B.J. Diak

The determination of solute clusters in dilute aluminum alloys using strain rate sensitivity

Abstract Precise, apparent activation volumes, V′, are determined at 78 K for aluminum solvent systems containing mostly Fe, Cr, Cu or Mg in solution. The relationship between 1/V′ and the flow stress correlates to the amount of solute in the aluminum matrix. It appears that only Fe solute manifests solute-defect interaction effects which approximate a random solid solution. All other solute elements exhibit a much reduced distribution of strain centres, suggesting that these solutes are clustered even though the dilute alloys were solutionized at temperatures considerably above the solvus temperature. Stress equivalence of the apparent activation volume is observed over a range of yield stress, σ0, as V′ ∝ σ0−0.95.

Recovery of tensile properties of AA5182 after cold rolling

An experimental AA5182 sheet was cold rolled 80%, and tensile specimens removed with orientations 0, 45 and 90 degrees to the rolling direction. Room temperature monotonic tensile tests were performed on the specimens in different recovered states obtained by isothermal annealing at 230°C from 0.1 to 10h. The tests were instrumented to measure instantaneous plastic strain ratio, and unloaded just after incipient necking, but before failure if possible. With annealing the flow curves are characterized by lower strains to the onset of jerky flow, the reappearance of yield point elongation, decrease in work hardening, and increasing ductility. The recovery in substructure was described using a constitutive parameter proportional to the mean slip distance.

Dislocation-defect analysis during micro-indentation using displacement rate changes

Abstract Micro-indentation with step-ramp displacement rate changes has been developed to nullify the machine and specimen compliance and to directly measure the apparent activation work, Δ w′, of the deforming material. The Cottrell-Stokes relation is obeyed for an eutectic solder, but in contrast, nominally pure Al in the annealed and pre-rolled states indicate that the type of obstacle barrier continuously changes as a function of strain rate. Thus it is deduced that for high homologous temperature testing, either dislocation-dislocation interactions or jog dragging are detected, whereas at lower temperatures other defects are generated especially when the total plastic volume under the indenter is minute. These results can be correlated to the evolution of debris at high strains detected by tensile tests.

Point defect generation, nano-void formation and growth. II. Criterion for ductile failure

Using the derived relation for point defect generation according to a new constitutive relation, the notion of nano-void formation at grown-in nano-particles is examined and its consequences deduced as the nano-voids grow in size with continued deformation. Assuming that void growth is due only to point defect accumulation, the analysis of fracture strains in tension of natural-aged AA6111 suggests that coalescence by micro-plastic activity occurs when the void diameter becomes about one third of the evolving inter-void spacing. Hence, the derived limit strain to incipient void-coalescence is inversely proportional to the square root of point defect generation as determined from the stress–strain data. Using this criterion, failure prediction maps can be constructed for strain modes of plane-strain and balanced bi-axial to result in the outer bounds of the forming limit diagram. Trial examinations with AA5754 and AA3003 show great promise.

Strain rate sensitivity of ultra-low carbon steels

Abstract Ultra-low carbon, interstitial free (IF) steels were prepared by an intermediate rolling reduction of 20% followed by a 3 or 9 day anneal at 400°C prior to further reduction and anneal. Previously described determination of the interstitial content using the Haasen plot intercept indicates that levels in the ppb range were attained. The strain rate sensitivity in the macro-plastic region defined by the slope of the Haasen plot increased after reducing solute levels below 1 ppm, and the work hardening mechanism was attributed to that of jogs and/or recombination of dissociated dislocations in bcc structures. A distinct strain rate sensitivity was found in the micro-strain region below 0.2% where the activation work was an order of magnitude larger than in the macro-plastic region indicating an activation distance of 4.5 b . This measurement suggests that during micro-straining, glissile dislocations intersect, but as the jog density increases with straining, the dislocation cores become sessile and some recombination mechanism activates.

Assessment of decomposition products in an Al-Mg-Si alloy by instrumented micro-indentation

Abstract Strain rate sensitivity measurements were made at 299 K by displacement controlled micro-indentation of an Al–Mg–Si alloy undergoing natural and artificial ageing at 299 and 463 K, respectively. The measured apparent activation volumes and apparent activation work are shown to discriminate differences in microstructural decomposition products expected for each ageing treatment that are not tractable from hardness measurements alone.

Point defect generation, nano-void formation and growth. I. Validation

The volume fraction of point defects generated as a function of plastic shear strain squared, γ2, was derived from crystal plasticity concepts. The evolution was determined from the stress–strain values using a new constitutive relation which replicates the measured behavior with at least two fitted loci. Assuming that nano-voids form by clustering of vacancies, the nano-void diameter was found to be proportional to their spacing and shear strain with the constant being characteristic of point defect production during deformation. The predicted amount of point defect generated was validated using the previously determined resistivity of [100] copper single crystals deformed at 4.2 K and annealed at 296 K. Similar analysis of super-pure polycrystalline copper data affirmed that the dynamic annihilation parameter extrinsically incorporated in the new derivation is larger due to formation of slip clusters. Moreover, the temperature dependence of the mean slip-distance to inter-forest spacing ratio at Stage II to III transition indicates that the thermally activated drag of vacancy-creating jogs occurs above 150 K. For polycrystalline aluminum deformed at 296 K, it was concluded that the nuclei of the nano-voids were not part of the evolving dislocation array but were embedded in the grown-in microstructure. This hypothesis is pursued in the accompanying paper, Part II, and its prediction results in a criterion for ductile failure.

Formation of Cube Texture in Nominally Pure Aluminum With Fine Particle Dispersion

An X-ray method to observe in-situ cube grain growth during recrystallization has been devised using a hot stage to measure the growth kinetics. Complementary studies, using electron channelling contrast, electron backscattered pattern and X-ray textural analysis, revealed that specific thermal-mechanical history can precipitate out Fe solutes such that the matrix is sufficiently pure to undergo continuous recrystallization even though a fine distribution of precipitate are initially formed. These precipitates control via Zener drag the grain size upon complete recrystallization. On the other hand, 2 to 3 ppm (atomic) of Fe is sufficient to impose discontinuous recrystallization where the final grain size is controlled by grain impingement. The apparent activation energies for grain growth during recrystallization can be separated into two categories, the one for continuous recrystallization is smaller than that for Fe diffusion in Al and the one for discontinuous process is larger. Examination of volume fraction for each textural component reveals that during recrystallization the amount of cube texture growth correlates best with the decrease of the brass component. The increase in volume fraction of cube texture during recrystallization corresponds very well to the hot stage observations of cube grain growth.

Influence of magnesium AZ80 friction stir weld texture on tensile strain localisation

Synchrotron diffraction was used to construct the first 2D texture maps of entire magnesium AZ80 friction stir welds and showed that basal slip is favoured along most of the advancing side interface, and to a lesser extent on the retreating side interface. Zones of grains optimally oriented for basal slip are known to be a major contributor to strain localisation leading to failure during transverse tensile tests. Profilometry results confirm that the basal plane orientation dominates strain localisation. Microtexture maps traversing the weld interfaces were used to describe the material flow within the weld using scatter from the ideal shear texture fibre. The current results are highly applicable to modelling the strength and ductility of these joints under transverse loading.

Nanovoid characterization of nominally pure aluminium using synchrotron small angle X-ray Scattering (SAXS) methods

A small angle X-ray scattering (SAXS) study of nanovoids in 99.988 and 99.995 at.% aluminium is presented. Absolute intensity calibration using a glassy carbon standard is used to extract the weak SAXS signature from nanovoids introduced by thermal quenching. SAXS analysis methods, including Guinier, Porod and Indirect Transform, are used to obtain values for the void–size, number distribution and volume fraction, as well as measures of the void-metal matrix interface structure in quenched aluminium samples. The SAXS analysis has identified a residual impurity effect on void formation and has been used to characterize trends in nanovoid size, number distribution and interface structure as a function of ageing time at elevated temperatures (artificial ageing). The work presented here, including identification of experimental tools that can be readily improved, demonstrates that SAXS studies are capable of providing precise characterization of nanovoid structure in aluminium. This level of information will be useful in developing phenomenological models of void nucleation and growth capable of linking atomic scale phenomena to macroscopic material properties.