Grain growth in samples of aluminum containing alumina particles

A study of the two-dimensional and three-dimensional grain size distributions before and after grain growth treatments has been made in samples having a range of oxide contents. In order to collect statistically useful amounts of data, an automatic image analyzer was used and the resulting data were subjected to a series of statistical tests which evaluate the difference between related distributions. Normal grain growth was observed in samples of the lowest oxide content; as the oxide level was increased, a significant narrowing of the grain size distribution was observed. In the latter samples the maximum or limiting grain size measured after prolonged heat treatment is approximately one half that predicted by Zener and is in broad agreement with the values calculated from the models due to Hillert and Gladman. Anomalous grain growth has been observed in samples where normal grain growth is restricted and where the initial grain size distribution is very wide.     

Effect of deformation heating and strain rate sensitivity on flow localization during the torsion testing of 6061 aluminum

The effect of deformation heating and strain rate sensitivity on flow localization during the torsion testing of 6061 aluminum was investigated both theoretically and experimentally. From the theoretical viewpoint, a simple analysis of the torsion test was carried out based on the torque equilibrium and one-dimensional heat transfer equations. The problem formulation was discretized to enable numerical solution of the governing equations and prediction of the effect of material properties on the development of deformation and temperature gradients. The analysis was validated by conducting high strain rate experiments on the 6061 alloy at a variety of temperatures. At low temperatures, at which the flow stress and temperature changes due to deformation heating are large and the strain rate sensitivity is low, marked flow localization occurred. The analysis modeled this behavior correctly, indicating that strain concentrations can occur solely as a result of the temperature gradients set up by heat transfer during testing, i.e. in the absence of geometric or deformation defects. At the higher temperatures, at which temperature changes due to deformation heating are small and the rate sensitivity is large, the flow remained nominally uniform until fracture intervened. The numerical simulations of these tests also showed good agreement with the observations.     

Effect of strain rate and temperature on hot workability and flow behaviour of duplex stainless steel

The flow behaviour and processing map of a duplex stainless steel were studied via hot compressive tests in a temperature range of 1223–1473?K and a strain rate range of 0.01–30?s^??1. The effect of strain rate and temperature on the hot workability, strain partitioning and dominant flow behaviour of the alloy was systematically investigated. It is found that the softening mechanism of each constituent phase differs from each other. The ferrite is softened by dynamic recovery and continuous dynamic recrystallisation (CDRX), while the austenite is softened only by the limited discontinuous dynamic recrystallisation (DDRX). At lower strain rates (0.01 and 0.1?s^??1), the strain is mainly accommodated by ferrite due to its excellent softening capability, which causes the apparent activation energy Q_p to decline continuously with the increase in true strain. In this case, plastic deformation of the austenite rarely occurs, and at this time, DDRX of austenite is not observed. When the strain rate increases, CDRX of ferrite is weakened at a relative low temperature, which prompts the strain transfer into austenite and induces the strain hardening due to its restricted softening. Accordingly, interactions between the strain hardening in austenite and weakened softening of ferrite leads to one or more platforms of Q formed at the medium stage of deformation (1–30?s^??1). The processing map shows that two flow instability regions appear at high strain rate due to the lack of sufficient response time for dynamic restoration at the early deformation stage. As the strain increases, dynamic softening mechanism is activated at a higher temperature, resulting in a gradually narrowed flow instability region. Differently, a decrease in temperature suppresses dynamic softening of the alloy with a high strain rate, which deteriorates the hot workability of the alloy and induces microcrack formation after straining of 0.8.     

The compressive failure of alumina containing controlled distributions of flaws

Alumina samples containing a controlled number and size of crack-line flaws were tested in simple compression. Crack extension and linkage were observed during loading by in situ scanning electron microscopy. Young's modulus, the stress for the initiation of crack growth and that for final failure were recorded. The observations, the most complete on a real ceramic system (as opposed to model materials like PMMA) confirm mechanisms which have been proposed for compressive fracture of brittle solids.     

Modeling particle fracture during the extrusion of aluminum/alumina composites

Particle fracture during the extrusion of a 6061/Al₂O₃/20p composite has been modeled using a modified comminution formulation. It has been assumed that the particles contain a Poisson distribution of flaws, and that the distribution is specific to the alumina and the method of production; the particle distribution in the extrudate was characterized by the Rosin-Rammler (RR) distribution. The model relates macroscopic deformation variables to fracture and, starting with the distribution in the as-cast material in each case, is able to predict with reasonable accuracy changes in size distribution for three extrusion ratios. Some discrepancy between prediction and experiment occurs at small sizes. This is believed to result mainly from inaccuracies in the measured data and effects of the continuous size distribution in representing a set of discontinuous data. The model is potentially generalizable to any particulate-reinforced metal matrix composite (MMC).     

Settling of multisized clusters of alumina particles in liquid aluminum

The clustering of alumina particles during their incorporation into pure liquid aluminum by stirring was studied experimentally. It was found that these particles together with entrained oxide films formed clusters of various sizes and shapes. When the stirring stopped, these clusters settled to form a loose sediment (containing about 0.18 volume fraction of alumina) on the bottom of the crucible. A mathematical model was developed to describe the settling of multisized clusters. Information regarding the size distribution of the clusters was obtained by matching the model results with the experimental data. The size distributions of the alumina clusters were comparable to those estimated from metallographic measurement.     

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5 · 10⁻⁵ s⁻¹≤ε≤1 · 10⁻² s⁻¹. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε≲10⁺⁴ s⁻¹), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed. This article is based on a presentation given in the symposium entitled “Dynamic Behavior of Materials-Part II,” held during the 1998 Fall TMS/ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

Influence of alloying elements on the strain rate and temperature dependence of the flow stress of steels

After a short introduction to the theoretical background of thermally activated glide of dislocations, a constitutive model is presented, which describes the temperature and strain-rate dependence of the flow stress. The properties of this constitutive equation were estimated for several plain carbon steels in normalized conditions, for quenched and tempered low-alloy steels, as well as for some high-strength low-alloy (HSLA) steels based on the temperature dependence and strain-rate sensitivity of the flow stress at temperatures 81 K≤T≤398 K and strain rates 5·10⁻⁵ s⁻¹≤ε≤1·10⁻²s⁻¹. The constitutive equation enables the extrapolation of flow-stress data to higher strain rates (ε<~10 ⁺⁴s⁻¹), which are in good agreement with the results obtained from high strain-rate deformation tests. The influence of solute-alloying elements on the thermal stress, the activation enthalpy, and the constitutive parameters will be discussed. This article is based on a presentation given in the symposium entitled ‘Dynamic Behavior of Materials-Part II,” held during the 1998 Fall TMS/ASM ASM Meeting and Materials Week, October 11–15, 1998, in Rosemont, Illinois, under the auspices of the TMS Mechanical Metallurgy and the ASM Flow and Fracture Committees.     

The effect of grain size, strain rate, and temperature on the mechanical behavior of commercial purity aluminum

Commercial purity aluminum AA1050 was subjected to equal channel angular extrusion (ECAE) that resulted in an ultrafine-grained (UFG) microstructure with an as-received grain size of 0.35 μm. This UFG material was then annealed to obtain microstructures with grain sizes ranging from 0.47 to 20 μm. Specimens were compressed at quasi-static, intermediate, and dynamic strain rates at temperatures of 77 and 298 K. The mechanical properties were found to vary significantly with grain size, strain rate, and temperature. Yield stress was found to increase with decreasing grain size, decreasing temperature, and increasing strain rate. The work hardening rate was seen to increase with increasing grain size, decreasing temperature, and increasing strain rate. The influence of strain rate and temperature is most significant in the smallest grain size specimens. The rate of work hardening is also influenced by strain rate, temperature, and grain size with negative rates of work hardening observed at 298 K and quasi-static strain rates in the smallest grain sizes and increasing rates of work hardening with increasing loading rate and grain size. Work hardening behavior is correlated with the substructural evolution of these specimens.     

Cast aluminum alloys containing dispersions of zircon particles

A process for preparing Al-alloy castings containing dispersions of zircon particles is described. Composites were prepared by stirring zircon particles (40 to 200 μm size) in commercially pure Al (99.5 pct)* and Al-11.8 pct Si melts and subsequently casting these melts in permanent molds. It was found to be necessary to alloy the above two melts with 3 pct Mg to disperse substantial amounts of zircon particles (25 to 30 pct). Further, it was possible to disperse up to 60 wt pct zircon by adding up to 5 pct Mg; however, the melts containing above 30 wt pct zircon showed insufficient fluidity for gravity diecasting and had to be pressure diecast. Microstructural studies of cast composites indicated the presence of a reaction zone at the periphery of zircon particles, and electron probe microanalysis showed concentrations of Mg and Si at the particle-matrix interface. Hardness, abrasive wear resistance, elastic modulus, 0.2 pct proof stress, and tensile strength of cast Al-3 pct Mg alloy were found to improve with the dispersions of zircon particles. Scanning electron micrographs of abraded and fractured surfaces did not show any evidence of particle pull-outs or voids at the particle matrix interface, indicating strong continuous bonding.     

Effect of friction,strain hardening,and strain rate hardening on the metal flow in rod extrusion

The metal flow in a rod extrusion is theoretically discussed in relation to a frictional condition, strain hardening and strain rate hardening. An existing rigid plastic FEM-program was modified in order to take a rate sensitivity of a material into consideration through a constitutive equation of the form. An interfatial friction was incorporated by a constant Coulomb-frictional coefficient. When the frictional coefficient is smaller than 0.1, a large effect of the friction does not appear. The larger the n-value is, the stronger the distortion of the grid in a billet becomes even under small frictional resistance. The effect of the m-value is similar qualitatively to that of the n-value. When the rate sensitive material is extruded at very low punch speed under large frictional resistance, the material is strained very irregularly all over the billet at an early stage.     

Effects of SiC whiskers and particles on precipitation in aluminum matrix composites

The age-hardening precipitation reactions in aluminum matrix composites reinforced with discontinuous SiC were studied using a calorimetric technique. Composites fabricated with 2124, 2219, 6061, and 7475 alloy matrices were obtained from commercial sources along with unreinforced control materials fabricated in a similar manner. The 7475 materials were made by a casting process while the others were made by powder metallurgy: the SiC reinforcement was in the form of whiskers or particulate. It was found that the overall age-hardening sequence of the alloy was not changed by the addition of SiC, but that the volume fractions of various phases and the precipitation kinetics were substantially modified. Precipitation and dissolution kinetics were generally accelerated. A substantial portion of this acceleration was found to be due to the powder metallurgy process employed to make the composites, but the formation kinetics of some particular precipitate phases were also strongly affected by the presence of SiC. It was observed that the volume fraction of GP zones able to form in the SiC containing materials was significantly reduced. The presence of SiC particles also caused normally quench insensitive materials such as 6061 to become quench sensitive. The microstructural origins of these effects are discussed.     

Effect of changing strain rate on stress-strain behaviour during high temperature deformation

Plane strain compression tests have been carried out on a ferritic stainless steel at a nominally constant temperature of 917°C and at strain rates of 0.15–5 s⁻¹, and on commercially pure aluminium and an Al-1% Mg alloy at 450°C and strain rates in the range 0.4–12 s⁻¹. Tests have been conducted both at constant strain rate and with strain rate increasing or decreasing from one constant rate to another in a controlled manner after steady state conditions had been established. A wide range of rates of change of strain rate have been studied, with initial and final strain rates differing by up to one and a half orders of magnitude.The ferritic stainless steel and the Al-1% Mg alloy follow a mechanical equation of state under all test conditions of changing strain rate in the sense that the flow stress is dependent only on the instantaneous strain rate and not on the way this strain rate is reached. On the other hand the commercial purity aluminium shows deviations from a mechanical equation of state, which increase systematically with the rate of change of strain rate but are the same magnitude for increasing and decreasing strain rate.     

碳纳米管和氧化铝颗粒在铜基复合材料中的协同作用

金属基复合材料中加入增强相,可以提高金属基体的力学性能和物理性能。碳纳米管和氧化铝颗粒作为常用的增强相,将二者同时加入金属基复合材料中,由于增强相之间的协同作用,可进一步提高其力学性能。加入0.8%(质量分数,下同)氧化铝颗粒、0.8%碳纳米管后,铜基复合材料的维氏硬度较之单独加入0.8%碳纳米管提高了5.4%;抗拉强度与单独加入0.8%碳纳米管和单独加入0.8%氧化铝颗粒相比分别提高了18.1%、41.6%;延伸率也分别提高了0.62倍、3.22倍。     

Monte carlo simulation of clustering of alumina particles in turbulent liquid aluminum

A Monte Carlo method was developed to simulate the formation of clusters of alumina particles suspended in a turbulent pure liquid aluminum melt. With this approach, the chaotic movements of small alumina particles suspended within eddies smaller than the Kolmogoroff microscale were treated in a similar way to Brownian motion, and clusters were assumed to form once these particles collided with each other. The results obtained from the simulation indicate that clusters form very quickly during vigorous stirring and that the formation kinetics at the very beginning of mixing follow a second-order behavior. Clustering has been observed previously in the SiC-Al system and was also observed in the Al₂O₃-Al system in the present work.