Biswadeep Saha

Acoustic emission and changes in dislocation structure and magnetostriction accompanying plastic deformation of [126]-oriented Fe-Ga alloy single crystals

Controlled compressive deformation of [126]-oriented Fe-20 at. % Ga alloy single crystal along [126] direction results in large and asymmetric changes in the magnetostriction of the sample. This is in contrast to a much smaller change in magnetostriction observed in [100]-oriented single crystal deformed along [001] direction. Deformation of [126]-oriented crystal along [126] direction involved operation of only one of the slip systems. This is confirmed by TEM examination that showed only a single set of dislocation array which introduces asymmetric strain modulation in the crystal. The [100]-oriented crystal deformation involved operation of multiple slip systems and formation of several sets of dislocation arrays which introduce more symmetric strain modulations. The results suggest that the nature of strain modulation introduced by the dislocation arrays has a strong influence on the magnetostrictive behavior. Several sudden load drops accompanied by acoustic emissions and formation of slip bands were observed during [126]-oriented crystal deformation, while no such load drops or audible acoustic emissions were seen during the [001]-oriented crystal deformation.

Influence of plastic deformation on the magnetostrictive behavior of [126]-oriented Fe–Ga alloy single crystals

Dislocation arrays were introduced through controlled deformation of Fe–20 at. % Ga alloy single crystal samples with ⟨100⟩ and ⟨126⟩ orientations. Alloy single crystals were obtained through vertical Bridgman growth process. Magnetostriction measurements show a large decrease in magnetostriction after deforming just past the yield point when only a single slip system is operative as in the case of [126]-oriented crystal deformation. Magnetostriction values showed a much lower decrease with deformation for the case of [100]-oriented crystal deformation, where eight different slip systems were operative and consequently eight different sets of dislocation arrays are expected. The results suggest that the nature of strain modulation introduced by the dislocation arrays has a strong influence on the magnetostrictive behavior of magnetostrictive alloys.

Effect of hydrogen and magnetic field on the mechanical behavior of high strength AISI 4340 steel

Presence of hydrogen in materials is known to affect their mechanical properties due to hydrogen embrittlement problem. Steels used in various applications are prone to be exposed to aqueous electrochemical environments, which may introduce hydrogen into the alloy. These alloys are also prone to be simultaneously exposed to magnetic field, which may affect the hydrogen embrittlement susceptibility of these alloys. Therefore, it is important to examine the effect of hydrogen and magnetic field on the mechanical behavior of iron-based alloys. In this work, the effect of hydrogen and magnetic field on the fracture behavior of high strength AISI 4340 steel was examined. Three-point bend test was used to study the fracture behavior. In all the cases, the samples tested with hydrogen charging show a drastic reduction in ductility and fracture stress values. The effect of magnetic field was seen to be negligible. The hydrogen embrittlement was characterized by a change in the fracture surface from a ductile-type fracture to a brittle cleavage-type fracture. Acoustic emission signals collected during the test corresponds to the fracture behavior.