The Role of Hydrostatic Pressure in Severe Plastic Deformation

Abstract

The contribution presents several experimental examples which show that the presence of an enhanced hydrostatic pressure—as compared to conventional large deformation modes—is one of the main features of severe plastic deformation (SPD). At the example of systematic high pressure torsion experiments with Cu at room temperature, strength measurements after deformation showed that the onset strains of deformation stages III, IV, and V are not affected by the pressure applied; however, the related onset flow stresses increase by at least 10 % of the values of low pressure torsion, per GPa of pressure increase. During deformation, increases of flow stresses by at least 40 % of the values of low pressure torsion, per GPa of pressure increase, have been found. From comparisons with tests on Ni, the increases appear to grow with the materials melting temperature. For a theoretical explanation of flow stress increases the pressure induced changes of i) the elastic moduli, and ii) the formation energy of lattice defects. While contribution i) is almost negligible, contribution ii) accounts for an increase of flow stress during deformation by about 15 % per GPa of pressure increase. The difference left to experiment has to be attributed to a third contribution, i.e., the pressure specific evolution of the structure. For this contribution, a modification of the model of Zehetbauer and Les[1–3] is introduced which is based on the pressure caused decrease of lattice diffusion. The latter is thought to restrict the diffusion controlled annihilation of dislocations, thus leading to a higher density of vacancies, dislocations and/or grain boundaries causing the higher stress level observed.

Universal Plot for Hardness Variation in Pure Metals Processed by High-Pressure Torsion

“…Xu et al 2) reported that the homogeneity for Al samples may evolve more rapidly at the lower pressures. Zehetbauer et al 17) showed that the flow stress of pure Cu increases with an increase in the applied pressure. Wetscher et al 18) reported a very small influence of the hydrostatic pressure on the resulting microstructure and the tensile strength of Armco Fe and Cu.…”

Evolution of strength and structure during SPD processing of Ti–45Nb alloys: experiments and simulations

“…The fits to experimental stress-strain curves could be achieved within less than 1 % deviation. The output parameters resulting from the fit procedures for the cases of CR&F, HPT and HE processing are summarized in [15,16,[18][19][20][21]29]). …”

“…The Zehetbauer's model [15,16,[18][19][20][21]] is a composite model, where screw and edge dislocations do not interact with each other because of different strain fields around the cores of dislocations. Thus, they are arranged in separate regions of cell interiors (areas of screw dislocations) and cell walls (areas of edge dislocations).…”

“…The latter were done using the dislocation-based constitutive large strain work-hardening model by Zehetbauer [18][19][20]. In several previous publications [8,15,16,21], it has been shown that the model is capable of predicting the strain-dependent evolution of the strength and also the structural features of material such as the dislocation density, the cell/cell wall size, the misorientation angle and the concentration of strain-induced vacancies of SPD-processed materials as the function of external hydrostatic pressure [15,16,22].…”

Механизмы Высокоскоростной Деформации Поликристаллической Меди

“…Этот результат согласуется с данными экспериментов [25]. Начальная концентрация вакансий в КК-Cu была принята рав-ной нулю, в УМЗ-Cu -10 −4 (согласно известным экспериментальным данным о концентрации вакансий в образцах, подвергнутых РКУП [26]). ДВ, обусловленный аннигиляцией вакансий, не наблюдается в КК-Cu.…”