Formation and evolution mechanisms of micropores in powder metallurgy Ti alloys

[Display omitted] •Pores in the P/M Ti6Al4V green bodies are categorized to three types according to their 3D morphological characteristics.•Pores features including morphology, size, orientation and connectivity are investigated quantitatively.•The evolution behavior of various types of pores are carefully studied and analyzed theoretically.•Pores evolution mechanisms are proposed and discussed with respect to sintering temperature. Mechanical properties and thermal deformation capabilities of powder metallurgy (P/M) Ti alloys are extremely susceptible to pores in the sintered parts. However, deciphering the pores evolution mechanisms is exceedingly challenging and heretofore this problem has yet been unequivocally understood. To address it, 3D X-ray computed tomography (XCT) is herein employed to perform this investigation. It is found that three types of pores encompassing branch-like pores (type Ⅰ), flat pores (type Ⅱ), and snatchy spherical pores (type III) form in the green compacts after the cold isostatic pressing, which is fundamentally governed by the irregularity of powder profiles. During sintering, type I pores are first to be decomposed at the junctions where pores branch triggered by stress concentration and subsequently undergo splitting and spheroidizing, till converting to (near) spherical pores. Type II and III pores are basically showing analogical behavior excluding the branch break-up step. Moreover, pores orientation distribution is turned out to be temperature independent, nevertheless, pores connectivity and localized porosity are closely related to the sintering temperature. Besides, a spot of pores is inspected inside partial large-sized master alloy powder particles, i.e., at 1200 °C, which possibly originates from the Kirkendall’s effect between the alloying elements and the Ti matrix.