The Temperature Dependence of Hardness of Ll2 Ordered Alloys

Hot hardness measurements were made on more than thirty polycrystalline binary Ll2 compounds in order to obtain sufficient and systematic data to answer the question as to why only a limited number of Ll2 alloys show a positive temperature dependence of strength. It was discovered that eleven alloys, Fe3Ga, Pt3In, Pt3Sn, Pd3Pb, Zr3In, Pt3V, Pt3Cr, Pt3Mn, Pd3Mn, Pt3Ti and Pd4Ti, exhibited the strength anomaly, in addition to the alloys Ni3Al, Ni3Si, Ni3Ga, Ni3Ge, Zr3Al and Co3Ti which are already known. The relation between phase stabilities relative to other geometrically close packed structures and the mechanical behavior of Ll2 compounds was discussed in terms of the electron concentration criterion and the atom size difference criterion. It was concluded that an Ll2 compound having a strength anomaly has a relatively low phase stability with respect to the DO19, or most likely the DO22 structure. This is consistent with the “Kear-Wilsdorf” mechanism. Suggestions have been made for further experimental work, with particular reference to the relation between phase stability and mechanical behavior in Ll2 compounds.