Low-temperature plastic strain of ultrafine-grain aluminum

Abstract

The microstructure and mechanical properties of ultrafine-grain (UFG) commercial-grade Al obtained by equichannel angular pressing (ECAP) are study in the temperature range 4.2–295K. Transmission electron microscopy and x-ray diffraction methods are used to show that as the number of passes increases, the grain size decreases, the grain shape becomes increasingly equiaxial, and the dislocation density inside a grain and the character of the intergrain boundaries change. An increase of the coherent scattering region and a decrease of the level of microdeformations indicate that pressing decreases the total density of imperfections of the crystal structure inside grains. As temperature decreases, the yield stress, plasticity, and strain hardening rate of UFG and coarse-grain polycrystals increase substantially. The deformation of UFG polycrystals at 4.2K becomes unstable (abrupt). The temperature dependences of the yield stress σy(T) of UFG and coarse-grain polycrystals, where the form of these dependences is characteristic for thermally activated detachment of dislocations from short-range potential barriers, are studied. The differences observed in the dependences σy(T) for UFG polycrystals are explained by a change in the nature of such barriers and the mechanism by which glide dislocations overcome them. The dependences of the yield stress σy and the coefficient of strain hardening θ on the grain size d are obtained. It is determined that the Hall–Petch relation describes the function σy(d) in the temperature interval 4.2–295K. The Hall–Petch coefficient increases as temperature decreases. The function θ(d) is monotonically decreasing at 295 and 77K but θ is independent of d at 4.2K. The experimental data are discussed within the framework of existing notions about the influence of the microstructure produced by ECAP on the evolution of the dislocation density during subsequent deformation.

Low Temperature Plasticity of an Ultrafine‐Grained Al–3Mg Alloy Prepared by Accumulative Roll Bonding

“…The rate of increase is reduced as test temperature decreases. The value of u % 800 MPa for ARB Al-3Mg alloy deformed by tension below 77 K up to about 8-10% is close to the work hardening coefficients reported for UFG pure Al after eight passes of ECAP (u % 1000 MPa) [9] and for AA6016 aluminum alloy after four cycles of ARB (u % 750 MPa). [10] The dependency of work hardening capacity and ductility of aluminum and Al-based alloys on the test temperature and the grain size is usually explained by the rate of dynamic recovery process supported by thermal fluctuations and grain boundary annihilation of dislocations.…”

“…[10] The dependency of work hardening capacity and ductility of aluminum and Al-based alloys on the test temperature and the grain size is usually explained by the rate of dynamic recovery process supported by thermal fluctuations and grain boundary annihilation of dislocations. [9,10] Nevertheless, in the present case, the refinement of grain size to 0.22 mm (which supports annihilation) seems to be not enough for ensuring sufficiently high rate of relaxation processes and necking at temperatures below 77 K. This explains the relatively high low temperature ductility of ARB processed alloy.…”

“…In UFG Al the amplitude of LTSD increases with decreasing grain size in the range down to 1 mm. [9] On the other hand the LTSD is suppressed in CG aluminum as the grain size decreases in the range 500-50 mm. [12] From Figure 1 it is seen that at 4.2 K in total %40 stress drops with amplitude up to 70 MPa (or 0.1s m ) occur in the flow curve, whereas, for CG Al-1.8%Mg with grain size of %30 mm there were %20 stress drops with amplitude of 4 MPa (or 0.01 s m ).…”

“…The athermal stress s y corresponding to the lines intersection is approximately 375 MPa which makes up about the half of flow stress measured at 20 K (see Figure 2b). This athermal stress usually increases with the grain refinement [9,10] and is attributed to interaction of dislocations with long range obstacles, [1] namely grain boundaries. [1,10] Such behavior of the strain rate sensitivity of deformation stress, i.e., the increase with decreasing grain size, was theoretically predicted in ref.…”

“…[6] Nevertheless, there is only limited quantity of work related to the behavior at lower temperatures. Several authors only reported enhanced strength and strain hardening rate of UFG Al as the deformation temperature decreases from RT to 4.2 K. [7][8][9][10] These and other mechanical features of such materials stimulate a great interest to study physical mechanisms of plastic deformation in the wide temperature range.In this work, the mechanical properties of ARB processed Al-3Mg were investigated by uniaxial tension in temperature range from 0.5 to 295 K. …”