Moustafa El-Tahawy

The Influence of Plastic Deformation on Lattice Defect Structure and Mechanical Properties of 316L Austenitic Stainless Steel

The effect of different plastic deformation methods on the phase composition, lattice defect structure and hardness in 316L stainless steel was studied. The initial coarse-grained γ-austenite was deformed by cold rolling (CR) or high-pressure torsion (HPT). It was found that the two methods yielded very different phase compositions and microstructures. Martensitic phase transformation was not observed during CR with a thickness reduction of 20%. In γ-austenite phase in addition to the high dislocation density (~10 × 1014 m-2) a significant amount of twin-faults was detected due to the low stacking fault energy. On the other hand, γ-austenite was gradually transformed into ε and α’-martensites with transformation sequences γ→ε→α’ during HPT deformation. A large dislocation density (~133 × 1014 m-2) was detected in the main phase (α’-martensite) at the periphery of the disk after 10 turns of HPT. The high defect density is accompanied by a very small grain size of ~45 nm in the HPT-processed sample, resulting in an very large hardness of 6130 MPa.

Evolution of the Dislocation Structure During Compression in a Mg–Zn–Y Alloy with Long Period Stacking Ordered Structure

Evolution of the dislocation structure in Mg97Y7Zn5 (at. %) alloy having long period stacking ordered (LPSO) structure was studied during compression tests. Two materials, an as-cast and an extruded one were deformed up to the applied strain of ~25%. The evolution of the crystallite size, the dislocation density and the population of the particular slip systems were determined by the evaluation of the X-ray diffraction peak profiles. A very low dislocation density with the order of magnitude 1012–1013 m−2 was detected in the compressed specimens. This dislocation density did not increase considerably with increasing strain. At the same time, a significant decrease of the crystallite size occurred during compression. These observations can be explained by the arrangement of dislocations into low energy dipolar configurations, such as kink walls, which do not contribute to the dislocation density measurable by X-ray diffraction peak profile analysis, however they yield a fragmentation of the crystallites.

Evolution of microstructure, phase composition and hardness in 316L stainless steel processed by high-pressure torsion

The evolution of phase composition, microstructure and hardness in 316L austenitic stainless steel processed by high-pressure torsion (HPT) was studied up to 20 turns. It was revealed that simultaneous grain refinement and phase transformation occur during HPT-processing. The γ-austenite in the initial material transformed gradually to ɛ-and α’-martensites due to deformation. After 20 turns of HPT the main phase was α’-martensite. The initial grain size of ~42 μm was refined to ~48 nm while the dislocation density increased to ~143 × 1014 m-2 in the α’-martensite phase at the disk periphery processed by 20 turns. The microstructure and hardness along the disk radius became more homogeneous with increasing numbers of turns. An approximately homogeneous hardness distribution with a saturation value of ~6140 MPa was achieved in 20 turns.