Title: Accommodation of Deformation Twins in Metals and Alloys

Speaker: Prof.S. Mahajan

Affiliation: Ira A. Fulton School of Engineering,

               Arizona State University, Tempe, AZ85287-9309, USA

 

Time:  10:00am, Thursday, 16 July 2009

Venue: Rm. 403, R&D Building

 

 

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.