Title: Interface energy trends for semicoherent Fe(001)/TMX(001) systems
using ab initio calculations

Speaker: Dan Fors, Chalmers University of Technology, Sweden

Date/Time: Wednesday, September 16, 2009, 2PM-3PM         

Location: CSRI Building, Room 90 (Sandia NM)

Brief Abstract: The abundance of interfaces plays a decisive role for the physical properties in materials. In particular, the strength in metallic alloys is closely connected to the distribution of point defects, precipitates and grain boundaries which effectively act as obstacles for dislocation migration. Understanding the stability and energetics for the underlying interfaces therefore holds the key to the ability to control the mechanical
properties of a metal.

In this talk I will concentrate on the interface energetics of nonshearable transition metal carbonitrides (TMX) in 9-12Cr steel alloys. The presence and time evolution of the fine dispersion of carbonitrides inside grains and at grain boundaries are believed to have a governing role for the long-term creep behavior. The particles are often found as thin nacl structured discs (50-100 ‰ in radius) where the flat side has a semicoherent character with respect to the iron matrix while the rim has a large relative misfit and an incoherent interface behavior. In order to understand the energetics at these interfaces I've performed a systematic ab initio investigation of the electronic and atomic structure for a set of semicoherent Fe(bcc)/TMX(nacl) systems (TM = Sc, Ti, V, Cr, Zr, Nb, Hf, Ta; X = C or N). Accurate treatment of the electronic structure available from density functional theory is combined with a Peierls-Nabarro model to account for the elastic displacements in the materials due to the periodic square dislocation network that is present at the interface. The electronic structure is found to be characterized by covalent Fe(3d)-X(2p) and metallic Fe(3d)-TM(d) bonds. The strength of the latter is further shown to be a plausible explanation for the interface energy trends along the 3d, 4d and 5d TM element rows. In addition, it is demonstrated that the elastic energy contained in the dislocation network gives a significant contribution to the interface energy and therefore must be included in order to acquire an accurate interface description.

CSRI POC: Ann E. Mattsson, (505) 844-9218