NiAl powder alloys: I. Production of NiAl powders

The influence of five methods of production of Ni₅₀Al₅₀ powder alloys on the processes occurring during reactive alloy formation of nickel monoaluminide during heating is considered. It is shown that, when powder mixtures obtained by agitation in ball mills and cladded composite powders with a low level of internal stresses are used, it is possible to produce a material with a nearly equilibrium phase composition in the course of reactive sintering due to an exothermic effect with the participation of a liquid phase (aluminum melt) in the reaction. The sintered material is porous and has an island structure. Mechanical alloying in a high-energy ball mill (attritor) results in the formation of layered Ni/Al granules with a developed interface and a high level of internal stresses and defects, which makes it possible to decrease the temperatures of initiation of reactive interaction by ∼300°C. This interaction develops in the solid phase according to a slow diffusive mechanism leading to the formation of intermediate nickel aluminides and hindering the achievement of equilibrium phase composition. The microingot granules (∼80 wt % particles 100–400 μm in size) produced by melt spraying by gases (N, Ar) has the composition of the melt, but grain boundaries are depleted of aluminum in comparison with the volume. The NiAl powders (∼90 wt % particles <40 μm in size) produced by combined hydride-calcium reduction are characterized by a highly homogeneous nickel and aluminum distribution, and their composition is close to equilibrium. These two types of powders are selected as the initial material for investigating the compacting and production of NiAl-based alloys.     

Structure and mechanical properties of Al-Fe system-base microcrystalline alloys produced by powder metallurgy methods

Translated from Poroshkovaya Metallurgiya, No. 4(340), pp. 93–98, April, 1991.     

A comparative investigation of the properties of ferritic stainless steel powders produced by various methods

Translated from Poroshkovaya Metallurgiya, No. 11(311), pp. 25–27, November, 1988.     

Reduction of porosity in oxide dispersion-strengthened alloys produced by powder metallurgy

In this work, the porosity in a iron-based oxide dispersion-strengthened (ODS) alloy, namely, PM2000, has been studied together with its recrystallization behavior. It has been found that pores are only found in coarse secondary recrystallized grains formed at the early stage of recrystallization. It is suggested that a lack of fast diffusion paths, particularly grain boundaries, will prevent gas trapped in the materials during mechanical alloying (MA) from diffusing away, and porosity is formed as a result. Thus, it is proposed that an extended anneal below the secondary recrystallization temperature will help to reduce the subsequent evolution of porosity in these materials, and the method has been demonstrated to work successfully in PM2000.     

Study of the sintering of nickel powders produced by different methods

Conclusions It was established in this investigation that, in the temperature range 500–700°C, carbonyl nickel powder is subject to more intensive sintering than electrolytic nickel powder. The theory of diffusional viscous flow is unsuitable for this case, and needs revision.It is shown that crystal lattice distortions, as determined by x-ray diffraction methods, do not cause an increase in the rate of sintering. Hypotheses are put forward for the possible mechanism by which diffusional mobility is increased in the case of carbonyl nickel.     

Cu-Zr and Cu-Zr-Cr alloys produced from rapidly quenched powders

The production of alloys by means of ingot technology leads to micro-and macrosegregation, separation of phases and impurities, often into large, brittle particles, and coarse grain size. Alloy development is frequently restricted because of the coarseness of the structure and the resultant imposed limitations on hot and cold plasticity. One ready means of avoiding these problems is to produce the alloy in powder or pellet form; this permits attainment of high cooling rates in the liquid and solid, minimizes segregation, alters phase separation and distribution advantageously, and results in significantly finer structures. The powders utilized to produce wrought shapes may be much coarser than press-and-sinter powders, leading to important processing economies without sacrifice of structure refinement. The alloy systems Cu-Zr and Cu-Zr-Cr were selected for the present study; these are high conductivity, high strength alloys of commercial interest. Cooling rates for these specific powders using nitrogen atomization varied from about 10³ to 10⁴°CJsec. Powders were cleaned, canned, and hot extruded to produce bar stock; mechanical testing was performed on both as-extruded and thermomechanically-worked samples, with highly beneficial improvements in strength, ductility, and high-temperature structural stability. V. K. SARIN, formerly Research Assistant, Department of Metallurgy and Materials Science, Massachusetts Institute of Technology, Cambridge, Mass.     

Properties of an aluminum-lead material produced by powder metallurgy methods processes

Conclusions The presence of lead and its oxides on the surface of the granules of the Al-Pb pseudoalloy leads to the formation of interphase boundaries in the form material and, consequently, impairs its mechanical and tribotechnical properties. The material with high antifriction properties was produced by extruding the mixture of the powders of aluminum and lead.Translated from Poroshkovaya Metallurgiya, No. 7(283), pp. 71–74, July, 1986.