metal Research of China,is a promising near-?titanium alloy,which is designed for use as disc and blade of high pressure compressor at service temperatures up to 600°C.Thus,Ti60 alloy have become promising candidates in the field of aerospace.In order to achieve a good balance of creep and fatigue peiperties,the forgings of Ti60 are designed to produce a tightly controlled bimodal microstructure.Acquisition of this type of microstructure involves a series of micorstructural evolution,including the growth of equiaxed primary?(?_p)during continuous cooling,the precipitation of plate?(?_s)and grain boundary?(GB?)during cooling,and the recrystallization of?_p and prior?grians during thermal-mechanical processing at high temperature.The detailed study of of these microstructures will help to better manipulate the microstructure and provide theoretical guidance for industrial processing.Therefore,it is an urgent need to comprehensively study the effect of thermo-mechanical processing to microstructure evolution of Ti60 alloy.The main research contents and conclusions are as follows:The evolution of microstructure at different cooling rates in a near?titanium alloy Ti60 were studied by optical micrograph,back scattered electron(BSE)images,high-resolution electron backscatter diffraction technique(EBSD)and electron probe microanalysis(EPMA).Microstructural observations indicated the size of?_p,the width of individual?_s,the thickness of boundary?layer,and the size of a colony increased with decreasing cooling rate.The?-rim phase observed by BSE image,which formed at the periphery of?_p during cooling and has an identical crystallographic orientation to the interior region of?_p analyzed by Kikuchi diffraction patterns,is considered to be evidence of epitaxial growth of?_p.The EPMA confirmed that contrast difference in BSE image within?_p is caused by the difference in composition of Al and Mo.The?-rim is light grey.The?-rim formed during cooling,which leads to lower Al and higher Mo concentration compared with the interior?_p.Thus,the atomic number of?-rim region increase and this region produce higher backscattered signal intensity in the BSE image.Hence,?-rim region apperar a brighter contrast.The further microanalysis of local composition indicated that epitaxial growth during continuous cooling is mainly controlled by the diffusional redistribution of aluminum and molybdenum atoms between?_p and?matrix.On this basis,the sizes of?_p were theoretically calculated after continuous cooling based on a diffusion-controlled model,and model predictions showed good agreement with experimental measurements.The evolution of plate?and boundary?during precipitation from?phase matrix of bimodal microstructure was studied by microstructural observation and crystallographic orientation analysis.The result shows that most grain boundary?(GB?)precipitates maintain a Burgers orientation relationship(BOR)with one of adjacent prior?grains.The small?colony structure near the GB?tend to form in the?grain that exhibits the BOR with the GB?.However,in some unique cases,the GB?did not maintain a BOR with either prior?grains due to preferential growth of equiaxed primary?(?_p).The length of grain boundary?is relatively limited.This is because?/?interfacial energy will be relatively high when BOR between them is deviated,which would be energetically unfavorable to the further growth of GB?.Additionally,?phase can nucleate on the subgrain boundary and simultaneously grow into the two adjacent?grains to form a continuous plate?.It was also found that both sides of some special high-angle prior?/?boundaries(e.g.49.5°/<110>)can form two colonies structure sharing common crystallographic orientation.The phase interface between?_p and?grain has an important influence on the precipitation of plate?.When an?_p grain and an adjacent?grain have a near BOR,the preferred?_s may form at?_p/?boundary with the similar orientation to the?_pThe evolution of equiaxed primary?phase(?_p)during thermal-mechanical processing of Ti60 alloy was studied.The recrystallization behavior and boundary splitting within equiaxed?_p were analyzed by crystallographic orientation and microstructure observations.The results showed that the formation of internal(sub)boundaries within the?_p was by the strong recovery.In most cases,it is difficult to obtain a refinement of equiaxed?_p by boundary splitting and?_p grains were still contiguous with peanut shape at relatively short heat treatment time(?24 h).The groove in the peanut structure was indicative of boundary of?/?.These observations were rationalized on the basis of the classical Mullins grooving analysis.The high temperature?grains undergo a series of complicated evolution during thermal-mechanical processing.It is found that the majority of the strains were accommodated by?phase.As the strain increases,the fraction of high-angle GBs gradually increases.The recrystallized grains formed gradually from subgrains by continuous recrystallization.Deformation in the?+?phase field does not form many new orientation?grains but slightly scatters?grain orientations around the orientations of big prior?grains in small strain samples.Each of different coarse macrograins corresponds to a family of prior?grains with similar crystallographic orientations with about 20°of spread,which are outlined by?_s plates with similar crystallographic orientations having near reflection contrast after etching.