Strain Path Reversal Research Articles

An Analysis of Deformation Microstructure and Subsequent Recrystallisation in Hot Deformed Aluminium Alloy AA5052 Using Forward and Reverse Torsion

The effects of strain path reversal on the microstructure in AA5052 have been studied using high resolution EBSD. Deformation was carried out using two equal steps of forward/forward (F/F) or forward/reverse (F/R) torsion at a temperature of 300°C and strain rate of 1s-1 to a total strain of 0.5. In both cases the deformation microstructure in the majority of grains analysed consisted of microband arrays clustering at specific angles to the macroscopic deformation axes. For the F/F condition microbands clustered around -20° and +45° to the maximum principle stress direction, whilst for the F/R condition significantly more spread in microband angle was observed. This suggests that the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path. This leads to the conclusion that instantaneous deformation mode determines the orientation of new microbands formed whilst a non-linear strain path history influences the range of misorientation angle in the material through the dissociation of previously formed microbands and the formation of new microbands at the new straining condition, leading to a lower level of misorientation angle. Analysis of material subjected to static annealing at 400°C for 1 hour appears to correspond with these observations as the F/F material was completely recrystallised with a fine grain structure whilst the F/R material had no major signs of recrystallisation.

The effect of strain-path reversal on cavitation during hot torsion of Ti-6Al-4V

The effect of strain-path reversal on cavitation behavior during the hot torsion testing of an alpha/beta titanium alloy, Ti-6Al-4V, with a colony-alpha microstructure was established. Optical microscopy was used to measure cavitation parameters such as cavity size, density, and area fraction. It was observed that when the torsion direction is reversed, the cavitation process is reversed as well: i.e., cavity shrinkage takes place. The experimental observations were interpreted in the context of previous models developed for the densification of porous bodies. For this purpose, the models were modified to treat the effect of colony orientation on the local stress state and the accommodation of the externally imposed strain, both of which affect the rate of densification/cavity shrinkage. A modified version of the AFRL PM-consolidation model was shown to provide reasonable estimates of the shrinkage kinetics. An alternate description of cavity shrinkage during reversed torsion, analogous to prior descriptions of cavity growth, was also developed. It was concluded that the absolute magnitude of the cavity shrinkage rate is smaller than its counterpart during growth because the local stress ratio is lower during reversed straining compared to that during forward straining.

An Analysis of Microband Orientation in Hot Deformed Aluminium Alloy AA5052 Using Forward and Reverse Torsion

The effects of strain path reversal on the macroscopic orientation of microbands in AA5052 have been studied using high resolution electron backscatter diffraction. Deformation was carried using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300°C and strain rate of 1s-1 to a total equivalent tensile strain of 0.5. In both cases microbands were found in the majority of grains examined with many having more than one set. The microbands appear to cluster at specific angles to the macroscopic deformation. For the forward/forward condition microbands clustered around -20° and +45° to the maximum principle stress direction and at ± 30-35° to the principal strain direction. For the forward/reverse condition significantly more spread in microband angle was observed though peaks were visible at ±35° with respect to principal stress direction and at -40° and +30° with respect to the principal strain direction of the reverse torsion. This suggests the microbands formed in the forward deformation have or are dissolving and any new microbands formed are related to the deformation conditions of the final strain path.

Strain Path Effects on Titanium Alloy Timetal-834 under Hot Working Conditions

The effects of strain path reversal under hot working conditions on the microstructure and crystallographic texture in the near-α titanium alloy Timetal-834 have been studied using high resolution electron backscatter diffraction (EBSD). The main objective of the work was to investigate the potential effect strain path may have on breaking up the well known clustering of similarly orientated primary alphas grains in the alloy, which significantly reduces its low cycle dwell fatigue lifetime. Deformation was carried out using forward torsion to an equivalent strain of 0.9 and forward/reverse torsion of two equal steps to produce a total strain of 0.9. The tests were performed at a typical industrial forging condition of 990°C (~50% alpha, ~50% beta) at an equivalent tensile strain rate of 2s-1. Investigation of the microstructure showed the primary alpha grains to align with the direction of torsion for the forward test and return to an equiaxed shape on strain reversal, though a significant numbers of deformation twins are formed and retained after the full strain reversal. Analysis of the texture of the starting material showed the typically clustering of primary alpha grains, which upon forward torsion and forward/reverse torsion did not break down. This indicates that during a typical forging operation the clustering of similarly orientated primary alpha grains inherited from the as-received billet will not be reduced. This suggests that improved in-service performance of this alloy can only be achieved by reducing the clustering upstream in the manufacture of the billet.

Effect of strain reversal on the dynamic spheroidization of Ti-6Al-4V during hot deformation

The effect of strain-path reversal on the kinetics of dynamic spheroidization during subtransus hot working was determined for Ti-6Al-4V with a colony α structure. Isothermal torsion tests were conducted at a temperature of 815 °C and a strain rate of 0.001 s−1; strain-path reversals were achieved by applying forward and reverse torsion sequentially. The kinetics of spheroidization were measured as a function of the local (macroscopic) strain for monotonic-deformation, reversed-torsion, and double-reversed-torsion tests. Strain-path reversal led to a reduction in the spheroidization kinetics compared with monotonic deformation for a given total strain. The slower rate of dynamic spheroidization associated with strain-path reversals was ascribed to a reduced rate of sub-boundary formation/lower sub-boundary energies, which drive the boundary splitting process, and less sharp α/β interface curvatures, which control the coarsening process that also contributes to spheroidization.

The effect of strain path reversal during hot rolling on austenitic stainless steel

During flat rolling the metal undergoes complex strain paths due to the interaction of the stock with the rolls. These changes in strain path affect the stored energy and hence the recrystallisation behaviour of the material on annealing. The current paper investigates the effect of the roll pass schedule on the recrystallisation kinetics of type 316L stainless steel. By changing the direction of rolling and the reduction on the second pass the effect of the deformation on the recrystallisation kinetics was determined following static annealing. The effect was found to be at a maximum after lower second pass deformation, with the maximum effect at 0.8 from the centre of the sample half thickness. A forward/forward pass gave an increase in the recrystallisation kinetics at this point with more homogeneous recrystallisation seen after a forward/reverse pass. The origins of these differences were investigated by the study of the sub-structure developed during deformation. The differences in the kinetics cannot be attributed to a single microstructural feature, but to a combination of all of them.