Farghalli A. Mohamed

Particulate reinforced metal matrix composites — a review

The physical and mechanical properties that can be obtained with metal matrix composites (MMCs) have made them attractive candidate materials for aerospace, automotive and numerous other applications. More recently, particulate reinforced MMCs have attracted considerable attention as a result of their relatively low costs and characteristic isotropic properties. Reinforcement materials include carbides, nitrides and oxides. In an effort to optimize the structure and properties of particulate reinforced MMCs various processing techniques have evolved over the last 20 years. The processing methods utilized to manufacture particulate reinforced MMCs can be grouped depending on the temperature of the metallic matrix during processing. Accordingly, the processes can be classified into three categories: (a) liquid phase processes, (b) solid state processes, and (c) two phase (solid-liquid) processes. Regarding physical properties, strengthening in metal matrix composites has been related to dislocations of a very high density in the matrix originating from differential thermal contraction, geometrical constraints and plastic deformation during processing.

The transition from dislocation climb to viscous glide in creep of solid solution alloys

Abstract There are two distinct and separate classes of creep behavior in metallic solid solution alloys. The mechanism of creep in Class I alloys appears to be some form of dislocation climb process, whereas the mechanism in Class II alloys appears to be a viscous glide process. By making assumptions concerning the nature of the climb and glide processes, and using existing experimental results for an Al-3% Mg alloy, it is shown that, to a, first approximation, the criterion for deformation by viscous glide is given by Bσ 2 k 2 (1 − v γ Gb 3 > T 2 e 2 cb 6 where B ∼ 8 × 1012, σ is the applied stress, k is Boltzmanns constant, v is Poissons ratio, γ is the stacking fault energy, G is the shear modulus, b is the Burgers vector, T is the absolute temperature, e is the solute-solvent size difference, and c is the concentration of solute atoms. The creep behavior of twenty-eight different solid solution alloys is analyzed, and it is shown that all alloys except one (Au-10% Ni) give results which are consistent with this criterion for viscous glide.

A dislocation model for the minimum grain size obtainable by milling

Primary among the processing techniques that are now available for synthesizing bulk nanocrystalline materials is ball milling, which produces nanostructures by the structural decomposition of large-grained structures as the result of severe cyclic deformation. It is well-documented that during milling, the grain size decreases with milling time, reaching a minimum grain size, dmin, which is a characteristic of each metal. In this paper, a dislocation model that predicts the value of dmin as a function of material parameters, such as hardness, melting temperature, and stacking fault energy, has been developed. The model is based on the concept that dmin is governed by a balance between the hardening rate introduced by dislocation generation and the recovery rate arising from dislocation annihilation and recombination. It is demonstrated that the model provides possible explanations for several recent observations regarding the characteristics of dmin.

Deformation mechanism maps based on grain size

A new form of deformation mechanism map is introduced based on grain size. Maps are developed for pure aluminum at two different homologous temperatures, and the relative contributions of the various deformation processes are estimated as a function of stress at different grain sizes. A simple method of constructing these maps is presented, requiring only a knowledge of the relevant constitutive equations for the various mechanisms and a minimum of calculation.

Interpretation of superplastic flow in terms of a threshold stress

In several recent experiments on the Zn-22% Al eutectoid and the Pb-62% Sn eutectic, a sigmoidal relationship between stress and strain rate is noted and the mechanical behaviour has been divided into three regions: low-stress region (region I), intermediatestress region (the superplastic region or region II), and high-stress region (region III). In region II, the stress exponent,n, is ≃ 2 and the apparent activation energy,Q, is close to grain-boundary diffusion,Qgb, but in both regions I and III the stress exponent and the activation energy increase (n > 2 andQ >Qgb). Analysis of the experimental data of the two superplastic alloys suggests that the transition in behaviour between region II and region I may not necessarily reflect a change in deformation process but can arise from the presence of a threshold stress which decreases strongly with increasing temperature. Based on consideration of various possible threshold stress processes during superplastic flow, it seems most likely that a threshold stress which depends strongly on temperature may result from impurity atom segregation at boundaries and their interaction with boundary dislocations.

Creep and substructure formation in an Al-5% Mg solid solution alloy

Abstract The creep behavior of Al-5% Mg divides into three distinct regions at a testing temperature of 827 K. In region III at τ ≳ 0.5 MPa, the stress exponent is 3.1, the activation energy is 149 ± 10 rmkJmole−1, and substructural observations reveal an essentially random distribution of dislocations. In region II at 0.2 M Pa ≲ τ ≲ 0.5 MPa, the stress exponent is 4.4, the activation energy is 139 ± 5 rmkJmole−1, and substructural observations reveal the formation of subgrain boundaries. In region I at τ ≲ 0.2 MPa, the stress exponent is 1.0 and the activation energy is 162 ± 30 rmkJmole−1. It is demonstrated that these three regions are consistent with viscous glide (class A), dislocation climb (class M), and Harper-Dorn creep, respectively.

Creep behavior of discontinuous SiCAl composites

Abstract A review of creep data of discontinuous SiCAl composites (whisker and particulate) shows that the creep behavior of these composites exhibits two main characteristics: (a) the stress dependence of the steady state (or minimum) creep rate, as described by the value of the stress exponent, is high and variable and (b) the temperature dependence of the steady state (or minimum) creep rate, which is measured by the creep activation energy, is much larger than that for self-diffusion in aluminum. These two characteristics are examined in the light of theoretical treatments describing the origin of high temperature strengthening in discontinuous metal matrix composites and dislocation models proposed for dispersion-strengthened alloys.

Creep at low stress levels in the superplastic Zn-22% Al eutectoid

Abstract A sigmoidal relationship between strain rate and stress was observed in a superplastic Zn-22% Al eutectoid alloy with grain sizes in the range of 2.1–7.5 μm. The relationship was independent both of the testing technique employed (whether constant stress or constant strain rate) and of the mode of deformation selected (whether tensile or shear). In the superplastic region (strain rates of ~10 −5 -10 −2 sec −1 ), the stress exponent was ~2.25, the exponent of the inverse grain size was ~2.3, and the activation energy was close to that for grain boundary diffusion. These results are in good agreement with the predictions of a model based on grain boundary sliding accommodated by the climb of dislocations into boundaries. At very low strain rates ≲10 −5 sec −1 ), the stress exponent was ~4.1, the exponent of the inverse grain size was ~2.4, and the activation energy was close to that for volume self-diffusion. These results are not consistent with any of the existing deformation mechanisms, but suggest that the sigmoidal relationship may arise through the sequential operation of two different processes

Factors influencing ductility in the superplastic Zn-22 Pct Al eutectoid

The maximum attainable ductility in the superplastic Zn-22 pct Al eutectoid depends critically on the imposed strain rate, the testing temperature, and the initial grain size. High ductilities are observed at intermediate strain rates, and there is a decrease at both higher and lower rates of strain. It is shown that i) the maximum ductility occurs at higher strain rates as the temperature is increased and/or the initial grain size is decreased, and ii) the maximum attainable ductility increases with increasing temperature and/or decreasing initial grain size. For specimens tested at different temperatures, similar macroscopic fracture characteristics are observed in specimens exhibiting a similar maximum flow stress. The experimental trends are qualitatively explained by relating maximum ductility to the maximum strain rate sensitivity and examining the influence of cavitation on the time to rupture.

Creep behaviour in the superplastic Pb–62% Sn eutectic

Abstract The creep behaviour of the superplastic Pb-62% Sn eutectic was investigated for grain sizes from 5·8 to 14·5 μm and at temperatures in the range from 336 to 422 K. The results showed a sigmoidal relationship between strain rate and stress. At intermediate strain rates (∼ 10−5–10−2 sec−1), the stress exponent was ∼1·65, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for grain boundary diffusion. At very low strain rates (≲10−5 sec−1), the stress exponent was ∼3·0, the exponent of the inverse grain size was ∼2·3, and the activation energy was similar to the value anticipated for lattice self-diffusion. The results are not entirely consistent with either of the two major theories of superplasticity, but suggest instead the sequential operation of two different deformation processes.