Precipitation in some Cu-Ni-Cr alloys

Abstract

An investigation has been made of the precipitation reaction in a series of Cu-Ni-Cr alloys which lie within the ternary miscibility gap. Symmetrical alloys, with a composition near the centre of the miscibility gap, decomposed by a spinodal mechanism to form a highly interconnected microstructure aligned along <100>. In asymmetrical alloys near the edge of the miscibility gap, phase separation occurred by nucleation and growth and discrete particles were formed. In the symmetrical alloys the initial stages of aging were associated with a rapid increase in hardness while the wavelength of the composition modulations remained constant. An interconnected microstructure rapidly evolved, and as coarsening proceeded the ‘effective wavelength’ increased but the hardness remained constant. Temperature-cycling experiments were carried out in which the amplitude of the composition modulations in the interconnected structure was varied while the effective wavelength was kept constant. These experiments proved that the yield strength of the interconnected structure was a sensitive function of the difference in composition of the two phases but relatively independent of the modulation wavelength. The interconnected microstructures of the symmetrical alloys gradually coarsened and eventually lost coherency. A hexagonal network ofinterface dislocations with Burgers vectors of the form a/2 <110> developed, and, simultaneously, the 〈100〉 planar interfaces rotated towards {110}. Alloys near the edge of the miscibility gaps decomposed by a nucleation and growth mechanism to form discrete particles of the minor phase.