Molybdenum alloy Mo-Ti-Zr-C adapted for laser powder bed fusion with refined isotropic microstructure and excellent high temperature strength

Abstract

The molybdenum‑titanium‑zirconium‑carbon alloy TZM is one of the few molybdenum alloys that can be processed crack-free in laser powder bed fusion (LPBF). However, the parts have a coarse-grained, columnar microstructure comprising epitaxially grown grains, and their strength is limited by residual porosity. Consequently, the mechanical properties do not reach the values of their conventionally powder-metallurgically produced pendants. In this work, the conventional alloy composition of TZM was adapted to the unique processing conditions in laser powder bed fusion by increasing the carbon content, and the effects on the microstructure and mechanical properties were investigated.Increasing the carbon content to 2.3 at.% yielded a significantly refined and isotropic microstructure. In addition, C led to the formation of a cellular subgrain structure consisting of (Mo, Ti) cells 0.4 ± 0.1 μm in size surrounded by a closed network of ternary (Mo, Ti) carbide. Residual oxygen impurities in C-modified TZM were partly dissolved in the ternary (Mo, Ti) carbide network and partly bound by Zr as nanometer-sized ZrO2 particles. These two mechanisms for binding oxygen minimized oxygen segregation—a major issue limiting the grain boundary strength in Mo and Mo alloys—and thus purified and strengthened the grain boundaries. The mechanical strength increase due to the increase in carbon content was 50% (0.5 at.% C vs 2.3 at.% C). At elevated test temperatures of 800 °C and 1200 °C, TZM-2.3 at.% C exceeded the ultimate tensile strength (UTS) of conventionally produced TZM by 24% and 16%, respectively.