An Investigation of Cavity Development during Superplastic Flow in a Zinc–Aluminum Alloy Processed Using Severe Plastic Deformation

Abstract

A Zn22% Al eutectoid alloy was processed by equal-channel angular pressing for 8 passes at 473 K and then tested in tension at 473 K over a range of strain rates. A highest elongation of 2230% was recorded at a strain rate of 1.0 © 10 ¹2 s ¹1 representing high strain rate superplasticity. Quantitative cavity measurements and a cavity volume analysis were undertaken to investigate the growth of internal cavities during superplastic flow. The results demonstrate a clear transition from the superplastic diffusion growth of cavities at the lower elongations to plasticity-controlled cavity growth at the higher elongations. A cavity growth diagram provides an excellent description of the cavity growth processes including the transition from superplastic diffusion growth to plasticity-controlled growth at cavity radii larger than ³2.1¯m.

The contribution of grain boundary sliding in tensile deformation of an ultrafine-grained aluminum alloy having high strength and high ductility

“…The formation of cavities elongated along the stress axis is consistent with other experimental reports after SPD processing [55] and it confirms the plastic flow mechanism of cavity growth in which the cavities become elongated in the direction of the applied stress [56]. There are numerous reports for the formation of cavity stringers or lines of adjacent cavities lying parallel to the tensile axis in conventional superplastic alloys [57][58][59] and there are also similar reports of aligned cavity stringers in UFG alloys processed by ECAP [60][61][62]. In practice, stringer formation in conventional alloys is generally attributed to a cascade void nucleation mechanism in which the cavities serve to partially accommodate the occurrence of GBS [63] and this explanation is consistent with the present results where it is apparent from Table 2 that the specimens exhibiting cavity stringers are those where the sliding contribution, n, is reasonably high.…”

Development of hardness homogeneity and superplastic behavior in an aluminum–copper eutectic alloy processed by high-pressure torsion

“…This reduction is relatively small by comparison with the grain refinements achieved in pure metals and single phase alloys but, as documented earlier for the Zn-22% Al eutectoid alloy [29,30], grain refinement is restricted in twophase alloys because of the difficulties in achieving a mixing of the two constituent phases. For example, experiments on the Zn-22% Al alloy using processing by ECAP showed a reduction in grain size from $ 1.8 to $0.8 mm [38] and this degree of grain refinement, by a factor slightly greater than 2, is comparable to the refinement achieved in the present experiments on the Al-33% Cu alloy. …”

Achieving room-temperature superplasticity in an ultrafine-grained Zn-22% Al alloy

“…Finally, it is worthwhile to mention that, although the influence may not be as critical as the threshold stress, the lower value of m is also caused in part by the level of GBS attributed to the microstructural homogeneity of Al-and Zn-rich phases in the sample. Early GBS measurement by TEM showed there is different levels of contributions of sliding to the entire plastic strain at individual interfaces during high temperature deformation and a maximum contribution was observed on the Zn-Zn interfaces and slightly less on the Zn-Al interfaces while the Al-Al interfaces exhibited a minimum contribution of GBS in a conventional Zn-22% Al alloy [51] and after ECAP [17,18,20]. In a very recent report, room-temperature plastic deformation by nanoindentation examinations revealed the homogeneity in distributions of the Zn and Al phases is strongly correlated with the change in the m value [40].…”

“…There have been numerous trials of grain refinement for improving superplastic properties of Zn-22 % Al alloys by means of the techniques of cross-channel extrusion (CCE) [11], equal-channel angular pressing (ECAP) [12], friction stir processing (FSP) [13], high-pressure torsion (HPT) [14] and thermo-mechanical controlling process (TMCP) [15]. These processing techniques led to refine grains of Zn-22 % Al alloys to the submicrometer range so that the alloys exhibited superplastic characteristics including excellent ductility at elevated temperature [16][17][18][19][20][21] and superplastic flow behavior even at room temperature [22][23][24][25][26][27][28][29][30][31]. Recent reports examined the capability of room-temperature superplastic properties in Zn-22 % Al alloys as tuned mass dampers to reduce seismic vibrations in building structures [32,33].…”