Terry R. McNelley

Superplasticity in a thermomechanically processed High-Mg, Al-Mg alloy

Superplastic elongations in excess of 400 pct have been observed in tension testing at 573 K (300 °C) and strain rate έ= 2 × 10-3 s-1 for a thermomechanically processed Al-10.2 pct Mg-0.52 pct Mn alloy. The thermomechanical processing consists of solution treatment and hot working, followed by extensive warm rolling at 573 K (300 °C), a temperature below the solvus for Mg in the alloy. This processing results in a fine subgrain structure in conjunction with refined and homogeneously distributed β(Al8Mg5) and MnAl6 precipitates. This structure does not statically recrystallize when annealed at 573 K (300 °C) but appears to recrystallize continuously during deformation at such a temperature and the resulting fine grain structure deforms with minimal cavitation. At temperatures above the Mg-solvus,e.g., 673 K (400 °C), recrystallization and growth occur readily resulting in rela tively coarser structures which deform superplastically but with extensive grain boundary sliding and cavitation.

The influence of thermomechanical processing variables on superplasticity in a High-Mg, Al-Mg alloy

The superplastic response of an Al-10.2 pct Mg-0.52 pct Mn alloy, warm rolled at 573 K (300 °C), may be enhanced by annealing at a lower temperature, 473 K (200 °C), prior to subsequent stressstrain testing at a warm temperature,e.g., 573 K (300 °C). This enhancement is attributed to recovery and precipitation during annealing effectively retarding continuous recrystallization and growth during subsequent deformation. The finer resultant structure more readily sustains superplastic flow processes. The warm-rolled structure, a refined subgrain structure in conjunction with fine β (Mg5Al8) and MnAl6 precipitates, will statically recrystallize only upon heating to a temperature above the Mg solvus, such as 713 K (440 °C). Such recrystallization results in a relatively fine-grained material but also suppresses warm-temperature superplasticity; deformation at 573 K (300 °C) takes place by dislocation flow in the recrystallized material. At temperatures above the Mg solvus, both warm rolled and recrystallized materials exhibit superplastic elongations but cavitate extensively as a result of deforming by boundary sliding.

Current issues in recrystallization: A review

Summary The authors have agreed that recrystallization is theformation of a new grain structure in a deformed material by the formation and migration of high angle grain boundaries driven by the stored energy of deformation. High angle grain boundaries are those with greater than a 10 to 15° misorientation. Recovery can be defined as all annealing processes occurring in deformed materials that occur without the migration of a high angle grain boundary. Grain coarsening can in turn, be defined as processes involving the migration of grain boundaries when the driving force for migration is solely the reduction of the grain boundary area itself. These definitions are consistent with some earlier definitions.

Strain-Path Effects on the Evolution of Microstructure and Texture during the Severe-Plastic Deformation of Aluminum

Microstructure and texture evolution during the severe-plastic deformation (SPD) of unalloyed aluminum were investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of aluminum with different purity levels (99.998 pct Al and 99 pct Al) were subjected to large plastic strains at room temperaturevia four different deformation processes: equal-channel angular extrusion (ECAE), sheet rolling, conventional conical-die extrusion, and uniaxial compression. Following deformation, microstructures and textures were determined using orientation-imaging microscopy. In commercial-purity aluminum, the various deformation routes yielded an ultrafine microstructure with a ∼1.5-µm grain size, deduced to have been formedvia a dynamic-recovery mechanism. For high-purity aluminum, on the other hand, the minimum grain size produced after the various routes was ∼20 µm; the high fraction of high-angle grain boundaries (HAGBs) and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous static recrystallization following the large plastic deformation at room temperature. The microstructure differences were underscored by the mechanical properties following four ECAE passes. The yield strength of commercial-purity aluminum quadrupled, whereas the high-purity aluminum showed only a minor increase relative to the annealed condition.

A microtexture investigation of recrystallization during friction stir processing of as-cast NiAl bronze

As-cast NiAl bronze (NAB) was subjected to friction stir processing (FSP). Orientation imaging microscopy (OIM) methods were used to obtain microtexture data in the stir zone (SZ) and along its periphery. At selected SZ locations, orientation data were obtained by convergent beam electron diffraction (CBED) methods in transmission electron microscopy (TEM). Random α phase textures were apparent in the SZ. The α grains tended to be equiaxed, exhibited annealing twins, and were refined to 1 to 2 µm at the edge of the SZ. The population of subgrain boundaries in α phase grains was highest near the plate surface in contact with the tool and decreased with depth in the SZ, reflecting deformation by the tool shoulder after the passage of the tool pin. Distinct shear texture components were apparent in the thermomechanically affected zone (TMAZ) outside of and along the periphery of the SZ. A texture gradient from the TMAZ into the SZ was apparent and was steeper on the advancing side and under the SZ center than on the retreating side. The apparent shear plane tended to align with the local interface between the SZ and TMAZ, while the shear direction tended to align with the FSP traversing direction. In this material, the SZ-TMAZ interface is a distinct boundary between recrystallized and deformed regions and the α-phase grain refinement reflects dynamic recrystallization and, in locations near the SZ-TMAZ interface, particle-stimulated nucleation (PSN) at undissolved Fe3Al particles.

An investigation by interactive electron backscatter pattern analysis of processing and superplasticity in an aluminum-magnesium alloy

Tensile specimens from an Al-lOMg-0.lZr alloy, thermomechanically processed (TMP) according to two different schedules and deformed superplastically at 300 °C, were examined using electron backscatter pattern (EBSP) imaging in the scanning electron microscope (SEM) with computer-aided analysis. The TMP schedules differed only in the duration of the interpass anneal (IPA) time between successive rolling passes. Microtexture, grain misorientation angle distribution, and grain boundary character were analyzed for both undeformed grip and deformed gage sections of these tensile specimens. Microtexture analysis revealed the same texture components, primarily brass and S, in the undeformed grip sections of both TMP materials, while analysis of deformed gage sections revealed only a decrease in texture with no new components evident. Material processed with a prolonged IPA time exhibited highly superplastic response and contained a high proportion of boundaries of 5 to 15 deg misorientation. Material processed with a short IPA time exhibited lower superplastic elongations and contained higher-order twin boundaries, suggesting that the twin chain mechanism is active during TMP. The observed difference in tensile behavior appears to be the result of extended recovery during TMP of the more ductile material evidenced by the development of a greater proportion of boundaries of 5 to 15 deg misorientation.

Deformation and Failure of a Superplastic AA5083 Aluminum Material with a Cu Addition

A modified AA5083 aluminum sheet material containing a Cu addition of 0.61 wt pct has been investigated under conditions relevant to commercial hot-forming technologies. This material was produced by continuous casting followed by industrial hot and cold rolling into sheet. Deformation and failure mechanisms at elevated temperatures were investigated through mechanical testing, thermal analysis, and microscopy. The effects of Cu addition are evaluated by comparisons with data from AA5083 sheet materials without Cu addition, produced both by continuous and direct-chill (DC) casting techniques. At low temperatures and fast strain rates, for which solute-drag (SD) creep governs deformation, the Cu addition slightly increases tensile ductility at 450 °C but does not otherwise alter deformation behaviors. At high temperatures and slow strain rates, for which grainboundary-sliding (GBS) creep governs deformation, the Cu addition decreases flow stress and, at 450 °C, improves tensile ductility. A strong temperature dependence for tensile ductility results from the Cu addition; tensile ductility at 500 °C is notably reduced from that at 450 °C. The Cu addition creates platelike particles at grain boundaries, which produce incipient melting and the observed mechanical behavior.

An EBSP investigation of alternate microstructures for superplasticity in aluminum-magnesium alloys

This study proposes to provide insight into alternative grain boundary structures in two aluminum-magnesium alloys processed to achieve superplastic behavior. A commercially processes superplastic 5083 aluminum alloy, SKY5083, and a laboratory processed, non-commercial superplastic Al-10Mg-0.1Zr alloy have been selected for examination. Although alloy content, processing routes, and deformation conditions vary for each material, a comparison of results may provide evidence that alternate grain structures and boundary misorientation distributions may support superplasticity in the GBS regime, depending on the TMP processing and alloy system chosen.

Precipitation and recrystallization during processing of a superplastic Al10Mg0.1Zr alloy

Microstructural evolution during thermomechanical processing (TMP) of a superplastic Al10Mg0.1Zr (weight percent alloy was studied. The TMP was carried out at 300 °C and consisted of 12 rolling passes corresponding to a total strain of 2.5. Superplastic elongations at 300 °C exceeded 1000% for an optimum processing schedule involving a 30 min interpass anneal (IPA) time. Precipitation of the intermetallic β-phase (Al8Mg5) was complete at an accumulated strain of 1.5 and the precipitate particle size and size distribution were found to be nearly constant for the remainder of the TMP. Particle stimulated nucleation (PSN) of recrystallization associated with prolonged IPA times accounted for the grain refinement necessary to support superplastic deformation. A model for PSN is applied to the data.

Adiabatic heating and the saturation of grain refinement during SPD of metals and alloys: experimental assessment and computer modeling

Severe plastic deformation methods include equal-channel angular pressing/extrusion, high-pressure torsion, and plane strain machining. These methods are extremely effective in producing bulk microstructure refinement and are generally initiated at a low homologous temperature. The resulting deformation-induced microstructures exhibit progressively refined cellular dislocation structures during the initial stages of straining that give way to refined, equiaxed grain structures at larger strains. Often, grain refinement appears to saturate but frequently coarsening is observed at the largest strains after a minimum in grain size is attained during SPD. Here, we summarize results on grain refinement by these processing methods and provide an analysis that incorporates adiabatic heating to explain the progressive refinement to intermediate strains and that may be followed either by an apparent saturation in grain refinement or by grain coarsening at the largest strains. This analysis is consistent with continuous dynamic recrystallization in the absence of the formation and long-range migration of high-angle boundaries.