The influence of forest dislocations on the mobility of the twin boundaries in Zn crystals

Abstract

The influence of the densities of the pyramidal and basal “forest” dislocations in Zn crystals on the value of the yield stress τk for the boundaries of the single twin layers, on the creep rate of the twin boundaries, v, and on the total displacement of the twin boundaries, δ, by pulsed loading was studied. It is shown that τk rises with the increase of the “forest” dislocation density in the {1122”︁ 〈1123〉 system, and v and δ decrease. It means that the pyramidal dislocations were the places of “pinning” for the twin dislocations. The increase of the “forest” dislocation density in the {0001} 〈1120〉 system leads to an increase of τk but v and δ also increase. The latter fact shows that the basal dislocations were not only the places of “pinning” for the twin dislocations but also the sources of them. The transition of the basal dislocations from the matrix in the twin was accompanied by the formation of perfect dislocations in the twin in the glide system {1100} 〈1123〉 and zonal twin dislocations on the boundary. The activation volumes for the process of creep depending on the densities of the pyramidal and basal “forest” dislocations were calculated from the values of the creep rate for the twin boundaries by different stresses.

Deformation twinning in Zinc-Aluminium single crystals after slip

“…The differences in the values of T~ may be also caused by different densities of the pyramidal and basal dislocations in samples used in the present work and in [2] and [lo]. This conclusion can be deduced from the result of Bashmakov et al [3] who have shown that the densities of the pyramidal and basal dislocations influence the twinning stress. The absence of a definite stress required to initiate twinning following some slip in these cases suggests the formation of stress concentrators necessary for twin nucleation.…”

“…A mechanism for twin growth in zinc is described [3,5,191 as the dissociation of two basal dislocations in (1012) twins according to the following dislocation reaction: (3) where h, is the Burgers vector of twinning dislocation, and index m and t corresponds to matrix and twin, respectively. Pyramidal dislocations may be considered as the pinning points of twin dislocations.…”

Time and spatial resolution of slip and twinning in a grain embedded within a magnesium polycrystal

“…It is therefore quite likely that for a wide class of grain orientations, "micro" yielding due to basal slip will precede twinning. This is likely to impact significantly on subsequent twinning events via a number of mechanisms; for example, by stimulating nucleation [11], by facilitating initial growth [12][13][14] and ultimately by impeding growth [15,16]. Bulk neutron methods have also shown that newly formed twins display relaxed [3] and even reversed [7] internal stresses.…”