John A. Wert

Grain refinement in 7075 aluminum by thermomechanical processing

A thermomechanical process for grain refinement in precipitation hardening aluminum alloys is reported. The process includes severe overaging, deformation, and recrystallization steps. Microstructural studies by optical and transmission electron microscopy of grain refinement in 7075 aluminum have revealed that precipitates formed during the overaging step create preferential nucleation sites for recrystallizing grains. The relationship between precipitate density following severe overaging and recrystallized grain density has been investigated; the results show that the localized deformation zones associated with particles larger than about 0.75 μ m can act at preferential nucleation sites for recrystallizing grains. The density of particles capable of producing nucleation sites for new grains is approximately ten times greater than the density of recrystallized grains. A close relationship between dislocation cell size after the deformation step and recrystallized grain density has also been established. Both quantities saturate for rolling reductions larger than approximately 85 pct. The grain size produced in 2.5 mm thick sheet by the optimum processing schedule is approximately 10 μm in longitudinal and long transverse directions and 6 μm in the short transverse direction.

Enhanced superplasticity and strength in modified Ti-6AI-4V alloys

Although Ti-6A1-4V displays extensive superplasticity at 1200 K, lower superplastic forming temperatures are desirable. A study was conducted with the goal of modifying the composition of the Ti-6A1-4V alloy to lower the optimum superplastic forming temperature. Computer modeling results and previous experimental data suggested that additions to Ti-6A1-4V of beta-stabilizing elements which have high diffusivity in the beta-phase would permit lower superplastic forming temperatures. A series of modified alloys with 2 wt pct additions of Fe, Co, and Ni was prepared for experimental evaluation. The modified alloys achieved desirable microstructures for superplasticity at 1088 K,i.e., the grain size was approximately 5 µm and roughly equal volume fractions of the alpha- and beta-phases were present at the deformation temperature. The superplastic properties of the modified alloys were measured at 1088 K and 1144 K. The modified alloys produced values of flow stress, strain rate sensitivity, and total elongation at 1088 K approaching those of the base Ti-6A1-4V alloy at its standard superplastic forming temperature of 1200 K. In addition to lowering the superplastic forming temperature, the β-stabilizing additions also increased room temperature strength levels above those normally found for Ti-6A1-4V. Based on the room temperature and elevated temperature tensile properties, addition of selected beta-stabilizing elements to Ti-6A1-4V simultaneously raises resistance to deformation at room temperature and lowers resistance to deformation at elevated temperatures. This reversal in behavior is explained by considering the effect of beta-stabilizer additions on the deformation mechanisms at room temperature and at elevated temperatures.

Identification of precipitates in 7075 Al after high-temperature aging

The precipitates that form in 7075 aluminum during overaging at 400°C have the C14 crystal structure. These precipitates are a solid solution of the isomorphous phases MgZn2 and MgAlCu. The mole fraction of MgZn2 in the precipitate is approximately 0.70 and the mole fraction of MgAlCu is approximately 0.30. The precipitates can be described as Mg(Zn2,AlCu).

Heating Rate Effects on Recrystallized Grain Size in Two Al-Zn-Mg-Cu Alloys

A method has previously been described whereby a fine and stable grain size may be achieved in conventional, heat-treatable aluminum alloy sheet by thermomechanical processing. The present work has examined the final recrystallization stage more closely. In particular, the effects of heating rate on recrystallized grain size have been determined and explained. It has been shown that heating rates greater than about 5 K . s-1 should be employed in the final recrystallization stage in order to obtain maximum benefit from the fine grain processing technique. The coarser recrystallized grain sizes obtained with slower heating rates are mainly due to early activation of the most highly favored nucleation sites. Thermal recovery of the matrix defect structure below the recrystallization temperature is an additional, though less significant, effect. The influence of the degree of cold work and the volume fraction of insoluble particles on recrystallized grain size is discussed in relation to the heating rate.

Intergranular fracture in an AlLiCuMgZr alloy

Abstract In the present study of fracture characteristics of an AlLiCuMgZr alloy, both underaged and overaged short transverse specimens failed by intergranular fracture. Approximately equal concentrations of K were found on all intergranular fracture surfaces, independent of aging treatment. This evidence alone does not allow a causal link to be established between intergranular fracture and grain boundary K segregation. However, previous investigators have demonstrated that Na can promote intergranular fracture in AlLi alloys [6], and K is thought to behave similarly. New results obtained in the present study are discovery of intergranular fracture in both underaged and overaged conditions, approximately equal concentrations of K on intergranular fracture surfaces in both aging conditions, and extensive intergranular fracture parallel to the tensile axis (delamination) in longitudinal tensile specimens.

Characterization of Fine-Grained Superplastic Aluminum Alloys

This paper summarizes the results of some recent research on a thermomechanical method of refining the grain size in precipitation hardenable aluminum alloys and illustrates the infuence of grain refinement on several material properties. Grain refinement is achieved by deliberately introducing a large number of nucleation sites for recrystallization and by controlling grain growth after recrystallization. Recrystallization to a relatively small and equiaxed grain size has been achieved in a number of commercial aluminum alloys using these concepts. The influence of the fine recrystallized grain size on such properties as superplastic deformation, room temperature tensile properties, fatigue life, and exfoliation corrosion resistance is discussed. The results show that refinement to a grain size of 8–14 µm is sufficient to develop extensive superplasticity and to yield a small increase in tensile properties in alloys such as 7075 and 7475.

Vacancy concentrations in quenched binary alloys

Abstract The effect of solute atoms on quenched-in vacancy concentrations has been studied theoretically using a simple model for vacancy annihilation during quenching. The model has been developed in generalized terms, making the results applicable to a wide variety of experimental conditions. For high quench rates, the quenched-in vacancy concentration is limited by the equilibrium concentration of vacancies at the quench temperature and solute effects are small. For low quench rates, the quenched-in vacancy concentration is determined by vacancy losses during quenching and solute effects can be enormous. Quantitative definition of high and low quench rates depends on vacancy properties, quench temperature, solute concentration and other parameters. The calculated results are in excellent accord with experimental results for pure Au and pure Pt.

Elimination of precipitate free zones in an Fe−Nb creep-resistant alloy

Precipitation of the Fe2Nb intermetallic compound has previously been found to cause substantial hardening during aging of Fe rich Fe-Nb alloys. However, the formation of a wide precipitate free zone adjacent to the grain boundaries caused a degradation of creep resistance. In an effort to decrease the precipitate free zone width, thereby improving the creep resistance, an extensive study was made of the precipitation behavior of an Fe-1.7 at. pct Nb(Cb) alloy quenched from the δ-phase field. The quenched alloy was found to decompose via a two step reaction during aging at temperatures below 550°C. The first step in the decomposition reaction is thought to occur by clustering of Nb atoms in the ferrite matrix, similar to the clustering of Mo atoms which is known to occur during aging of Fe-Mo alloys. The second step in the reaction is not well understood. The precipitate free zones were formed by solute depletion in the vicinity of the grain boundary and the subsequent difficulty of nucleation of the Fe2Nb precipitates in the regions of lowered solute concentration. Using two step aging treatments, an initial low temperature step to develop the Nb atom clusters followed by a higher temperature step to cause Fe2Nb precipitation, the precipitate free zones were eliminated from the aged alloys. The origin of this effect is thought to be the heterogeneous nucleation of Fe2Nb precipitates on the clusters developed during the initial aging step.

Grain Refinement and Grain Size Control in Superplastic Forming

Fine grain size is a microstructural requirement for superplastic alloys. We now have a general understanding of the methods of grain refinement and of restriction of grain coarsening. Phase transformation, recrystallation, deformation of duplex microstructures, and phase separation in duplex alloys can produce grain refinement, while particle dispersions and partitioning in duplex microstructures are useful methods for restricting grain coarsening. The methods selected for grain refinement and grain coarsening control for a specific alloy depend on the character of the alloy. Application of several of these methods to grain refinement and grain coarsening control in high-strength aluminum alloys is discussed.