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A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 8 1 9 reproducibility and chemical compo- sition issues. Dimensional control of large parts becomes more difficult due to shrinkage. Parts with flat surfaces to rest on the setter are consistent dimen- sionally, but parts with multiple sur- faces that require setters in complex shapes become less practical as size increases. Further, large overhanging areas are difficult to control dimension- ally due to gravity. Packing density of ti- tanium powder mixes will increase with the development of new binders, which will decrease shrinkage and reduce di- mensional problems. Typical proper- ties of titanium injection-molded parts are 0.14-0.32 w/o oxygen, 96-98% rela- tive density, 120-135 ksi ultimate tensile strength, and 9-14% elongation. RAPID SOLIDIFICATION AND MECHANICAL ALLOYING Rapid solidification (RS) involves rapid quenching (high cooling rate) of a liquid to the solid state, which extends normal solubility limits and refinement of the microstructure. Figure 5 shows the effect of cooling rate on beta-phase grain size in Ti-6Al-4V. An example of microstructural refinement in Ti-6Al-4V with increased cooling rate is shown in Fig. 6. Due to the fine beta-phase grain size, alpha phase produced after ag- ing is predominantly at the beta grain boundaries and exhibits an equiaxed morphology. This leads to high ductil- ity with significant superplasticity (i.e., at low temperatures and relatively high strain rates). Mechanical alloying (MA) is a sol- id-state powder processing technique involving repeated cold welding, frac- turing, and rewelding of blended pow- der particles in a high-energy ball mill to produce a homogeneous material. The microstructures of Ti3Al + 2Er RS and MA powder compacts after HIP at 850° and 1000°C, respectively, are shown in Fig. 7 [4] . THERMOHYDROGEN PROCESSING In the thermohydrogen process- ing (THP) technique, hydrogen is in- troduced into a titanium alloy as a temporary alloying element [5] . The hy- drogenated alloy is heat treated and/or worked and the hydrogen is removed to less than specification levels (125 ppm for Ti-6Al-4V) by a simple vacuum an- neal. The effects of hydrogen include: • Stabilization of the high tempera- ture allotromorphic beta phase, thereby lowering the beta-to-alpha phase transformation temperature (beta transus temperature) Fig. 5 — Effect of cooling rate on the beta-phase grain size in Ti-6Al-4V alloy. Fig. 6 — Equiaxed alpha phase produced in rapidly solidified Ti-6Al-4V alloy after anneal- ing at 965°C (1770°F). Fig. 7 — Microstructures of Ti3Al + 2Er powder compacts made of (a) rapidly solidified [RS] powders and (b) mechanically alloyed [MA] powders after HIP at 850°C and 1000°C, respectively. RS powders result in a fine dispersion of Er 2 O 3 , which could potentially provide good elevated temperature properties. MA powders result in a small grain size and dispersion of fine particles. Fig. 8 — Refinement of microstructure using thermohydrogen processing (THP): (a) as-cast Ti-6Al-4V, (b) after THP treatment. Beta-phase regions (black), alpha-phase regions (white). (b) (a) (b) (a)