Precipitation in a Ni–Al–Mo Directionally Solidified Eutectic Composite
The most recently developed high temperature materials are known as in-situ composites or directionally solidified eutectics. Since the composites are formed during solidification, they have excellent morphological stabilities up to near their melting temperatures. The Ni-Al-Mo system has received the greatest attention for recent several years(1)~(3) since the alloy is expected to form a structure consisting of ductile α-Mo fibers aligned in aductile γ+γ' matrix†. However variations in microstructures and the resultant strength by thermal treatments have not been investigated in earlier papers(1)~(3). In a separate paper(4) the present authors have reported the detailed microstructural analysis of as-grown composite and the microstructural and strength variations of the composite due to quenching from high annealing temperatures. The present paper describes the microstructural changes occurring during ageing of a Ni-12.7at%Al-21.6at%Mo alloy composite after quenching from 1533K studied by transmission electron microscopy
Short range order in Ni-Mo, Au-Cr, Au-V and Au-Mn alloys☆
Abstract The nature of short range order (sro) has been investigated in the series of alloys Ni4Mo, Ni3Mo, Au3Cr, Au4Cr, Au3Mn and Au4V, which exhibit different long range ordered (lro) structures, by transmission electron microscopy and diffraction. The examination of the sro state, at temperatures above the critical temperatures (Tc) for the order-disorder transformations and also in the samples that were quenched from above Tc, showed diffuse scattering peaks centered near {1 1 2 0} positions in the f.c.c. reciprocal lattice, whose shape changed from one system to another. Evidence for diffuse scattering near superlattice positions corresponding to the D1a structure was obtained in the sro state of Au-Cr, Au-V and Ni-Mo alloys. Additional diffuse scattering was observed near Ni2Mo positons in the Ni-Mo alloys. The diffraction patterns of Au3Mn above Tc showed evidence for diffuse scattering near the superlattice positions corresponding to the Watanabe type two-dimensional long period superstructure. Inclusion of pairwise interaction parameters up to third nearest neighbors in the high temperature statistical mechanical model of Clapp and Moss partially accounts for the shapes of {1 1 2 0} sro spots found in Ni-Mo and Au-Mn alloys, but not in Au-Cr and Au-V alloys. The shapes of the diffuse sro scattering in the latter alloys, however, can be interpreted in terms of a distribution of various types of imperfectly ordered microdomains existing above the critical temperature. For a particular stoichiometry, the most frequently occurring microdomain may or may not correspond to the equilibrium long range ordered (Iro) structure of the alloy.
Study on Anti-Phase Domains in Cu 3 Au by Means of Electron Diffraction and Electron Microscopy
It has been confirmed by electron diffraction that anti-phase domains in Cu 3 Au giving rise to diffuse superlattice reflections grow up during isothermal annealing below the transition temperature of order-disorder, evaporated films of nearly stoichiometric compositions being used. This fact means that anti-phase domains in Cu 3 Au are not in an equilibrium state unlike the so-called periodic anti-phase structure. Electron-microscopic images of domain boundaries of alloy films with 26 atomic per cent gold have been observed. These images also confirm the isothermal growth of domains. The irregular configuration of domain boundaries as observed by Fisher and Marcinkowski in electropolished specimens of Cu 3 Au has been confirmed also in the evaporated films. It is concluded, however, that some correction should be made on the domain distribution of Cu 3 Au suggested by these authors, on the basis of dark field images of electron micrographs formed by different kinds of superlattice reflection in the prese...