We report the first observation of reversible equilibrium crystalline–metastable crystalline phase transformation in mechanically alloyed Ni–Ge powder mixtures. The formation of the equilibrium NiGe and metastable NiGe2 phases was investigated using x-ray diffraction and scanning electron microscopy methods. It was clearly shown that milling of the blended elemental powders first resulted in the formation of the equilibrium NiGe phase and continued milling led to the formation of the metastable NiGe2 phase. However, on milling for a longer time, the metastable phase transformed back to the equilibrium NiGe phase. The formation mechanisms of the stable and metastable phases and the reversibility of the phase transformations have been explained on the basis of the thermodynamic stability of the different phases and the contribution of defect concentration and surface energy effects to the free energy of the milled powder.

The crystallization behavior of melt-spun ribbons and bulk samples of the Cu36Zr48Al8Ag8 glassy alloy on heating is presented here. The crystallization kinetics and structural changes in the Cu36Zr48Al8Ag8 glassy alloy were studied using x-ray diffraction, transmission electron microscopy, differential scanning, and isothermal calorimetry methods. A clear comparison is made of the differences in the crystallization kinetics of the melt-spun ribbons and the copper-mold-cast bulk rod samples. It was suggested that the kinetics of crystallization in the rod sample, at any given temperature, are somewhat different than in the ribbon samples, probably because of size and free volume effects. Differences in the crystallization behavior of this alloy with other Cu-Zr-Al-Ag alloys have also been discussed.

Formation of a homogeneous nanocrystalline CuIn0.7Ga0.3Se2 alloy was achieved by mechanical alloying of blended elemental Cu, In, Ga, and Se powders in a planetary ball mill. X-ray diffraction and transmission electron microscopy and diffraction techniques were employed to follow the structural evolution during milling. It was observed that, depending upon the milling conditions, either a metastable cubic or a stable tetragonal phase was produced. The grain size of the mechanically alloyed powder was about 10 nm. The mechanically alloyed powder was consolidated to full density by hot isostatic pressing the powder at 750 °C and 100 MPa for 2 h. Irrespective of the nature of the phase in the starting powder, the hot isostatically pressed compact contained the well-recrystallized tetragonal CuIn0.7Ga0.3Se2 phase with a grain size of about 50 nm.

Blended elemental W–7 wt.%Ni–3 wt. %Fe powders were mechanically alloyed in a planetary ball mill and a heavy duty attritor. The structural and morphological characteristics of the as-milled powders were characterized by x-ray diffraction and electron microscopy techniques. A ternary solid solution of Fe and Ni in W formed in both the cases; the amount of Ni and Fe dissolved is higher in the attritor-milled powder than in the ball-milled powder. Morphologically, the W-rich particles in the attritor-milled powders were spheroidal, smaller in size, and showed a narrower size distribution than in the ball-milled condition. These differences led to increased density in the attritor-milled powder compact and significant differences in chemical composition of the matrix region and W-rich grains. Transmission electron microscopy investigations of the matrix region in the compacts revealed the presence of Fe2W precipitates uniformly distributed in an Ni(Fe, W) matrix.

Nanocrystalline materials have a grain size of only a few nanometers and are expected to possess very high hardness and strength values. Even though the hardness/strength is expected to increase with a decrease in grain size, recent observations have indicated that the hardness increases in some cases and decreases in other cases. A careful analysis of the available results on the basis of existing models suggests that there is a critical grain size below which the triple junction volume fraction increases considerably over the grain boundary volume fraction and this is suggested to be responsible for the observed softening at small grain sizes.

The solid solubility of magnesium in titanium under equilibrium conditions is reported to be extremely small. Mechanical alloying of a mixture of titanium and magnesium powders resulted in the formation of nanocrystalline (10–15 nm in size) grains of Ti–Mg solid solution. This solid solution has a metastable fcc structure with a = 0.426 nm and contains about 3 wt.% (6 at.%) magnesium in it. It is suggested that the fcc structure has formed as a result of the heavy mechanical deformation of the hep structure introduced during milling. High temperature annealing of the metastable solid solution led to its decomposition forming the equilibrium phases, viz., elemental titanium and magnesium.

Results of in situ hot-stage electron microscopic investigations on the transformation behavior of icosahedral Mg32 (Al,Zn)49 and decagonal Al–Co quasicrystalline phases are presented. It is shown that in both the cases the transformation begins with the formation of fine holes. This has been interpreted on the basis of the lower density of the quasicrystalline phase. Differences in the transformation behavior between the icosahedral and decagonal phases are highlighted.

An Al-5 wt. % Co alloy has been rapidly solidified from the liquid state, resulting in the formation of a slightly supersaturated aluminum solid solution and the monoclinic Al9Co2 phase. High-resolution electron microscopy and electron diffraction have been extensively employed to characterize the as-solidified as well as the annealed foils. The high-resolution micrographs of the as-solidified solid solution showed the presence of clustering and D 1a - type ordering on an extremely small microscopic scale. Annealing at temperatures above 623 K resulted in the precipitation of Al9Co2 in the form of platelets arranged in a Widmanstätten pattern. Orientation relationships between the Al9Co2 platelets and the matrix have been established. Guinier-Preston zonelike platelets on close-packed {111} planes, however, have also been observed upon electron irradiation in the annealed alloys, presumably due to the enhanced diffusivity of cobalt atoms.