Papers by Keyword: Schmid Factor

Abstract: In the present work, the crystal structure and microstructure of martensite in Ni-Mn-In alloys and the variant rearrangement behavior under the external mechanical loading were investigated. Results show that the martensite has an incommensurate 6M modulated structure (I2/m (α0γ)00) with the modulation wave vector q = 0.3437c*. Microstructure of martensite is in plate shape and self-organized with four orientation variants that are alternately distributed and related in either type-I, or type-II or compound twin relation. The variant interfaces are in coincidence with their corresponding twinning plane and then should be considered coherent. Under the uniaxial compression, the loading located in the common positive Schmid factor zone of the three types of detwinning systems might be favorable to obtain the single variant state. This study is expected to offer a fundamental information of crystal structure, microstructures and variant rearrangement behaviors in Ni-Mn-In alloys, so as to understand the underlying mechanisms of their multifunctional magneto-responsive properties.

Abstract: Tensile properties and fracture behavior of an AA6060 alloy were investigated at room temperature (295K) and cryogenic temperature (77K). It was found that both ultimate tensile strength and elongation increased with decreasing temperature. In particular, the latter increased with increasing strain rate at 77K. The changes in mechanical properties were thought to be due to a higher working hardening rate at low temperature, while the effect of strain rate on strain hardening was obscure at both temperatures. The hardness after tensile testing at 77K increased due to an increased accumulated dislocation density. Fracture occurred in a semi-ductile transgranular manner at 295K, while a mixture of intergranular and transgranular mode with less slip localization occurred at 77K. Moreover, a decreased testing temperature led to a decreased size of dimples. The rotation of grain orientations can lead to increased Schmid factors and change of the latter was strongly dependent on the deformation temperature, which was clarified by compression tests.

Abstract: In this study an advanced method for investigation of the microstructure such as electron backscattering diffraction (EBSD) together with in-situ tensile test in a scanning electron microscope (SEM) has been used at room temperature and 300°C. EBSD analyses provide information about crystallographic orientation in the microstructure and dislocation structures caused by deformation. The in-situ tensile tests enabled the same area at different strain levels to be investigated. For the same macroscopic strain values lower mean misorientation angles of individual grains at the elevated temperature indicated that less residual strains at grain levels are gained compared with room temperature. The influence of grain size on plastic deformation on microscopic level, where larger grains tend to accumulate more strains, is discussed.

Abstract: A new selection criterion to explain the activation of the twinning variant is proposed. This criterion is based on the calculation of the deformation energy to create a primary twin. The calculation takes into account the effect of the grain size using a Hall-Petch type relation. This criterion allows to obtain a very good prediction for the variant selection. The calculations are compared with the experimental results obtained on T40 (ASTM grade 2) deformed by Channel Die compression.

Abstract: We postulated a softening model involving grain rotation that results in diffusion-accommodated grain-boundary sliding. This numerical model was used to compute the proportion evolution of grains within shear bands and was also employed to predict the softening of nanocrystalline materials considering non-homogeneous plastic deformation due to shear bands. The effect of softening mechanism for total stress-strain relation and the grain size and mean maximum Schmid factor effect was also considered in our model.

Abstract: Both EBSD and AFM methods were used to investigate the active slip systems and fatigue crack initiation behavior in face-centered cubic polycrystalline metal, austenitic stainless steel, SUS316NG, under cyclic torsional loading. Most active slip planes are the primary slip planes having the largest Schmid factor. Grains with slip band cracks or transcrystalline cracks have larger Taylor's factors. On the basis of EBSD and AFM observations, h, the depth of intrusion vertical to the surface, S, and the component of the slip displacement perpendicular to the surface trace, SB, showed a sharp increase at the onset of crack initiation. The critical value of SB at crack initiation was 170 nm.

Abstract: The superiority of creep in Ti-48at%Al alloy with fully transformed lamellar structure to that in Ti-50at%Al alloy with γ single phase is characterized by the extension of transient stage. This extension of the transient stage derives by the retarding effect of α2 plate on the onset of the accelerating stage, through suppressing the dynamically recrystallization which is the main reason of the accelerating stage. This superiority in Ti-48at%Al alloy will become more clear by investigating the creep of the single crystal designated as the PST crystal, because of removing the grain boundaries which is the formation site of dynamic recrystallization. By using the PST crystal, the predominant deformation using primary slip plane of γ plate will continue, because the α2 plate restricts the operation of other slip planes. In PST crystals with the angle between the stress axis and the lamellar plates, designated as φ, less than 45°, the uniform deformation will proceed, because of the decrease in creep rate due to the decreasing in Schmid factor through the monotonous decrease in φ. But these suppositions have not confirmed. In this study, the deformation manner of the PST crystals with φ of less than 45° is investigated by the analyzing of creep curve, macrostructure and inverse pole figure of the PST crystals interrupted the creep tests at 1148K/68.6MPa at the strains of 0.20 and 0.65. Inverse pole figures of PST crystal are obtained using SEM-EBSD method. By accepting the creep deformation, the stress axes of the PST crystals move for [001]-[1, – 11] line with slip system of (111)<1, – 01>, and after reaching at this line, the stress axis turn to [1, – 11] pole position with (111)<1, – 10> slip system. The change in stress axis is not homogeneous in gauge portion accepting small strain, by subjecting the further creep deformation up to the onset of the accelerating stage, this heterogeneous deformation in gauge portion disappeared.

Abstract: To investigate the effect of external loads arising from differential thermal expansion between a substrate and a surface mount component, specimens with a simulated surface mount component (nickel) on a copper substrate having a 1 mm2 joint area and solder thickness of about 100 µm were prepared to induce extrinsic shear in joints undergoing thermomechanical fatigue (TMF) cycling. The specimens were fabricated stress free and later clamped to a copper block to cause a significant reversal in sign of the shear imposed on the solder joint during TMF cycling for 20 minutes at 150°C and 3.5 hr at -15°C. The evolution of surface damage and microstructure was examined using SEM and Orientation Imaging Microscopy (OIM). The joints were almost single crystals. However, the orientations of the tin in each joint is different, leading to different resolved stresses on a given slip system. The joint with the largest resolved shear aligned with the crystal caxis showed the most damage. Low angle tilt boundaries developed, and sliding was observed on boundaries near 7 and 14° that have a coincident site lattice. Schmid factor analysis was carried out in regions that showed ledges or grain boundary sliding. Slip on (110) planes correlated well with some of the ledges.