J.D. Embury

Study of the mechanical properties of Mg-7.7at.% Al by in-situ neutron diffraction

Abstract Internal stresses in tension and compression in an extruded and artificially aged Mg-7.7at.% Al alloy have been determined by in-situ neutron diffraction. Measurements were made on grains having the c axis normal to, parallel to and at 62° from the stress axis, and on the reinforcing phase. The results are consistent with basal slip and {1012} twinning. The second-phase particles of Mg17Al12 bear much higher stresses than the magnesium matrix does. A simple mean stress model proposed by Brown and Clarke is shown to describe adequately the strengthening due to the particles. Twinning manifests itself clearly through variations in the integrated intensity of diffraction peaks during loading. Most of the observed variations in scattered peak intensity can be explained by referring to the lattice reorientation produced by {1012} twinning. The calculated stress tensors corresponding to these intensity variations have been used to show that a critical resolved shear stress criterion is applicable for {...

Structure and properties of AlSiC foam

Abstract The mechanical response of composite foam AlSiC has been examined in compression and indentation. The foam has a closed cell structure and is made of aluminium matrix with SiC particles dispersed in it. The cell walls have a complex microstructure consisting of non-uniform distribution of particles, voids and cavities as well as micro-segregation and precipitates resulting from dendritic solidification. Consequently, the mechanical response is complex. In compression, deformation localizes in a band which extends outward with increasing strain. A similar response is observed in indentation, where localization takes place near the indenter and deformation proceeds radially outward. The mechanism of deformation in individual cell walls is identified to be a combination of processes, such as debonding at the particle/matrix interface, particle pull-out and microvoid coalescence in the ductile matrix. The growth of cracks in the cell membranes is associated with a wide damage zone, resulting in high specific energy absorption capacity.

The influence of hydrostatic pressure on the ductility of AlSiC composites

Abstract Tensile tests with superimposed hydrostatic pressures were performed on two types of metal matrix composite: 2014 Al with 20% SiC particles and 2124 Al with 14% SiC whiskers. In the materials with SiC particulate, the ductility increases rapidly with pressure and the mode of damage initiation is by particle fracture. Materials containing SiC whiskers exhibit a different fracture mode involving whisker matrix decohesion, and strain localization which results in shear fracture.

Tensile and bending properties of AA5754 aluminum alloys

The ductility and bendability of AA5754 automotive sheets have been investigated as a function of iron content. The influence of pre-strain, introduced by cold rolling, on the bendability has also been examined. The low iron containing alloy (0.08 wt.%) showed development of surface instability in the form of small undulations on the tensile surface. In contrast, the high iron containing alloy (0.30 wt.%) showed cracks on the tensile surface. The development of damage during bending has been studied by carrying out in-situ bend tests in a scanning electron microscope. High iron containing alloy exhibits damage at the iron rich particles which leads to drastic reduction in the bendability.

A note on the deformation of fine grained magnesium alloys

This paper reports that there is increasing interest in the use of magnesium alloys both from the viewpoint of their strength to weight ratio in monolithic form and as a matrix for metal matrix composites. One strengthening mechanism which has received little attention in Mg is that due to reducing the grain size. Some early work explored the Hall-Petch formalisms for polycrystals of different grain sizes and recent work showed that vary fine grained materials of a commercial alloy AZ91 (composition Mg - 9% Al - 1% Zn - 0.2% Mn) can be produced by rapid quenching and subsequent consolidation. This approach has the advantage of producing bulk material which can be studied by a variety of testing methods although the product is a two phase material containing particles of Mg{sub 17}Al{sub 12}.

An approach to materials processing and selection for high-field magnet design

In this paper, the physical principles and design criteria for materials selection for high-field, pulsed magnets is discussed. Selection charts are developed based on the design constraints of minimizing ohmic heating and resisting Lorentz forces. Using the selection charts, the merits of various materials are compared. The effects of processing techniques on the material properties and, therefore, the performance of the magnet design are illustrated with the use of selection charts. This leads finally to a discussion of development vectors for composite winding materials.

ROLE OF DAMAGE ON THE FLOW AND FRACTURE OF PARTICULATE REINFORCED ALLOYS AND METAL MATRIX COMPOSITES

Abstract A large number of models exist to predict the tensile behaviour of two-phase materials. They fail, however, to predict the behaviour at high strain because they do not properly account for the role which damage accumulation plays during plastic flow. A new model is proposed here which incorporates damage. It uses an incremental self-consistent method developed previously. In the new application, the model treats damaged and undamaged regions as distinct continuous “phases” and allows the relative volume fraction of each phase to vary during deformation. The stress redistribution due to damage during the evolution is carefully analysed. The model considers the influence of particle size via a Weibull analysis and incorporates the results of existing FEM calculations. The model agrees well with existing experimental data and provides a new method to evaluate the role of microstructural parameters.

The shearable–non-shearable transition in Al–Mg–Si–Cu precipitation hardening alloys: implications on the distribution of slip, work hardening and fracture

A systematic study has been conducted to evaluate the nature of the dislocation–precipitate interaction and its relationship to the mechanical properties for a commercial Al–Mg–Si–Cu alloy. A variety of experimental techniques employed including transmission electron microscopy, slip line observations and macroscopic work hardening behaviour. The results from this work indicate that a clear transition in macroscopic behaviour of the alloy can be observed when the precipitates become large enough so that they are not sheared by dislocations. Direct observations using a transmission electron microscope (TEM) indicate that the precursor to the Q phase becomes impenetrable to dislocations when its equivalent diameter is above 2.5–3.0 nm. The transition from shearable to non-shearable precipitates manifests itself in a number of ways including: (i) a change in the local distribution of slip from a banded to a more homogeneous structure and (ii) a characteristic change in macroscopic work hardening behaviour. In addition, observations on intergranular fracture suggest that the distribution of slip and the intrinsic fracture properties of the grain boundary are critical in controlling this process. Finally, an integrated view of the relationship between the basic dislocation–precipitate interaction and the global response of the alloy is rationalized.

Basic aspects of the co-deformation of bcc/fcc materials

Abstract Materials consisting of a hard phase embedded within a softer, ductile matrix represent an industrially important class of materials. Recent work has shown that in many cases such two-phase materials can be co-deformed to produce alloys with strengths far surpassing those that would be predicted from a simple ‘rule of mixtures’ estimate. Alloys comprising a bcc phase embedded within an fcc matrix are representative of materials exhibiting this type of behaviour and may be considered as excellent model systems for the study of the co-deformation process. Here, the important processes associated with co-deformation will be reviewed and new results from the co-deformation of a Cu–Cr and a Ni–W alloy will be described.

Deformation of copper single crystals to large strains at 4.2K I. Mechanical response and electrical resistivity

Abstract The deformation of Cu single crystals at 4.2 K was studied by simultaneous measurements of mechanical and electrical properties. The aim was to extend the study of low-temperature deformation to higher strains than are usually employed; thus most of the crystals were stretched to failure. The deformation can be divided into three distinct regions. In region A (which includes stages I and II of plastic deformation) the crystal deforms by slip. In region B there is twinning but no slip; at the end of region B the specimen is 70% twinned. The crystal enters region C as a fine layer structure consisting of twin and parent lamellae; in this region there is no more twinning and deformation proceeds by slip. Examination of the deformation-induced resistivity and resistivity annealing confirms the conclusion that flow stress in region A is due to dislocation accumulation and indicates that in region C other obstacles, such as twin-parent interfaces, are also important. The annealing data, together with other evidence, suggest that elimination of short vacancy dipoles and loops by pipe diffusion accounts for the recoverable component of the resistivity.