Title: Formation of Deformation Twins in Metallic Crystals

Speaker: Prof. S. Mahajan

Affiliation: Ira A. Fulton School of Engineering,

               Arizona State University, Tempe, AZ85287-9309, USA

 

Time:  10:00am, Tuesday 14, July 2009

Venue: Rm. 403, R&D Building

 

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.