A.K. Ghosh

Cavity formation and early growth in a superplastic Al–Mg alloy

Knowledge of the exact physical mechanism of cavity formation and early growth is important for the prediction of the extent of internal damage following superplastic deformation. To this end, the early stages of cavitation in a superplastic Al–Mg–Mn–Cu alloy have been experimentally studied and reported here. Small cavities (<0.5 μm) were detected by scanning electron microscopy and the number of cavities per unit volume was monitored by image analysis through optical microscopy. Before deformation, some cavities were seen at the particle–matrix interfaces. However, during tensile deformation in the temperature range of 450–550°C (and strain rates ∼10−4 to 10−2 s−1), additional cavities emerge and grow. Most cavities are observed at the interface between particles and the matrix from submicrometer size range, and grow initially along the interface. This suggests that early cavity growth is by matrix/particle decohesion, possibly starting from interfacial defects, and this growth has rapid kinetics. The density of observable cavities increases with strain, i.e. “nucleation” is continuous. The number of cavities increases at higher strain rates and at lower test temperatures. This is due to the higher flow stresses, reduced strain-rate sensitivity and poorer diffusional accommodation process, which assist in the initial growth of the submicrometer and nanoscale interface defects. But the evidence for diffusional cavity growth in the initial stages was not found.

Cavity growth during superplastic flow in an Al-Mg alloy: I. Experimental study

Cavitation caused by superplastic straining of a fine-grained Al–Mg–Mn–Cu alloy under uniaxial tension has been quantitatively evaluated. Tensile tests were conducted at constant true strain rate in a range of 10−4s−1 to 10−2s−1 at constant temperatures between 450°C and 550°C. Care was exercised to achieve precision in strain-rate control in these tests since strain-rate and temperature history do affect the extent of cavitation. Measurements of the number and size of cavities were made by using image analysis on tested specimens viewed by optical microscopy and further supported by SEM. With increasing strain, the cavity population density increases, a result seen previously but not studied in detail. Cavity growth was also monitored carefully and found to be due to the plastic deformation of the matrix surrounding the cavity. The total volume fraction of cavities which is the product of the above two components was found to increase exponentially with strain. The dependencies of cavity volume fraction on strain-rate and temperature are not straightforward, however. Based on experimental observations of decohesion between matrix and non-deformable particles, continuous nucleation of new cavities, and data related to plasticity-based growth of cavities, attempts are made to explain these complex effects.

Grain elongation and anisotropic grain growth during superplastic deformation in an AlMgMnCu alloy

Evolution of grain morphology in a fine grained Al-Mg-Mn-Cu alloy during uniaxial superplastic deformation is studied quantitatively. Grains undergo elongation, as well as dynamic grain growth along all directions, the separation and analysis of which is attempted here. Based on such analysis, the computed true grain growth rate along directions transverse to the tensile axis are found to exceed that parallel to the tensile axis. Possible mechanisms for this new observation are suggested. A unique relationship between grain boundary sliding rate and dynamic grain growth rate is found at different applied strain rates; this indicates that grain boundary sliding and grain boundary migration rates are inherently connected through the same mechanism.

Cavity growth in a superplastic Al-Mg alloy: II. An improved plasticity based model

The extent of cavity growth estimated from a combination of diffusional and plasticity based growth models generally underestimates the actual cavity growth in superplastic alloys. It has been shown that in a fine grain Al–Mg alloy, cavity growth begins by matrix/particle debonding at grain boundary particles (Mat. Sci. Forum, Trans. Tech. Pub. 304–306 (1999) 609), and also from pre-existing voids. In this study, cavity growth beyond interface decohesion is modeled in which deformation of the matrix surrounding the cavity is free from interface constraint, but it still experiences an accelerated local deformation rate. Stress and strain-rate in this region are intensified due to the perturbed flow field near the cavity, and not relaxed during the time frame for superplastic forming. This local deformation around the cavity is a function of strain-rate sensitivity, m, the level of strain concentration, and the cavity spacing. Two important effects not previously considered: (i) local stress concentration around the cavities, and (ii) continuous nucleation of new cavities, have been included in this work. Using this model that is suitable for low overall cavity volume (i.e. no cavity coalescence), faster growth rate is predicted for single cavities when strain-rate sensitivity is low and/or the population density of cavities is low (generally at slow strain-rates). By combining the predicted growth rate of individual cavities with the emerging cavity population density determined experimentally, a quantitative understanding of the various complex dependencies of cavitation has been obtained.

Interface shear properties and toughness of NiAl/Mo laminates

Mechanical properties at the interface between NiAl intermetallic and Mo ductile phase in sandwich laminates have been studied under a variety of stress states involving primarily shear. A shear/compression test for laminates of NiAl/Mo/NiAl has been used in which a double-notched specimen with cut-through metal interlayer is loaded to failure in compression. Because of the large elastic mismatch between NiAl and Mo, a primarily shear loading generates a significant amount of normal stress in this test. Interface shear response and toughness obtained using this technique are compared for NiAl/Mo laminates with interfacial segregant, Cr, Re and C layers fabricated by high temperature diffusion bonding. A microstructural investigation of the interfacial reaction zone and the shear failure path is conducted to identify failure mechanisms. Failure strength and related interfacial toughness is examined using an FEM model of local stress state close to the notch in the bimaterial shear/compression specimen geometry. A trend of increasing bond strength with reducing notch spacing is identified and analyzed in terms of local stress state, the onset of microyielding and crack shielding. Thus, a new fracture stress locus has been developed for bimaterial interfaces. Residual stress due to CTE thermal expansion mismatch is measured and the contribution of thermal residual stress to the failure behavior of the laminate is explored. The combined effect of stress concentration at crack tip, residual stress and slip discontinuity across interface have been used to explain the apparently brittle loading response and easy debonding of Mo fibers in eutectic NiAl/Mo alloys.