Neil E. Paton

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.

Microstructural Influences on Superplasticity in Ti-6AI-4V

The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio, grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching” is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural parameters such as grain aspect ratios, grain size and grain size distribution.

Hydride habit planes in titanium-aluminum alloys

The habit plane of hydride precipitates in pure Ti has been previously reported as being predominantly the {10-10} prism plane, with precipitation also taking place on {10-11} planes, while habit planes in a Ti-3 pet Al alloy were {10-11} {11-21} and {10-12}. No habit planes near the basal plane have been reported in Ti alloys, although the {10-17} plane (14 deg from the basal plane) has been observed in certain zirconium alloys. In this work, Ti single crystals containing up to 6.6 wt pet Al were charged with hydrogen and the ef-fect of Al on hydride habit plane observed with both transmission and optical microscopy. Hydrides with a basal and near-basal habit plane were found in the higher Al content al-loys and it is suggested that the increased propensity for basal slip in these alloys is the reason for the change in habit plane. The implications of these observations in under-standing hydrogen induced fracture in Ti alloys are discussed, and possible modifica-tions to the conclusions of other investigators in this field suggested.

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.

Attainment of full interfacial contact during diffusion bonding

The rate-limiting step in the diffusion bonding of materials such as Ti−6 A1−4V is the complete elimination of porosity from the bondline. Here a model for predicting the time to complete bonding based on the mechanisms of porosity removal is developed. Bonding is pictured as a two-stage process: in the first stage long-wavelength surface asperities are flattened by plastic flow, and in the second stage voids are reduced in size by a combination of plastic flow and vacancy diffusion. Both stages have been treated analytically, and the resulting predictions of bonding time are in general agreement with the times usually required for industrial diffusion-bonding operations. In addition, a limited number of experiments have been conducted to verify some basic predictions of the model. As expected, the calculations have shown that the bonding die must be carefully designed so that the local bonding pressure is never unreasonably low. The results also demonstrate that the presence of long-wavelength surface asperities—a result of most surface preparation procedures —can significantly lengthen the bonding times.

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.

Model of sustained load cracking by hydride growth in Ti alloys

This paper presents a solution to the two-dimensional time and temperature dependent diffusion problem adjacent to a crack tip. This solution is used in addressing the problem of sustained load cracking in titanium-aluminum-hydrogen alloys. The theoretical solution presented indicates a maximum in sustained load cracking rate as a function of temperature. Experimental results confirm the presence of this maximum growth rate as a function of temperature. However, the temperature at which the maximum occurs is somewhat lower than that calculated theoretically. The importance of the result is that sustained load cracking rate in titanium alloys containing as little as 100 ppm hydrogen increases by several orders of magnitude as temperature decreases from room temperature to approximately −70 °C.

Formation characteristics of the α/β interface phase in Ti-6Ai-4v

The conditions for formation of the interface phase, or interfacial layer, in Ti-6Al-4V have been studied systematically. The interface phase does not grow during isothermal treatments, but rather grows only during cooling from elevated temperatures to about 650°C. The width of the interfacial layer is a function of the cooling rate, having a maximum of about 4000Å at 28°C/h. The interface phase forms initially with an fcc structure which subsequently transforms to hcp α phase. A theoretical description of the mechanisms of interface phase formation is presented.

The influence of microstructure on mechanical properties in Ti-3AI-8V-6Cr-4Mo-4Zr (Beta-C)

The effect of microstructure on mechanical properties of a metastable beta titanium alloy has been investigated, with emphasis on the influence of α-phase precipitation on tensile strength and ductility. A commercial alloy, Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C) has been aged to produce both Type la (obeying the Burgers orientation relation) and Type 2α (not obeying the Burgers relation) as hardening precipitates in the beta matrix. Although a direct comparison of Types 1 and 2α could not be made, there did not seem to be any direct correlation between precipitate type and mechanical properties. Rather, it has been found that overaging to produce a coarse distribution of relatively large (>1000Å) noncoherent α precipitates provides the best combination of strength and duc-tility. The same strength can be achieved with a much finer distribution of small (∼100-200A) coherent α precipitates, but with much reduced ductility.

The influence of α/β interface phase on tensile properties of Ti-6Al-4V

The effects of α/β interface phase on room temperature tensile properties of Ti-6Al-4V having an equiaxed primary α microstructure have been studied systematically. Due to the conditions under which it grows, manipulation of the interface phase width also results in alteration of the volume fraction of primary α in the alloy. Tensile yield strength and elongation were correlated to interface phase width and volume fraction primary α. The relative individual influence of each of these microstructural features on properties is not unambiguously clear, but evidence indicates that yield strength increases with increasing interface phase width when the interface phase exceeds about 250 nm, and elongation decreases with increasing interface phase width up to about 250 nm, and is unaffected at widths above 250 nm. It is speculated that the interface phase raises yield strength and lowers elongation by acting as a barrier to slip.