R. Lin Peng

Intergranular strains and plastic deformation of an austenitic stainless steel

Intergranular strains due to tensile plastic deformation were investigated in a sheet material of austenitic stainless steel. The objective was to study the development of residual intergranular strains in samples unloaded from the intermediate and large plastic deformation regimes for which few theoretical and experimental studies were available. By using neutron diffraction, residual lattice strain distribution as a function of sample direction was mapped for a number of crystallographic planes. Deformation microstructures were examined by both transmission electron microscopy and the electron back scattering pattern technique. Residual intergranular strains were observed in samples deformed significantly beyond the elastic limit and the strains varied with sample directions as well as the amount of applied plastic strain. In addition, a different tendency of intergranular strain evolution was observed after large plastic deformation, which could be attributed to the change of dominant plastic deformation mode from slip to mechanical twinning. The results are discussed based on the observed deformation microstructure studies.

Micromechanical behaviors of duplex steel : in situ neutron diffraction measurements and simulations

Recently, much attention has been paid to studying the micromechanical behavior of multiphase materials by virtue of their extensive engineering applications. A strong heterogeneity in stress exists in two-phase materials during loading due to the different thermal expansion coefficients and the respective mechanical properties of each individual phase. For one duplex stainless steel under uniaxial compression, the distributions of microstrains were characterized with the strain response of multiple reflections to the applied external stress for each phase by employing in situ neutron diffraction experiments. Based on these, the anisotropic elastoplastic properties of the duplex steel on microscales, i.e. phase size and grain size, were modeled using a two-phase viscoplastic self-consistent (VPSC) model involving lattice rotation (texture evolution). Good agreement between predictions and neutron diffraction measurements was found. The stress partition between phases and orientated grains was discussed to characterize the phase stress and the grain-orientation-dependent stress, thus the particular micromechanical properties of two-phase materials may be explored.

Direct evidence of detwinning in polycrystalline Ni–Mn–Ga ferromagnetic shape memory alloys during deformation

In situ time-of-flight neutron diffraction and high-energy x-ray diffraction techniques were used to reveal the preferred reselection of martensite variants through a detwinning process in polycrystalline Ni–Mn–Ga ferromagnetic shape memory alloys under uniaxial compressive stress. The variant reorientation via detwinning during loading can be explained by considering the influence of external stress on the grain/variant orientation-dependent distortion energy. These direct observations of detwinning provide a good understanding of the deformation mechanisms in shape memory alloys.

Characterisation of residual stress distribution in clinching joints of carbon steel by diffraction methods

Abstract Sheet joints of carbon steel fabricated by two different clinching methods, namely TOX and Eckold, have been investigated. The holding force of the joints was determined by shear tension tests and the deformation microstructure was characterised using optical microscopy. The surface residual stress and mean residual stress distributions as a function of increasing distance from the outer diameter of the interlock button were mapped by X-ray and neutron diffractometry, respectively. The Eckold joints showed more severe joint distortion in the form of global sheet bending, but nevertheless possessed higher shear tension strength than the TOX joints. Characteristic residual stress distributions depending on the clinching method were found in both the TOX and Eckold joints. The observed residual stress distributions have been attributed to the different die construction employed by the two methods, which permitted different degrees of plastic deformation during clinching.

In situ neutron diffraction study of micromechanical interactions and phase transformation in Ni–Mn–Ga alloy under uniaxial and hydrostatic stress

This paper deals with the experimental study of stress-induced phase transformation in a polycrystalline Ni-Mn-Ga alloy under uniaxial compression and its powder under hydrostatic compression. In situ neutron diffraction experiments were employed to follow changes in the structure and lattice strains caused by the applied stresses. Large lattice strains that are dependent on the lattice planes or grain orientations were observed in the parent Heusler phase for both the bulk material and the powder sample. The development of such anisotropic strains and the influence of external load conditions are discussed in the paper.

Measurements of textures and stresses in Ni2MnGa ferromagnetic shape-memory alloys‖

The neutron diffraction method and the synchrotron high-energy X-ray diffraction technique provide global and local measurements of textures and residual stresses in both structural and functional materials under various environments. In this paper, we present the in situ measurements in ferromagnetic shape-memory alloys during phase transformation, which reveal the good “memory” of textures and stresses.