Structure and mechanical properties of boron-doped cubic zirconium trialuminides

Abstract

Cubic (L12) ternary zirconium trialuminides macroalloyed with Cu(Al5CuZr2), Mn(Al66Mn9Zr25), and Cr(Al67Cr8Zr25) (atomic percent) and doped with 50 and 100 ppm boron were fabricated by induction melting. Their as-cast microstructures are characterized by a small amount of porosity (1 to 2 pct) and second phase (2 to 3 pct). Boron seems to slightly enhance porosity (up to 3.3 pct) in Al5CuZr2 +100 ppm B alloy, and it also promotes some compositional inhomo-geneity in Al66Mn9Zr25 alloy. Vickers microhardness and compressive properties at room temperature (RT), peak strength temperature (500 °C to 600 °C) and 900 °C were investigated. Microcracking development was also investigated in Al5CuZr2 +100 ppm boron alloy exhibiting a stepped load-deflection curve. Vickers microhardness strongly depends on load, similarly to boron-free cubic ternary zirconium and titanium trialuminides, and increases in a systematic way with increasing boron content which seems to indicate a solid solution strengthening effect. At RT, 0.2 pct offset yield strength is not increased by the boron doping in most of the alloys studied except for Al66Mn9Zr25 + 50 ppm B alloy. Permanent deformation (apparent ductility) at ultimate compressive strength is not enhanced by boron doping. In Al5CuZr2 +100 ppm B alloy microcracks start nucleating and proliferating in the elastic region of load-deflection curve in characteristic “bursts” accompanied by a “click” sound and the appearance of a discernible step on the load-deflection curve. Pre-existing pores are observed to be active centers of microcracking.