F. Gregori

Plastic Deformation in Laser‐Induced Shock Compression of Monocrystalline Copper

Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock‐compressed copper, albeit at pulse durations in the μs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle‐Grady relationship.

The Effect of Dislocations in Grains on Texture Formation in Strain Induced Boundary Migration

In order to elucidate the predominance of Goss grains after SIBM in electrical steel sheets, Goss, D-Cube and {111}<112> grains after temper rolling of 5 and 9% reduction were observed by TEM. In 5% strain the amount of dislocations in Goss grains was the smallest of the three orientations. In 9% strain dislocations in Goss grains were distributed more heterogeneously than the other two types of grains. It is considered that {111}<112> grains have large amounts of dislocations owing to high Taylor factors and the differences of microstructures between Goss and D-Cube grains are due to orientation stabilities. Goss grains are speculated to be easy to recover and therefore they are predominant after SIBM.

Laser‐Induced Shock Compression of Copper and Copper Aluminum Alloys

Single crystal copper and copper 2‐wt% aluminum alloy with [134] and [001] orientations are compressed by means of a high energy short pulse laser. Pressures ranging from 20 GPa to 60 GPa are achieved. The shocked samples are recovered and the residual defect substructure is analyzed by transmission electron microscopy. Results show systematic differences depending on orientation and stacking fault energy. Samples with orientations [001] are symmetrical with simultaneous activation of eight slip systems. This leads to a higher work hardening rate. The [134] orientation is asymmetrical with one dominating slip system, and thus a reduced work hardening rate due to a prolonged easy glide region for dislocations. These differences in work hardening response affect the stresses required to achieve the twinning threshold pressure. The effects of stacking fault energy on the defect substructure and threshold twinning are also characterized. Experimental results are rationalized in terms of a constitutive descr...

The influence of microstructural features of individual grains on texture formation by strain-induced boundary migration in non-oriented electrical steels

In order to elucidate the predominance of near-Goss grains after Strain-Induced Boundary Migration (SIBM) in electrical steel sheets, the microstructures of grains of orientations close to Goss, D-Cube, and {111}〈112〉 were observed by TEM after temper rolling of 5xa0% (which is the optimal strain for the development of a strong Goss-type texture) and 9xa0% reduction (selected to emphasize the differences between orientations and facilitate their analysis). After 5xa0% strain, the dislocation density seems to be the smallest in the Goss grains. After 9xa0% strain, this difference in dislocation density is still present but also accompanied by a difference in dislocation arrangement between the Goss grains and the others. These microstructural features are put in regards to some calculations of rotation rates of individual orientations and analyses of slip system activities performed with the Taylor model. The analysis of all these data allow us to establish a strong link between dislocation microstructure and stability of orientation and to conclude that this correlation may be responsible for the favored growth of the near-Goss grains by SIBM during annealing after 5xa0% temper rolling.

Laser Induced Shock Defects in Copper Aluminum Alloys: Stacking Fault Energy Effects on the Slip-Twinning Transition

Copper and copper aluminum (2 and 6 wt% aluminum) with two orientations, [001] and [ 34 1 ], were subjected to high intensity laser shocks (energy levels of 40-300 J; energy densities of 15-70 MJ/m and durations of 2.5 ns). The defects created were characterized by transmission electron microscopy. The slip-twinning transition was determined quantitatively in terms of both orientation and stacking fault energy. The threshold twinning pressure for copper oriented to [001] decreases with decreasing stacking fault energy from 40 GPa for pure copper to less than 20 GPa for copper-2 wt% aluminum. For the [ 34 1 ] orientation, pure copper twinned at pressures on the order of 60 GPa, and whereas the copper-2 wt% aluminum alloy readily twinned at pressures less than 40 GPa. The results are rationalized in terms of a criterion in which slip and twinning are considered as competing mechanisms. A constitutive description using a modified MTS (mechanical threshold stress) constitutive equation applied incorporating slip and twinning in terms of orientation, stacking fault energy, temperature rise due to shock heating, and strain rate. The predictions are in agreement with experiments. The constitutive description provides a rationale for the experimental results; the calculated thresholds for [001] and [ 34 1 ] are, respectively: 17 GPa and 25 GPa for pure copper, 9 GPa and 13 GPa, for Cu-2wt % Al and 1 GPa and 2 GPa for Cu6wt% Al.

Reverse Taylor Tests on Ultrafine Grained Copper

Reverse Taylor impact tests have been carried out on ultrafine grained copper processed by Equal Channel Angular Pressing (ECAP). Tests were conducted on an as‐received OFHC Cu rod and specimens that had undergone sequential ECAP passes (2 and 8). The average grain size ranged from 30 μm for the initial sample to less than 0.5 μm for the 8‐pass samples. The dynamic deformation states of the samples, captured by high speed digital photography were compared with computer simulations run in AUTODYN‐2D using the Johnson‐Cook constitutive equation with constants obtained from stress‐strain data and by fitting to an experimentally measured free surface velocity trace. The constitutive response of copper of varying grain sizes was obtained through quasistatic and dynamic mechanical tests and incorporation into constitutive models.