Mg alloy AZ31B-H24 sheet was investigated through microstructural characterization and mechanical testing. Tension tests were conducted across a broad range of strain rates for temperatures from 22 up to 450 °C, as were gas-pressure bulge tests for several forming pressures at 350 °C. Tensile data were used to calculate flow stresses, tensile elongations, activation energies for creep and the Lankford coefficient, or R-value. The AZ31B material exhibits strong plastic anisotropy (R=7) at room temperature because of its basal crystallographic texture. Plastic anisotropy decreases with increasing temperature but demonstrates no sensitivity to strain rate until recrystallization occurs. Upon recrystallization, the R-value becomes sensitive to strain rate and continues decreasing with increasing temperature until it reaches a minimum (R=1–3) near approximately 300 °C. Plastic anisotropy at these high temperatures is greatest at the fastest strain rate. This sensitivity to strain rate is attributed to a competition between dislocation-climb creep, which produces anisotropic flow and dominates at fast strain rates, and grain-boundary-sliding creep, which produces isotropic flow and dominates at slow strain rates. The mechanistic understanding developed for plastic flow in AZ31B was implemented in a material constitutive model for deformation at 350 °C. A new aspect of this model is the inclusion of dislocation pipe diffusion as a potential accommodation mechanism for dislocation-climb creep. This model was validated against independent gas-pressure bulge test data through the predictions of a finite-element-method simulation, and the model provided quite accurate predictions of the experimental data.

Spall strength was measured as a function of composition and microstructure in three Al materials: a high-purity Al (Al HP), a commercial-purity Al (AA1100) and an alloy of Al containing 3 wt.% Mg (Al–3Mg). The Al HP and AA1100 materials were tested as single-crystal sheets, and the Al–3Mg alloy was tested as polycrystalline sheets having a variety of controlled grain sizes. A high-intensity laser produced shock loadings to create tensile strain rates ranging from 2 × 106 s−1 to 5 × 106 s−1, which caused spall fracture. Crystallographic orientation, relative to the direction of shock propagation, does not discernibly affect spall strength in the Al-HP material. Intermetallic particles, associated with impurity elements, initiate microstructural damage during tensile shock loading and reduce spall strength of the AA1100 material below that of the Al-HP material. The spall strength of the Al–3Mg is lowest among the three materials, and this is a result of the decreased ductility during spall fracture caused by the Mg solid-solution alloying addition. Grain size affects fracture character of the Al–3Mg material, but does not discernibly affect spall strength; the fraction of ductile transgranular fracture, versus brittle intergranular fracture, increases with grain size.Highlights► Three aluminum materials were subjected to laser-induced spallation tests. ► High-purity aluminum exhibits greater spall strength than commercial-purity. ► Spallation damage preferentially develops at large intermetallic particles. ► The addition of magnesium to aluminum as a solid solution decreases spall strength. ► Spallation fracture preferentially follows grain boundaries.

Abnormally large grains have been observed in Al-Mg alloy AA5182 sheet material after forming at elevated temperature, and the reduced yield strength that results is a practical problem for commercial hot-forming operations. The process by which abnormal grains are produced is investigated through controlled hot tensile testing to reproduce the microstructures of interest. Abnormal grains are shown to develop strictly during static annealing or cooling following hot deformation; the formation of abnormal grains is suppressed during plastic straining. Abnormal grains grow by static abnormal grain growth (SAGG), which becomes a discontinuous recrystallization process when abnormal grains meet to form a fully recrystallized microstructure. Nuclei, which grow under SAGG, are produced during hot deformation by the geometric dynamic recrystallization (GDRX) process. The mechanism through which a normally continuous recrystallization process, GDRX, may be interrupted by a discontinuous process, SAGG, is discussed.

Filaments with diameters of the order of 1 μm were observed on fracture surfaces of a fine-grained aluminum–magnesium alloy following superplastic deformation in air. No filaments were observed on fracture surfaces of the same material after superplastic deformation at identical temperature and strain rate in vacuum. Filaments contain oxygen and more magnesium than does the as-received specimen surface. These data support the theory that filaments are formed on fracture surfaces by growth of magnesium-rich oxide.

The effect of microstructure on cavitation developed during hot deformation of a fine-grained AA5083 aluminum-magnesium alloy is investigated. Two-point correlation functions and three-dimensional (3-D) microstructure characterization reveal that cavitation depends strongly on the mechanism that controls plastic deformation. Grain-boundary-sliding (GBS) creep produces large, interconnected cavities rapidly during plastic straining. Solute-drag (SD) creep produces isolated cavities with less total volume fraction at a given strain. The 3-D microstructure data reveal adjacency between various microstructural features. Cavities are observed to be preferentially adjacent to large Al6(Mn,Fe) particles and to Mg-Si particles of all observed sizes. These data suggest that cavities preferentially nucleate at Mg-Si particles and at large Al6(Mn,Fe) particles. This result may be applied to reduce cavitation in commercial hot-forming operations utilizing aluminum-magnesium alloys.

Fine-grained AA5083 aluminum sheet is used for hot-forming automotive body panels with gas pressure in the superplastic forming (SPF) and quick plastic forming (QPF) processes. Deformation under QPF conditions is controlled by two fundamental creep mechanisms, grain-boundary-sliding (GBS) and solute-drag (SD) creep. The failure mechanisms of AA5083 materials under QPF conditions depend strongly on these deformation mechanisms and on the applied stress state. Failure can be controlled by flow localization, cavitation development or a combination of both. There is interest in using continuously cast (CC) AA5083 materials instead of direct-chill cast (DC) materials in QPF operations as a means of reducing material cost. However, CC and DC AA5083 materials can produce significantly different ductilities under hot forming. Rupture-based forming-limit diagrams (FLDs) have been constructed for a CC AA5083 sheet material under hot-forming conditions. Forming limits are shown to be related to the controlling deformation mechanisms. Differences between FLDs from DC and CC AA5083 materials are investigated. The differences in FLDs between these materials are related to differences in cavitation development.

Laser-shock-induced spall failure is studied in thin aluminum targets at strain rates from 2 to 5 × 106 s−1. Targets were prepared from high-purity aluminum in the recrystallized condition and a low-impurity aluminum alloy containing 3 wt pct magnesium in both recrystallized and cold-rolled conditions. The effects of material and microstructure on spall fracture morphology are investigated. Recrystallized pure aluminum produced spall fracture surfaces characterized by transgranular ductile dimpling. Recrystallized aluminum-magnesium alloy with a 50-μm grain size produced less ductile spall surfaces, which were dominated by transgranular fracture, with some isolated transgranular ductile dimpling at fast strain rates. Transgranular ductile dimpling regions disappeared in recrystallized alloy specimens with a 23-μm grain size tested at faster rates. Cold-rolled alloy material produced spall failure surfaces consisting of brittle intergranular and transgranular fractures. Measured spall strength increases with increasing ductile fracture character. Spall failure preferentially follows grain boundaries, making grain size an important factor in spall fracture surface character.

A plate of ultrahigh-carbon steel (UHCS) was processed by hot and warm rolling, according to the Wadsworth–Sherby mechanism, to produce damask surface markings. The surface markings produced by this industrial processing method are similar to those of historical Damascus steels, which are also of hypereutectoid composition. The microstructure of the UHCS with damask contains fine, spheroidized carbides and a discontinuous network of proeutectoid carbides along former-austenite grain boundaries, which give rise to a surface pattern visible with the unaided eye. Tensile tests at room temperature measured tensile strengths and ductilities, which depend on sample orientation relative to the rolling direction of the plate. Hot and warm rolling causes a directional microstructure, giving rise to both an elongated, directional damask pattern and a directional dependence for strength and ductility. A maximum tensile ductility of 10.2% was measured at 45° relative to the rolling direction. The plate material was subjected to heat treatments creating pearlitic and martensitic microstructures, which retain visible damask patterns.