Powder metallurgy titanium by the Hydrogen Sintering and Phase Transformations (HSPT) process

Hydrogen has been investigated for decades as a temporary alloying element to refine the microstructure of Ti-6Al-4V, and is now being used in a novel powder metallurgy (PM) method, Hydrogen Sintering and Phase Transformations (HSPT) process. In earlier work from the literature, pseudobinary phase diagrams of (Ti-6Al-4V)-H have been studied and developed, but they are not well established due to limitations of their methodologies, and the microstructural refinement mechanism of alloying with hydrogen is still under debate.;In this study, in situ studies of phase transformations during hydrogenation and dehydrogenation of (Ti-6Al-4V)-H alloys were conducted using high energy synchrotron X-ray diffraction (XRD), thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). The eutectoid phase transformation of beta ↔ &agr; + delta was observed in the (Ti-6Al-4V)-H alloy via in situ synchrotron XRD at 211 °C with a hydrogen concentration of 37.5 at.% (measured using TGA-DSC). The relationships of hydrogen composition to partial pressure and temperature were investigated in the temperature range of 450 to 900 °C. Based on these results, a partial pseudobinary phase diagram of (Ti-6Al-4V)-H is proposed for hydrogen compositions up to 60 at.% in the temperature range of 100-900 °C.;The phase transformation sequences in HSPT were directly revealed by in situ synchrotron XRD in this study. Combined with the analysis of the microstructure and phases by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM), the microstructure refinement in Ti-6Al-4V is found to be the results of (1) the precipitation of ultra-fine &agr;/&agr; 2 within coarse beta grains during isothermal holding at a moderate temperature, and (2) the eutectoid transformation of beta→&agr;+delta at a low temperature (~200 °C).;The application of HSPT in manufacturing PM commercial pure (CP) titanium was also investigated. The microstructure refinement of CP-Ti in HSPT was not as significant as that of Ti-6Al-4V, since the grain grow in CP-Ti was much faster than that in Ti-6Al-4V in the dehydrogenation step. However, the HSPT samples still exhibited finer grain microstructures than vacuum sintering, and showed a 14% improvement in tensile strength and a 20% improvement in ductility (as measured by % elongation) compared to vacuum sintered samples.