Topic: Formation of Deformation Twins in Metallic Crystals
Abstract
Slip and twinning are two processes by which metallic crystals can respond to stress-induced shape change. The slip phenomenon has been studied extensively during the last five decades. As a result, it is relatively well understood. On the other hand, the formation of deformation twins in metallic crystals is not clear. The central question is how do partial dislocations required on consecutive twinning planes arise during the deformation? We will address this issue in this talk.
We envisage that slip precedes twinning. In the case of the BCC crystals, the dissociation of ?<111> screw dislocations governs the nucleation of three-layer embryonic twins. The coalescence of these nuclei located at different levels within a slip band could lead to microscopic twins, i.e., a portion of the slip band is converted into a microscopic twin. The twins in the FCC crystals appear to form in a similar fashion. However, in this case the formation of embryonic twins involves two types of dislocations having co-planar Burgers vectors which appear to interact repulsively.
We correlated the crystallography of slip with those of deformation twins in BCC and FCC crystals. Based on these results, we developed circumstantial evidence in support of the above ideas and will be presented. Furthermore, the influence of metallurgical variables, such as deformation temperature, strain-rate, alloying and orientation, on deformation twinning will be rationalized in terms of the above hypotheses.
Topic: Accommodation of Deformation Twins in Metals and Alloys
Abstract
Imagine a situation where slip and twinning are occurring concomitantly in a metallic material. Since slip bands and twins are associated with shear displacements, it is relatively easy to visualize that stress concentrations could develop at twins terminating within a crystal, a slip band impinging on a twin, that is, slip- twin interactions, twin-twin intersections and twin-grain boundary encounters. We experimentally investigated such interactions in BCC and FCC materials. We will present these results in this talk. We will show that the strains associated with a twin terminating within a crystal are relaxed by slip ahead of the twin, i.e., emissary slip, a term coined by Sleeswyk. The strains of a slip band or a BCC twin can be propagated across an existing twin by slip through the twin. The situation in the case of FCC is more complicated. We observe both slip and twinning in crossed twins.
We developed generic ideas to rationalize the above interactions. There are two basic tenets of our approach. First, the twinning partials bounding a non-coherent twin boundary can coalesce to form slip dislocations. This can explain the origin of emissary slip. We can also analyze twin-twin interactions in terms of the propagation of slip dislocations across the crossed twin. Second, slip or twin planes which are activated within the crossed twin must contain the line of intersection of a slip or twin plane with the coherent twin boundary of the crossed twin, enabling dislocations to move from the matrix into the twin and out. We will show that the proposed dislocation reactions for strain transmission are energetically unfavorable and would require stress concentration.
Curriculum Vitae-S.Mahajan
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