Carlos H. Cáceres

Casting defects and the tensile properties of an AlSiMg alloy

Abstract Samples containing either entrapped dross and oxide films, gas porosity or small drilled holes have been used to study the effect of different types of defects on the tensile behaviour of an Al 7Si 0.4Mg casting alloy. The tensile properties show little or no correlation with the bulk porosity content, especially in the case of samples containing dross and oxide films. In contrast, the decrease in tensile ductility and strength correlates with the area fraction of defects in the fracture surface of the samples. The experimental results are in agreement with the predictions of a simple analysis based on models for the growth of a plastic instability in a tensile sample.

Pseudoelastic behaviour of cast magnesium AZ91 alloy under cyclic loading-unloading

Abstract Large stress–strain hysteresis loops are observed under cyclic loading after a small plastic prestrain. Loops have been observed in sand-cast material in a variety of tempers under tension or compression, and in high-pressure die-cast material with different cross-section thickness tested in tension. The loops are first observed after a nucleation strain of between 0.001 and 0.01% and grow to a maximum width after 1–2% plastic strain, becoming slightly narrower afterwards. When fully developed, the loops add a large (0.3–0.45%) pseudoelastic strain to the elastic strain, effectively decreasing the elastic modulus of the alloy by up to 70%. In sand-cast material of a given temper, the effects tend to be more pronounced in compression than in tension. Further, the effect is slightly larger in die-cast or aged sand-cast as compared to as-cast sand-cast material. The phenomenon is discussed in terms of the partial reversal of { 1 0 1 2 } twins upon unloading.

Damage by the cracking of silicon particles in an Al-7Si-0.4Mg casting alloy

The cracking of Si particles during plastic deformation has been studied for different microstructures produced by varying the solidification rate and length of solution treatment. The number of cracked particles increases with the applied strain. The larger and longer particles are more prone to cracking. In coarser structures particle cracking occurs at low strains, while in finer structures the progression of damage is more gradual. Between 3 and 10% of the particles crack prior to fracture. The stresses in the particles can be calculated using current models of dispersion hardening and the particle cracking can be described by Weibull statistics. Assuming that fracture occurs when a critical level of damage is attained, the ductility of the alloy can be expressed as a function of the dendrite cell size and the average size and aspect ratio of the cracked Si particles.

Effects of solidification rate and ageing on the microstructure and mechanical properties of AZ91 alloy

Sand-cast plates of a commercial AZ91C alloy have been used for the study. Varying the solidification rate by placing large cast-iron chills in the mould produced a range of secondary dendrite arm spacing (SDAS) within the cast plates. The plates were solution heat-treated. quenched and aged at 165 degreesC for up to 350 h. The SDAS (mum) varied with the solidification time, t(f) (s), as SDAS = 5.3 t(f)(0.43). The tensile ductility in the as-quenched (T4) condition did not depend on the solidification rate whilst in the T6 condition it tended to decrease for slowly solidified material (SDAS > 50 mum). The yield strength and hardness increased and the ductility decreased with ageing. The fracture mode changed from predominantly transgranular in the T4 condition to predominantly intergranular in the T6 condition. The properties of the sand-castings are compared with those of high-pressure diecastings and the possible strengthening mechanisms are discussed. A number of areas that require more research are pointed out

On the mechanism of strain-enhanced grain growth during superplastic deformation

Superplastic deformation is often accompanied by grain growth, the rate of which depends on both strain and strain-rate, and is usually well in excess of that found in the absence of deformation. Two models for this process have been developed. In the first, which is most applicable to single phase materials, we assume that the deformation enhancement of grain growth is due to the damage created at triple junctions by grain boundary sliding. A geometrical model is used to show how the recovery of this damage by boundary migration, enhances the normal grain growth process. A second model, more suitable to microduplex alloys, is based on the postulate of Holm el al. [8, Acta metall. 25, 1191 (1971)] that superplastic flow enhances the coarsening of particles which pin grain boundaries. Experimental data for a variety of materials give excellent agreement with these models.

The deformation and fracture behaviour of an AlSiMg casting alloy

Abstract By changing the solidification rate, chemical modification and length of solution treatment we show that the ductility of the Al7Si0.4Mg casting alloy depends on the dendrite cell size and the size and shape of the silicon particles. For the strontium-modified alloy the ductility has a minimum at intermediate cell sizes, the fracture mode being transgranular for the larger cell sizes and intergranular for the finer cell sizes. For the unmodified alloy, the ductility also has a minimum at intermediate cell sizes and the fracture mode is, again, transgranular at large cell sizes and intergranular at fine cell sizes. The ductility of the large cell size unmodified materials is low, being dominated by the large elongated silicon particles. If the unmodified alloy is solution treated for shorter times it is more brittle because the silicon particles are more elongated.

Solid solution strengthening in concentrated Mg–Al alloys

Solid solution effects on the hardness and flow stress have been studied in alloys with Al contents between 1 and 8 wt%. The hardness increases with the Al content as Hv10 (kg mm−2)=29+3 Al (wt%). After correcting for grain size strengthening effects, the (0.2%) proof strength increases linearly with cn, where c is the solute atom concentration and n=1/2not, vert, similar2/3. The results suggest that strengthening of the basal plane controls the solid solution strengthening in polycrystals of concentrated Mg–Al alloys.

Large strain behaviour of a superplastic copper alloy—I. Deformation

Abstract The effect of strain hardening (which results from strain induced grain growth) on the flow behaviour of a superplastic copper alloy, Coronze 638, has been analyzed. To do this, we measure the strain induced grain growth as a function of strain and strain rate. Then, by using a constitutive equation relating grain size and flow stress, the effective hardening rate can be calculated. The work shows that grain growth at low strain rates causes sufficient hardening to compensate for the low strain rate sensitivity, thus preventing the development of sharp necks. The possible effect of cavitation on the hardening rate is also evaluated. A re-evaluation of the literature confirms our conclusion that a sharp neck develops at low strain rates (region I of the σ−ϵ curve) only for those materials which exhibit ver little strain-enhanced grain growth.

The influence of microstructure on the Bauschinger effect in an Al-Si-Mg casting alloy

The Bauschinger effect has been studied in an Al-7% Si-0.4% Mg casting alloy for a range of dendrite cell sizes and aspect ratios of the Si particles. The internal stresses increase linearly with the imposed pre-strain for strains up to 0.007, gradually saturating thereafter. For a given value of the cell size the internal stresses reach a higher saturation value when the Si particles have a larger aspect ratio. At constant aspect ratio of the Si particles, smaller cell sizes result in higher internal stresses at large strains. Current models for dispersion hardening can be used to calculate the stresses in the particles. The calculations are in very good agreement with the experimental results.

Strain hardening behaviour and the Taylor factor of pure magnesium

Taylor orientation factors for strain hardening in textured and random polycrystals of magnesium were derived from the ratio of the strain hardening rates of polycrystals to that of single crystals deforming by equivalent polyslip. For polycrystals with textures that inhibit basal and prismatic slip while favouring pyramidal polyslip, the Taylor factor is estimated to be between 2.1 and 2.5, increasing to about 4.5 for randomly textured polycrystals. The micromechanics of strain hardening in polycrystals are discussed.