| Description |
Hydrogen sintering and phase transformation (HSPT) is a novel low-cost process for powder metallurgy (PM) titanium alloys. The microstructure obtained from the HPST process has very fine α grains, as well as other common features of a PM product, such as residual pores and large α grains. This research is aimed at evaluating the microstructure and mechanical behavior of Ti-6Al-4V alloy made from TiH2 powders with different size distributions and a variety of processing parameters. Pneumatic isostatic forging (PIF) was carried out under β-tarsus to eliminate residual porosity without coarsening the grains. Tensile, fatigue, and crack growth properties of HSPT processed alloys, as well as wrought Ti-6Al-4V alloys, were studied in this research to investigate the microstructure- property relationships, with an emphasis on the effect of microstructural inhomogeneity on fatigue behavior. Tensile properties of the HSPT Ti-6Al-4V alloys were found equivalent to or exceeded those of mill-annealed alloy. The low cycle fatigue (LCF) life was also found equivalent to that of wrought Ti-6Al-4V after PIF. However, the high cycle fatigue (HCF) performance was still inferior to that of wrought material due to the large α phase grain/grain boundary α phase plate, which was found to initiate cracks from inside of the specimen in HCF range. It is also shown that with the optimization of the distribution of powder sizes and the processing parameters, the properties of HSPT processed Ti-6Al-4V alloys are much superior to those of Ti-6Al-4V alloys made by other conventional iv blended elemental (BE) methods, and equivalent to best properties found in alloys produced by pre-alloyed method in hot iso-static pressed (HIPed) and heat treated condition. The S-N behaviors of different alloys were studied. Crack initiation types, sizes, and locations were found to alter the fatigue failure mechanism and the shape of S-N curves. The quantitative correlations between crack initiation size, location, and the S-N curve shape were also revealed in this study. Fatigue crack growth tests were carried out using miniature single edge-notched tension (SENT) specimens to evaluate the crack growth behavior. Crack growth stages were classified based on the characteristics of fractographs of both fatigue failed samples and crack growth samples. Microstructurally small crack growth rate was calculated from rough area size and fatigue life consumed in this region. The predicted S-N curves at HCF region based on crack growth rate at different stages for Ti-6Al-4V alloys with different crack initiator sizes match very well with the current HCF data. |
