The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—I. Young's modulus

AlCr and AlCrFe alloys were quenched from the vapour phase onto substrates held at 230–370°C. Up to 5.6at.%Cr (10.7 wt%) was retained in solid solution. The effect of chromium content (C_Cr) on Young's modulus (E) was measured; E increased at a rate of 5.7 GPa/at.%Cr. The addition of iron up to 0.5at.% (1.0 wt%) had no detectable effect on Young's modulus of the AlCr binary alloy. Conversion of chromium in solid solution to intermetallic particles did not affect the modulus. The specific modulus (E/ρ where ρ = density) increased with chromium content and E/ρC_Cr, was 1.35 MNm/kg at.%Cr. Vapour quenched AlCr alloys have greater potential for improving the modulus of Al-alloys compared with AlLi or Al powder alloys containing iron, manganese or chromium.     

The influence of Li on the nucleation of defects of quenched AlLi alloys

The nucleation and kinetics of defects formed by quenching in two AlLi alloys having Li content of 1.7 and 3.74 at.%Li have been studied by positron annihilation spectroscopy. It has been found that the defect formation is sensitive to the aging time at the temperature from which the samples are quenched. This fact has been related to the Li loss experienced by the alloys aged at high temperature. The quenched-in defects have been identified as vacancy-Li clusters and dislocation loops. The latter are formed by the collapse of the Li-rich vacancy complexes and are very sensitive to the Li content; as a consequence, the loops are decorated by Li-rich zones and are revealed as very effective positron traps in comparison to the vacancy-Li complexes, giving rise to an enhanced trapping.     

Microstructure and mechanical properties of RS NiCrAl melt-spun alloys

The microstructure and mechanical properties of three melt-spun NiCrAl alloy ribbons have been studied in the as-cast condition as well as after thermal treatments. The microstructure of the alloys is dendritic-microcellular in as-cast condition and phases present for 10 at.% Al and 30 at.% Al alloys are as is predicted by the equilibrium phase diagram. In the 20 at.% Al alloy, γ' has frozen in metastable form and partial ordering takes place during cooling in the solid state. After thermal treatments the ribbons generally maintain a refined microstructure; α phase precipitates are always found in β and γ' phases in 20 and 30 at.% Al alloys. The hardness of the alloys increases with aluminum content. The tensile strength at room temperature is related to the phases present in the material for each state of treatment. The alloys are brittle, a higher ductility always being obtained in the as-cast condition.     

Excess aluminium effect on the composition and microstructure of Mo–Al alloys from aluminothermic reduction of MoO3

The effect of aluminium excess on the composition and microstructure of Mo–Al alloys from aluminothermic reduction of metallurgical-grade MoO₃ was investigated. Aluminium excess ranged from 0 to 30% (in wt-%) with respect to the stoichiometric amount. The results showed that molybdenum yield increases for higher excess aluminium. The microstructure and composition of the alloys were investigated using scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and X-ray diffraction (XRD). The following phases were observed in the microstructures: Mo_ss, Mo₃Al, and Mo₃Al₈, besides particles of Al₂O₃. The results also showed that metallurgical-grade MoO₃ leads to a product containing significant amounts of silicon.     

The G.P. zones in AlCu alloys—II

Measurements on an absolute scale of the diffuse X-ray scattering by an aged single crystal of Al-1.94 at.% Cu have been made at a synchrotron source. Aging times were chosen to correspond to a mixture of G.P.1 and G.P.2 states, and to G.P.2 itself. The results indicate that the transition is in reality a coarsening reaction. There is not a unique structure for G.P.2 different than that for G.P.1. The G.P.1 state is a mixture of almost pure Cu single layer and multilayer platelets with their broad faces parallel to {100} planes of the Al-rich matrix. With increased aging time, these regions thicken and the number of single layer regions decreases. The so-called extra reflections in the G.P.2 state are actually thickness fringes. The strains around a multilayer zone are similar for all aging treatments, oscillate with distance from a zone, and are ~10% near the zone boundary, where the A1 planes collapse toward the zone.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—II. Haz microstructure and strength evolution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part II the heat and material flow models presented in the first paper (Part I) are utilized for prediction of the HAZ subgrain structure and strength evolution following welding and subsequent natural ageing. The modelling is done on the basis of well established principles from thermodynamics, kinetic theory and simple dislocation mechanics. The models are validated by comparison with experimental data, and are illustrated by means of novel mechanism maps. These show the competition between the different process variables that contribute to microstructural changes and strength losses during friction welding of AlMgSi alloys and AlSiC metal matrix composites.     

Coherent precipitation effect on thermo-power of AlCu alloys

In situ thermoelectric measurements of AlCu alloys are described for several types of heat treatments. The thermoelectric power shows deviations from the solid solution matrix during the sequence of precipitations occurring in these alloys (i.e. GP zones, θ′',θ′ and θ). The sign and the amplitude of these deviations can be modeled on the basis of the HEW theory for the resistivity maximum. Extension to the diffusion thermopower displays a size and morphology (isotropic, platelike) effect in the electron Bragg scattering by the coherent precipitates, corresponding rather well with the experimental data.     

Rapidly solidified AlCu alloys—I. experimental determination of the microstructure selection map

Laser rapid solidification experiments have been performed on AlCu alloys of hypereutectic composition 36, 40 and 44 wt% Cu. By taking thin foils from the surface of the laser traces it has been possible to study the resulting growth morphologies using TEM; the microstructural orientation allowing the morphologies to be correlated with their local growth velocities. Microstructural orientation allowing velocity range 0.01–2.0 ms⁻¹ have been studied, and eutectic (both with and without oscillatory) instabilities), dendritic, cellular, banded and planar front growth have all been observed. By combining these results with earlier observations made on alloys with lower Cu concentrations, it has been possible to produce a microstructure selection map for AlCu alloys. The map correlates microstructure to growth velocity and composition in the ranges 0.01–2.0 ms⁻¹ and 0–44 wt% Cu, i.e. the major part of the binary AlAl₂Cu eutectic (0–54 wt% Cu). As well as providing an interesting overview of solidification structures, several features of this map can be used to gain information on the AlCu phase diagram.     

Microstructural influence of Mn additions on thermoelastic and pseudoelastic properties of CuAlNi alloys

The thermoelastic and mechanical properties of CuAlNiBMn shape memory alloys have been studied as a function of manganese concentration and of heat treatment. Below a limiting value of manganese content, the loss of thermoelastic and pseudoelastic properties has been observed, in particular in the quenched specimens. The partial transformation and its degradation during thermal cycling observed in the low manganese content alloy has been attributed to the lower degree of B2 order achieved during the quench, leading to slower kinetics of DO₃ ordering. The accomodation of strains between martensite variants and between the martensite/austenite phases appear to need dislocation accumulation at their interfaces. The presence of dislocations observed during the reverse transformation seem responsible for the degradation of the transformation and the loss of pseudoelastic properties of this alloy.     

The martensitic phases and their stability in CuZn and CuZnAl alloys—II. The transformation between the close packed martensitic phases

The stability between the close packed martensitic structures 6R, 18R and 2H in CuZnAl single crystals is analysed. Experimental results for high electron concentrations (e/a > 1.50) and data from the literature for lower e/a are used, covering a wide range of compositions. Numerical values for the stacking fault energies and the differences of enthalpy are reported. The thermodynamic stability of the orthorhombic 18R structure with respect to 6R and 2H in a wide composition range is established.     

The effect of hydrogen on the crystallization behavior of amorphous PdSi alloys

Differential scanning calorimetry, X-ray diffractometry and Optical metallography were used to study the effect of hydrogen atmosphere and surface treatment on the crystallization process in PdSi metallic glasses. In Pd₈₀Si₂₀ metallic glasses, the surface treatment by mechanical polishing with 1200 SiC induces surface crystallization in which preferred nucleation at the surface occurs, while homogenous crystallization occurs in as-quenched samples without surface grinding. In amorphous Pd₈₅Si₁₅, the crystallization occurs simultaneously from the sample surface and in the bulk. Surface treatment by mechanical polishing does not change the crystallization morphology. These results were not affected by the atmosphere of argon or hydrogen. It is shown that the surface crystallization, which is dominant in the initial stages of the transformation, is revealed by the low temperature shoulder of the main peak of crystallization in a DSC trace. The process of crystallization of amorphous Pd₈₀Si₂₀ and Pd₈₅Si₁₅ is found to be impeded by absorbed hydrogen, this having been measured by the increase of both crystallization temperature and activation energy for crystallization process in a hydrogen environment. Such crystallization behavior can be explained by assuming that hydrogen reduces the free volume available for rearrangements of host atoms of the metallic glasses.     

The effect of conditions of preparation on the strength of WC — Co hard alloys and the relationship between the strength of these alloys and their structure and composition

Conclusions This paper presents a review of investigations which elucidated the effect of the technological conditions of obtaining the initial tungsten powder and the grinding and sintering conditions on the strength of a hard alloy during bending. An analysis of the results of these investigations shows that the differences in the strength of alloys differing in carbide grain size is primarily due to various manufacturing conditions and not to differences in grain size.On applying various methods of regulating the size of the carbide grain, sharply different changes in alloy strength are observed. The influence of the size of the grain was not noted, since it was suppressed by the greater effect of the technological factors.On excluding the effect of changing the sintering temperature (and other variable technological factors) the dependence of the alloy strength on the cobalt content changes its form: the maximum strength between 15 and 25% cobalt, noted in many researches, is not observed in this case. The strength increases continually up to 50% cobalt.The dependence on grain size with various methods of regulating carbide dispersity, and the dependence of the strength on the cobalt content on excluding the effect of the technological factors differ substantially from the regularities described by Gurland [1].The theoretical theses of Gurland are ungrounded, inasmuch as the strength relations are at variance with them, when the effect of the technological factors is excluded.The relations actually existing may be explained on the basis of the skeleton structure of the carbide and cobalt phases.     

Microstructures and mechanical properties of rapidly solidified CuCr alloys

Rapidly-quenched CuCr alloys have been prepared by melt-spinning and the microstructures and mechanical properties examined as a function of alloy content and subsequent annealing treatment. Mechanical properties have been successfully measured by tensile testing on the ribbon samples and it is shown that this method is more suitable than hardness testing for a proper evaluation of the materials. An alloy containing 2% Cr has been prepared in the totally solid-solution state, whilst a 5% Cr alloy contains large chromium particles distributed uniformly throughout the ribbon and fine-grain-sized CuCr solution. These large chromium particles are deduced to arise by a uniform, primary-solidification mode prior to, and independent of, the nucleation and growth of the copper from the lower ribbon surface. The increased solid solubility obtained allows extensive precipitation and strengthening following ageing: the primary chromium particles play an important role both in strengthening and in restricting grain coarsening.