Understanding competing fatigue mechanisms in powder metallurgy Ti–6Al–4V alloy: Role of crack initiation and duality of fatigue response

Multiple mechanisms of fatigue crack initiations from pores leading to short-term and long-term fatigue life have been observed in a powder metallurgy (PM) Ti–6Al–4V alloy made by the hydrogen sintering and phase transformations (HSPT) process. The pores in the sintered alloy could be classified into two groups; type-I being caused by particle interstices that did not fully close during sintering (sinter-pores) and the type-II originating from larger voids that did not close during powder packing (cave-pores). The fatigue life trends can be classified by the type and the location of pore from which the crack initiated. Interestingly, the two types of pores, and the two different locations (surface versus interior) from which the cracks initiated lead to four distinct S–N trends in the fatigue data. When the stress amplitude level and the pore size were accounted for through initial stress intensity factors, the fatigue data reduced to dual S–N fatigue curves. The results offer new insights into how pores, especially their type and size, and location control the fatigue behavior of PM Ti–6Al–4V alloy. Large improvements in fatigue life can be obtained if the large cave-pores are eliminated or their volume density reduced to very low levels.