The Impact Of Heat Treatment On Microstructure And Mechanical Properties Of Ti-5al-5mo-5v-1cr-1fe Alloy

Ti-5Al-5Mo-5V-1Cr-1Fe is a deep hardenable(α+β)alloy,which used below 400℃for fan and compressor discs.The development of microstructure on ageing of an(α+β)titanium alloy containing Ti-5Al-5Mo-5V-1Cr-1Fe in theβand(α+β) solution-treated and quenched conditions has been investigated.Some of the samples were heat treated at 900℃for lh following by water quenched.A fully recrystallized equiaxedβgrain structure was obtained but grain growth was significant.Solution treatment in the(α+β)phase field led to a bimodal microstructure consisting of globular primaryαin aβmatrix,with grain boundaryαin discontinuous form.Ageing at 300℃gave uniformωprecipitation in the matrix.The orientation relationship between theωandβphases is {0001}_ω,//{111}_β,<11(?)0>_ω,//<110>_β.In (α+β)condition,bothωandαphase particles precipitated in theβmatrix.The (2/3)<111>_βlongitudinal displacement waves producesωphase structure by yielding a local ordering of the displaced {111}_βplanes.During recrystallization in theα+βphase field for the as-forged alloys,theβstabilizers such as Mo,V,Fe,Cr localized in theβtransformed structure.Thus theωandαprecipitations were accelerated.Ageing below 400℃,the presence of very fineωparticles were detected by electron diffraction and the formation of fineαplatelets is preceded by the precipitation ofωphase particles.Theαplatelets align as groups in the shape of significantly fine laths.This suggested the possibility thatωphase particles can act as strong nucleation sites for hexagonalαphase and can be utilized for the microstructural control ofα/βduplex phase structure to improve the mechanical properties.Ageing above 500℃,high densities ofαprecipitates occurred on grain boundaries and in grains.Theαprecipitation took first place on the preferred site of boundaries and was then followed by a matrix nucleation.It was proposed that grain boundary precipitate has a near rational orientation relationship with respect to one of the adjacent matrix grains but an irrational relationship with respect to the second grain.This implies that the grain boundary precipitate nucleates coherently as much as possible with the rational matrix grains.Therefore in the recrystallized matrix,a high-angle grain boundary plane plays a major role in variant selection for grain boundary nucleation.The microhardness increased with increasing ageing temperature,passed through maxima,and then decreased.Forβsolution-treated condition the microhardness reached its maximum value at the ageing temperature of 450℃while forα+βsolution-treated condition at 350℃due toωparticles for the nucleation ofαprecipitations,leading to very fineαprecipitations in the structure.At all ageing conditions,maximum microhardness occurred after about 8h.After ageing for up to 8h,there were only slight changes in microhardness with ageing time.The alloys that heat treated inα+βfield and aged always presented higher values than that ofβcondition,which was attributed to finerβgrain size andαprecipitation.Aging at higher temperature than 400℃leads to largerαprecipitates which would be expected to be more detrimental to strength.After long term exposure,a large amount of coarseαprecipitates formed in grains or on grain boundaries also lead to a reduction in strength.