Murray W. Mahoney

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.

Superplasticity in a high strength powder aluminum

The superplastic properties of a rapidly solidified, high strength P/M Al alloy and the same alloy reinforced with SiC particulates (SiCp) have been studied. To prepare superplastic test materials, a matrix alloy powder of composition 7.2Zn-2.4Mg-2Cu-0.2Zr-0.12Cr-0.2Co (Kaiser PM-64) and the powder mixed with 10 to 20 vol pct SiCp (~5 μm diameter) were thermomechanically processed to very fine equiaxed grain structures of ~6 μm and ~8 μm, respectively. Superplasticity in these materials was evaluated by characterizing (1) high temperature stability, (2) dynamic grain growth, (3) strain rate sensitivity, (4) flow stress behavior, (5) cavitation and cavitation control, and (6) total superplastic strain. It was observed that the PM-64 alloy could achieve a total elongation of over 800 pct, while the SiCp reinforced alloy could attain an elongation greater than 500 pct before failure. Also, it was shown that with the use of hydrostatic pressure during superplastic flow, cavitation could be controlled. Observations were made of the effect SiCp reinforcement particles had on the superplastic flow stress behavior. Interpretations are proposed to explain the role of particulates during superplastic straining.

Creep of titanium--silicon alloys. [Ti--5 Zr--0. 5 Si with and without 5 Al; Ti-11; IMI-685]

Operative creep mechanisms in laboratory melts of Ti-5Zr-0.5Si and Ti-5Al-5Zr-0.5Si have been investigated as a function of microstructure, creep stress, and temperature. From creep rate data and transmission electron microscopy results, it has been shown that an important creep strengthening mechanism at 811 K in Si bearing Ti alloys is clustering of solute atoms on dislocations. All of the alloys investigated showed anomalously high apparent activation energies and areas for creep, and a high exponent (n) in the Dorn equation. In addition, the effect of heat treatment was investigated and it is shown that the highest creep strength was obtained by using a heat treatment which retained the maximum amount of silicon in solution. This is consistent with the proposed creep strengthening mechanism. An investigation of the creep behavior of several other Si containing alloys including two commercial alloys, Ti-11 and IMI-685 indicated similar results.

Control of Superplastic Cavitation by Hydrostatic Pressure

It has been shown that the application of hydrostatic gas pressures during superplastic deformation of fine grained 7475 Al can entirely prevent the intergranular cavitation normally encountered at atmospheric pressure. A critical ratio of hydrostatic pressure to flow stress may be defined for each superplastic forming condition above which virtually no cavitation occurs. In superplastic deformation conditions where intergranular cavitation plays a significant part in final tensile rupture, the superplastic ductility may be improved by the application of hydrostatic pressures. Similarly, detrimental effects of large superplastic strains on service properties may be reduced or eliminated by the application of suitable hydrostatic pressures during superplastic forming. In this case, superplastically formed material may have the same design allowables as conventional 7475 Al sheet.

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.

Development of forming limits for superplastic formed fine grain 7475 AI

Forming limits in conventional sheet metal forming are given by strain levels obtainable prior to the onset of a localized neck or tear in the sheet. While the external appearance of such a neck is not observed in superplastic metals until strains become quite large, the formation of internal cavities could dictate the tolerable levels of strain in formed components. In this paper, these useful strain limits for a superplastic 7475 Al alloy have been explored. The approach used was to establish the influence of strain state (uniaxial, plane strain, and balanced biaxial) on the inception and growth characteristics of cavities and to correlate the extent of cavitation with material properties. Based on these data, it was then possible to establish strain states for which little or no loss in properties was observed, and thereby to define forming limits for superplastic forming this material. These results, coupled with comparisons against strains developed in actual parts as well as analytically predicted strains, show that a wide range of structural parts can be superplastically formed within the constraints of the recommended forming limits.

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.

The effect of microstructure on the hot salt stress corrosion susceptibility of titanium alloys

It was the purpose of this study to identify what metallurgical processes could be applied to commercial structural titanium alloys to increase the hot salt stress corrosion (HSSC) threshold stress and therefore increase their range of application. Toward this purpose Ti-6A1 and Ti-6A1-4V were evaluated as a function of microstructural variables. Specifically, it was shown that both increasing amounts of cold work and increasing grain size decrease HSSC resistance of Ti-6A1. Also, for Ti-6A1-4V it was shown that preferred orientation can have a profound effect on the HSSC resistance. Crack initiation time, crack growth rate, and stress rupture life were evaluated in Ti-6A1-4V as a function of applied stress at 727 K. These results indicate that HSSC cracking can be described by a critical resolved shear stress criterion, and that increased high temperature creep resistance and decreased room temperature notch rupture strength combine to increase HSSC susceptibility and embrittlement.

A back-stress-based rationale for creep behavior during dynamic aging

Abstract Recent creep results on a titanium-base alloy were analyzed in terms of a creep equation relating the creep rate to an effective stress which is defined as the applied stress less an average back stress dependent on the nature of the strengthening mechanism operative during creep of the alloy. It is shown that the back stress at low strain rates is close to the Orowan—Ashby stress required to bypass silicide precipitates dynamically formed on mobile dislocations during creep in a titanium alloy containing small additions of silicon. At higher strain rates, when the dynamic precipitation process is not observed to occur, the back stress values appear to be close to the estimated values of the drag stress due to silicon atmospheres around moving dislocations. The true creep activation energies in both strain rate regions, calculated by correcting the apparent activation energies for the temperature dependence of the elastic modulus and the appropriate back stress, appear to be of the order of magnitude expected for diffusion in vacancy-controlled lattice diffusion in a titanium alloy.