Transmission Electron Microscopy Investigations of Phase Transitions in SPD Materials

H. Peter Karnthaler ,  Christian Rentenberger ,  Thomas Waitz 

University of Vienna, Institute of Materials Physics, Wien, Austria

Abstract

When methods of transmission electron microscopy (TEM) are applied to nanostructured alloys processed by severe plastic deformation (SPD) like high pressure torsion (HPT) special methodological points have to be considered. In HPT materials the density of grain boundaries is so high that in the TEM images frequently overlapping grains are causing moiré patterns that must be interpreted correctly. In the following, two examples are given of studying phase transitions in HPT materials. In polycrystalline L12 ordered Ni3Al a two phase structure is formed by HPT deformation at about 8000 %. The structure consists of veins of disordered nanograins that are embedded in the ordered coarse-grained structure. The observations show that inhomogeneous deformation leads to localized disorder and a high density of dislocations. Dynamic recovery causes a heterogeneous formation of the nanocrystalline structure. The nanocrystalline veins showing only a weak texture grow by the formation of nanograins generated at their interface. At strains above 50.000% the whole volume transforms to the nanocrystalline structure [CR & HPK, Acta Mater. 53 (2005) 3031]. In NiTi, a shape memory alloy, the phase transformations caused by the HPT deformation were investigated. Prior to HPT the specimen was in the cubic B2 austenite phase whereas after a HPT deformation of about 1000%, strain induced B19’ martensite containing shear bands of an amorphous phase were observed. The latter fill the whole volume at larger strains. Since the amorphous phase formed by HPT contains several nanocrystalline debris that can act as nucleation sites, it can be successfully used to make a bulk nanocrystalline structure by devitrification. The nanocrystalline structure of NiTi processed this way shows a new size-dependent martensitic transformation path based on atomic scale twinning that is not observed in the large-grained material [TW & HPK, Acta Mater. 52 (2004) 5461].

 

Presentation: invited oral at E-MRS Fall Meeting 2005, Symposium I, by H. Peter Karnthaler
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-05-31 13:15
Revised:   2009-06-07 00:44