Phase Selection In Undercooled Melts Of Cu-Ge Peritectic Alloys

Phase selection in undercooled melts of peritectic alloys is of much interest because it is helpful to an understanding of non-equilibrium solidification, and of great value in industries. It occurs in undercooled melts of many engineering materials such as permalloy, stainless steel, rare earth permanent magnet materials, and rare earth high-temperature superconducting materials. However, a clear and general understanding of it is missing in literature, and therefore, much work should be done in this respect.In the present thesis work, peritectic Cu-14.8%Ge composition and hyperperitectic Cu-18%Ge composition were chosen as model alloys for the purpose of investigating phase selection in undercooled melts. They were undercooled and rapidly solidified using the electromagnetic levitation technique. Parallelly, glass and metal substrate-quenching experiments and drop tube experiments were carried out on the same compositions, aiming at a comparative investigation of influences of undercooling and cooling rate on phase selection.In the EML experiments, Cu-14.8%Ge and Cu-18%Ge compositions were undercooled below liquidus by an amount of 157K to 244K and by an amount of 124K to 241K, respectively. Theαphase was crystallized primarily in both compositions. In the glass substrate-quenching experiments, the peritectic Cu-14.8%Ge composition was undercooled by an amount of 32K to 174K, and crystallized into primaryαphase; the hyperperitectic Cu-18%Ge composition was undercooled by an amount of 179 to 294 K, and crystallized into primaryζphase for undercoolings above 280 K. In the metal substrate-quenching experiments, both compositions were solidified via primary crystallization of the peritecticζphase. In the drop tube experiments, all samples of the peritectic Cu-14.8%Ge composition and most of the samples of the hyperperitectic Cu-18%Ge composition were crystallized into primary a phase, whereas few samples of the composition Cu-18%Ge were crystallized into primaryζphase, at least at the initial stage ofthe solidification process.The above results showed that primary crystallization of the peritecticζphase in peritectic Cu-Ge alloys is fairly difficult. However, it is able to become a primary phase even under slow cooling conditions, provided that a liquid undercooling of greater than 280 K is achieved. It. is thus concluded that with respec to the phase selection in undercooled Cu-Ge alloys, undercooling is a decisive factor, whereas cooling rate is a beneficial factor.