| Ti-6Al-4V alloy(TC4 alloy)is anα+βtitanium alloy,that has been widely used in the aerospace field due to its high specific strength,good corrosion resistance,and excellent fatigue performance.With the improvement of the design indicators and performance requirements of the new generation of aviation equipment,higher requirements have been put forward for the mechanical performances of titanium alloys.The microstructure of alloys determined the mechanical performances.Numerous studies are therefore conducting a comprehensive understanding of the microstructure-related property of the TC4 alloy to meet design requirements.Grain boundaryα-phase(GB-α)is a primary or secondaryαphase precipitated on the prior-βgrain boundary,which has attracted much attention due to its important influence on properties.The presence of continuous GB-αis detrimental to the ductility,fracture toughness,and fatigue performance.In contrast,discontinuous GB-αcould increase fracture resistance.Selective laser melting(SLM)is an innovative technique to build complex three-dimensional(3D)parts in a bottom-to-top manner from design,that leads to a wide range of applications in the aerospace,nuclear,and medical industries.However,due to the rapidly cooling rate(10~5-10~7 K/s)of the SLM process,GB-αis mostly not identifiable at the micro-scale in as-fabricated states.Furthermore,the extremely fast cooling rate could also lead to the presence of theα’martensite phase and large residual stress in as-fabricated parts.These adverse factors on the alloy’s performance are expected to mitigate by post-fabrication heat treatment,which could decompose theα’martensite phase and release residual stress.But post-heat treatment process inevitably promotes the rapid nucleation and coarsening of the GB-α.In addition,the unique extremely thermal gradient of the SLM process could lead to the long columnar prior-βgrains along the build direction,which causes mechanical property anisotropy.GB-α,as the decoration of the prior-βgrain boundaries,is known to be the origin of property anisotropy in SLM titanium alloys.Although the GB-αformation mechanism in conventionally manufactured Ti alloys,which is normally correlated with the chemical driving force and crystallographic orientations,has been well established,the GB-αgrowth behavior in SLMed Ti alloys has rarely been reported.Furthermore,previous studies have not considered that microstructural features govern the deformation(compare with GB-αrelated failure).Understanding how the continuous or discontinuous of GB affects the compatibility requirements necessary for titanium alloys deformation,and how these factors could be related to the mechanical performance in SLMed titanium alloys,is crucial.In this study,GB-αprecipitation and coarsening behavior in SLMed TC4 was investigated and compared with those in as-cast TC4.We investigated the effects of heat treatment on the microstructure and tensile performances of SLMed TC4 along with the vertical and horizontal directions.Finally,the deformation behavior and dislocation motion in GB-αwith different morphologies in a SLMed TC4,and how this could be related to the failure mechanisms,were fully investigated.These findings could help rationalize the GB-αmorphology evolution and benefit the further mechanical property manipulation in SLMed titanium alloys.The following conclusions can be drawn from this study:1)Grain boundaryα-phase precipitation and coarsening behavior in TC4 processed by SLM were investigated and compared with those in as-cast TC4 alloy.GB-αin SLMed TC4 tended to precipitate at the triple junctions(TJs)of the priorβ-grain boundaries during the subsequent annealing process.GB-αfollowed Burgers orientations relationship with one of the two adjacentβgrains Kinetic analysis on GB-αshowed that the coarsening mechanisms varied in SLMed TC4 at different annealing temperatures(bulk diffusion at 750℃and interface reaction at both 850 and 950℃),while were consistent in as-cast TC4(interface reaction at all the three heat treatment temperatures).The inconsistent coarsening mechanisms were attributed to the GB-αcurvature difference caused by the differentαlamellae nucleation mechanisms.2)The correlation between mechanical performances and GB-αwith different morphology of SLMed TC4 was investigated.The morphology of GB-αis mainly dependent on the HT temperature,the GB-αexhibits the discontinuous morphology when HT temperatures increase to near theβtransus.Tensile performances are very much dependent on HT temperature.With the increase of HT temperature,tensile strength declines and the total elongation rises because of acicularα’martensite transformation to the coarserαlath.However,the Water-quenching samples present different trends,which is attributed to the fact that the high cooling rate leads to the high-temperatureβphase being transformed toα’martensite again.Furthermore,the anisotropic ductility and the discontinuous ratio of GB-αcould be correlated.With the discontinuous ratio of GB-αlarger than 0.6,the anisotropic ductility of SLMed TC4 could be eliminated.Fatigue testing found that the continuous GB-αis a potential crack pathway,resulting in poor fatigue performance;while discontinuous GB-αinhibits the fatigue cracks propagating along the grain boundary,causing an increase in the fatigue performances.3)A detailed investigation into the deformation behavior of GB-αwith different morphologies in a SLMed TC4 was studied.We found premature dislocation activities and strain accumulation within continuous GB-αwith the perpendicular tensile loading.The damaging behavior in continuous GB-αsamples with the transversal tensile loading was the deformation-to-failure was mainly attributed to theαlaths within the prior-βgrains,instead of the continuous GB-α.This different deformation mechanism fully supports that the presence of the continuous GB-αis one of the causes of the anisotropic performance in SLMed TC4.In contrast,discontinuous GB-αcould fully accommodate uniform deformation with the matrix microstructure under the loading direction both perpendicular and parallel with the prior-βgrain boundaries,which provides an approach that eliminates the mechanical property anisotropy in SLMed TC4. |
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