In-(12∼14 at%)Pb合金の時効過程における相変化

Abstract

Phase changes during aging of indium-(12-14 at%) lead alloys have been studied by means of X-ray diffractometry and metallographic techniques. These indium alloys, when cooled from a high-temperature phase and then aged at room temperature, show four kinds of phase change depending on the concentration of lead as follows: (a) fct(c⁄a>1)→fct(c⁄a>1)+fct(c⁄a<1), (b) fco→fco+fct(c⁄a<1)→fct(c⁄a>1)+fct(c⁄a<1), (c) fct(c⁄a<1)→fco+fct(c⁄a<1)→fct(c⁄a>1)+fct(c⁄a<1) and (d) fct(c⁄a<1)→fct(c⁄a>1)+fct(c⁄a<1). The fco phase is a metastable one and is not expected from a conventional equilibrium phase diagram of the In-Pb alloy system. In a composition range where the fco phase appears, the coexistence of fco and fct(c⁄a<1) phases is always found. These phase changes are well explained in terms of thermodynamics, by assuming a free energy-composition curve for each existing phase. When the fco phase precipitates in the fct phase, i.e. in the case of (c), a banded surface relief structure due to {110}fct,c⁄a<1 twins appears. This precipitation is a kind of the Bainite reaction and takes place by a mechanism involving macroscopic cooperative movements of atoms, being quite similar to a martensitic transformation in indium-rich alloys. The features of structural changes during aging are also discussed on the basis of the Landau theory concerning structural phase transformations.