Hierarchical microstructure of a titanium alloy fabricated by electron beam selective melting

Here, a near alpha-type Ti6.5Al2Zr1Mo1V alloy has been fabricated by electron beam selective melting (EBSM). Near-equiaxed grains existed in the first few layers, whereas elongated columnar prior beta grains almost parallel to the building direction formed in the subsequent built layers. Interspacing of beta phase gradually decreased as the build height increased. Martensite alpha' with twins and dislocations emerged and microhardness value reached the maximum in the top region, whereas only alpha/beta phase appeared in other regions in the EBSMed sample. Multiple phase transformations can be observed with the change of peak temperatures during each thermal cycle. With a sufficient dwell time, martensite alpha' in the middle and bottom regions in-situ decomposed into alpha + beta and coarsened by the heat conduction from the subsequent layers. Fine beta precipitates nucleated heterogeneously inside alpha' plates and at plate-plate interfaces during the subsequent EBSM process. Considering the phase transformation during the heating process and the cooling process, the existence time of different phases was combined with cycle heating and cooling to clarify the dynamic evolution of microstructure under complex thermal history of EBSM, favoring the fabrication of high-performance titanium alloy components. (C) 2020 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.