Strain hardening during high pressure torsion deformation
Introduction
Recent investigations [1], [2], [3] have shown for various materials deformed by different severe plastic deformation (SPD) methods like high pressure torsion (HPT) or equal channel angular pressing (ECAP), a saturation of both the microstructural refinement and the yield strength. The question always arises, are these changes already present during the deformation or are they the result of post-deformation recovery or recrystallisation. If there are no changes of the developed microstructure after the deformation, then in situ measurements of the flow stress during the deformation should reflect the characteristics of the results from mechanical tests after the deformation. The aim of this study is to compare the mechanical behaviour during and after the deformation by HPT. The in situ measurement permits, furthermore, to estimate the effect of the hydrostatic pressure on the flow stress during HPT.
Section snippets
Test equipment
Fig. 1 shows a sketch of our tool. The sample lies inside the cavity, in the gap between the two anvils a burr is formed during the compressive loading. The torque is measured directly above the sample on one of the anvils by means of strain gauges. This measured torque M consists of the torque necessary to deform the sample Md plus a torque arising in the area of the burr Mb.
Samples
To investigate the influence of the hydrostatic pressure on flow stress of severely plastic deformed samples during and
Torque versus number of turns
In Fig. 2, the measured torques versus angle of twist for copper and for Armco iron for different hydrostatic pressures are shown. In addition, the fitted curves are indicated. At the beginning, a pronounced strain hardening is clearly visible, after a certain number of turns, a saturation takes place, somewhat earlier in iron than in copper. The necessary torque to deform the sample increases with pressure. However, it has to be noted that these curves contain the torque for the deformation
Discussion
The results from the in situ measurement of the torque curves, tensile tests, microhardness measurements and SEM micrographs have clearly shown that for both materials, saturation in structural size and strength takes place when they are deformed by high pressure torsion up to very high strains. This saturation and the resulting microstructure can be explained by a steady state deformation, where new structural elements are continuously formed (for more details, see Ref. [1]), the current
Conclusion
- (i)
Depending on the material, saturation of strain hardening and grain refinement takes place at γ of about 40 and 20 for Armco iron and copper, respectively.
- (ii)
Only a very small influence of the hydrostatic pressure on the resulting microstructure and the tensile strength on Armco iron and copper samples deformed by high pressure torsion could be observed.
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2022, Progress in Materials ScienceCitation Excerpt :At present, no investigations were carried out in order to highlight the effects of the applied pressure or the rotation speed on the texture evolution in FCC materials. Nevertheless, experimental results show that there is only a minor difference in the microstructure and mechanical properties evolution as a function of the applied pressure [15,145,161]. There is also evidence that the microstructural and mechanical properties variation of the HPT-processed materials originate from a combination of the shearing strain achieved in torsion and the compressive strain introduced by the imposed pressure [145].
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