Structure and mechanical properties of boron-doped cubic zirconium trialuminides

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.