Metals A and B form a terminal solid solution , cubic in structure. The variation of the lattice parameter of with composition, determined by quenching single-phase alloys from an elevated temperature, is found to be linear, the parameter varying from 3.6060 to 3.6140 in containing 4.0 weight percent B. The solvus curve is to be determined by quenching a two-phase alloy containing 5.0 weight percent B from a series of temperatures and measuring the parameter of the contained . How precisely must the lattice parameter be measured if the solvus curve is to be located within +/- 0.1 weight percent B at any temperature