Study of Microstructure Evolution of Ti60 Titanium Alloy during Controlling of Bimodal Structure

Author:Gao Xiong Xiong

Supervisor:zeng wei dong


Degree Year:2018





Due to high strength-to-weight ratio,near-αtitanium 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-αtitanium 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 primaryα(α_p)during continuous cooling,the precipitation of plateα(α_s)and grain boundaryα(GBα)during cooling,and the recrystallization ofα_p and priorβgrians 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 nearαtitanium 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 ofα_p,the width of individualα_s,the thickness of boundaryαlayer,and the size of a colony increased with decreasing cooling rate.Theα-rim phase observed by BSE image,which formed at the periphery ofα_p during cooling and has an identical crystallographic orientation to the interior region ofα_p analyzed by Kikuchi diffraction patterns,is considered to be evidence of epitaxial growth ofα_p.The EPMA confirmed that contrast difference in BSE image withinα_p is caused by the difference in composition of Al and Mo.Theα-rim is light grey.Theα-rim formed during cooling,which leads to lower Al and higher Mo concentration compared with the interiorα_p.Thus,the atomic number ofα-rim region increase and this region produce higher backscattered signal intensity in the BSE image.Hence,α-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 betweenα_p andβmatrix.On this basis,the sizes ofα_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 plateαand boundaryαduring precipitation fromβphase matrix of bimodal microstructure was studied by microstructural observation and crystallographic orientation analysis.The result shows that most grain boundaryα(GBα)precipitates maintain a Burgers orientation relationship(BOR)with one of adjacent priorβgrains.The smallαcolony structure near the GBαtend to form in theβgrain that exhibits the BOR with the GBα.However,in some unique cases,the GBαdid not maintain a BOR with either priorβgrains due to preferential growth of equiaxed primaryα(α_p).The length of grain boundaryαis relatively limited.This is becauseα/βinterfacial energy will be relatively high when BOR between them is deviated,which would be energetically unfavorable to the further growth of GBα.Additionally,αphase can nucleate on the subgrain boundary and simultaneously grow into the two adjacentβgrains to form a continuous plateα.It was also found that both sides of some special high-angle priorβ/βboundaries(e.g.49.5°/<110>)can form two colonies structure sharing common crystallographic orientation.The phase interface betweenα_p andβgrain has an important influence on the precipitation of plateα.When anα_p grain and an adjacentβgrain have a near BOR,the preferredα_s may form atα_p/βboundary with the similar orientation to theα_pThe evolution of equiaxed primaryαphase(α_p)during thermal-mechanical processing of Ti60 alloy was studied.The recrystallization behavior and boundary splitting within equiaxedα_p were analyzed by crystallographic orientation and microstructure observations.The results showed that the formation of internal(sub)boundaries within theα_p was by the strong recovery.In most cases,it is difficult to obtain a refinement of equiaxedα_p by boundary splitting andα_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 ofα/α.These observations were rationalized on the basis of the classical Mullins grooving analysis.The high temperatureβgrains undergo a series of complicated evolution during thermal-mechanical processing.It is found that the majority of the strains were accommodated byβphase.As the strain increases,the fraction of high-angle GBs gradually increases.The recrystallized grains formed gradually from subgrains by continuous recrystallization.Deformation in theα+βphase field does not form many new orientationβgrains but slightly scattersβgrain orientations around the orientations of big priorβgrains in small strain samples.Each of different coarse macrograins corresponds to a family of priorβgrains with similar crystallographic orientations with about 20°of spread,which are outlined byα_s plates with similar crystallographic orientations having near reflection contrast after etching.