Tsuyoshi Mayama

Biaxial ratcheting deformation of type 304 stainless steel: Effect of memorization of back stress

Abstract This paper treats both experiments and simulations of biaxial ratcheting. The experiments are conducted using a tubular specimen of type 304 stainless steel at room temperature. The specimen was subjected to cyclic shear straining under the axial superposed stress. The experiments show that the biaxial ratcheting strain was affected by the cyclic shear strain amplitude, the shear strain rate and the superposed stress level. Larger biaxial ratcheting strain occurred in the case of tensile superposed stress compared with that in the case of the compressive superposed stress. Moreover, even under the zero superposed stress, biaxial ratcheting strain occurred in the axial direction due to the cyclic shearing straining. Finally, the biaxial ratcheting behaviours were simulated by the unified constitutive model proposed by the authors. The characteristic features of the biaxial ratcheting behaviour, especially the axial strain due to the cyclic shear straining superposed on the zero axial stress, are well simulated by the constitutive model.

Microstructure dependence of fatigue crack propagation behavior in wrought magnesium alloy

This paper deals with the fatigue crack propagation behavior of rolled AZ31B magnesium alloy (grain size: approximately 40 ?m). Fatigue crack propagation tests were performed on single edge notched tension specimens at a stress ratio of R = 0.1 and a frequency of 10 Hz at room temperature. Loading axes were parallel to the rolling direction; fatigue cracks propagated parallel to the transverse direction (L-T specimen), parallel to the short transverse direction (L-S specimen). Loading axis was perpendicular to the rolling direction; fatigue cracks propagated parallel to the transverse direction (S-T specimen). The crack growth rate (da/dN) of the L-S specimen was several times lower than that of the L-T specimen in the examined stress intensity factor range (?K). Fracture surfaces of the L-T and L-S specimens showed many steps parallel and perpendicular, respectively, to the macroscopic crack growth direction. The da/dN of the S-T specimen was higher than that of the L-T and L-S specimens in the examined ?K. The fracture surface was covered by quasi-cleavage facets independent of macroscopic crack growth direction, and the fracture surface roughness at low ?K was larger than that at high ?K.

Deformation Behavior of a Magnesium Alloy Sheet with Random Crystallographic Orientations

In the present study, the deformation behavior of a cast Mg alloy sheet that had random crystallographic orientations was studied both experimentally and numerically. Although the crystallographic orientations were random, the stress-strain curve was asymmetric between tension and compression: the flow stress under tension was higher than that of compression. Moreover, the stress-strain curve exhibited a strain path dependency: a slightly sigmoidal curve occurred under tension following compression, while it did not occur under compression following tension. Clearly, such tendencies were similar to those observed in rolled Mg alloy sheets although the tendencies were less pronounced in the cast Mg alloy sheet. A crystal plasticity finite-element method was used to understand the mechanism of these results. Simulation results showed that the asymmetry and the strain path dependency in the stress-strain curves occurred in the cast Mg alloy sheet because of the asymmetry in the activity of twinning between tension and compression as in the case of rolled Mg alloy sheets.

Thermal Stability and Mechanical Properties of Extruded Mg-Zn-Y Alloys with a Long-Period Stacking Order Phase and Plastic Deformation

Lightweight Mg alloys with excellent shock-absorption properties are being actively adopt‐ ed for use in electronic information devices and automotive parts [1]. For such structural ap‐ plications, Mg alloys need to have adequate ductility, thermal stability, and strength. However, Mg alloys often exhibit low ductility and low tensile yield strength at room tem‐ perature and above as a result of a scarcity of slip systems in their hexagonal close-packed structures [2]. Effective ways of improving the ductility and tensile yield strength of Mg al‐ loys include grain refinement [3] and control of the texture [4]; these techniques promote prismatic slips and facilitate the creation of large plastic deformations. Recently, alloys of Mg with transition metals (TMs), such as Co, Ni, Cu, or Zn, and rare-earth (RE) metals, such as Y, Gd, Tb, Dy, Ho, or Er, have been found to show superior mechanical properties to those of other Mg alloys [5,6]. A characteristic of these Mg–TM–RE alloy systems is the for‐ mation of a long-period stacking order (LPSO) phase in as-cast materials and/or after heat treatment. In the present study, we examined the annealing properties, tensile properties, thermal stability, and rolling workability of high-strength extruded Mg96Zn2Y2 alloys. Mg96Zn2Y2 alloy contains an LPSO phase as a secondary phase in the dominant α-Mg phase [5,6]. In general, Mg alloys with LPSO phases are known to have greatly enhanced mechani‐ cal properties, whereas their ductility can be maintained only by extrusion and/or plastic de‐ formation treatments of the cast metal. It has been suggested that kink deformations in the LPSO phase and microstructural refinement in the α-Mg phase occur during extrusion de‐ formation [7]. The tensile yield strength, microstructure, fatigue properties, and thermal sta‐

Crystal Plasticity Analysis on Compressive Loading of Magnesium with Suppression of Twinning

The compressive loading behavior of single crystals and bicrystals of magnesium without consideration of deformation twinning has been investigated by crystal plasticity finite element analysis with the aim of fundamental understanding of kink band formation in magnesium alloys with long period stacking ordered structure (LPSO) phase. The basal plane of the single crystal model is set to be parallel to the compressive direction. The result of the compressive loading analysis of single crystals indicates the significant influence of suppression of twinning on the activation of nonbasal slip systems and stress-strain behavior. The compressive analysis of symmetric bicrystal is also performed to clarify the influence of the angle between basal plane and the loading axis. The influence of the introduction of grain boundary and the slight change of crystal orientation is discussed in terms of activated deformation modes.

Influence of grain boundary on activation of slip systems in magnesium: Crystal plasticity analysis

Crystal plasticity finite element analysis method considering the accumulation of geometrically necessary (GN) dislocations was applied to monotonic loading of pure magnesium bi-crystal. The deformation mechanisms considering in the present analysis method are basal slip , prismatic slip , 1st order pyramidal slip , 2nd order pyramidal slip and tensile twinning . Tensile twinning is incorporated into crystal plasticity analysis assuming that twinning plane and direction of shear by twinning are equivalent to slip plane and slip direction, respectively. Critical resolved shear stresses (CRSSs) for each slip system in the literatures were used. Analysis model is designed to investigate the influence of grain boundary on the activation of slip systems. That is, one grain consisting of bi-crystal (grain A) had the crystal orientation whose Schmid factor for prismatic slip is 0.5. The crystal orientation of the other grain (grain B) was slightly deviated from that of grain A. The result of the calculation of tensile loading of the bi-crystal showed that both grains are deformed by the multiple slip of basal slip system, which resulted in the formation of GN dislocation bands.

Quantitative Evaluation of Dislocation Structure Induced by Cyclic Plasticity

In the present study, a new approach is conducted to evaluate dislocation structure induced by cyclic plasticity. First, cyclic plastic loading tests are carried out up to 100 cycles with three different small strain amplitudes on SUS316L stainless steel at room temperature. The test result presents the dependence of the strain amplitude on cyclic hardening and softening behaviors. Specifically, it is found that the cyclic loading test with strain amplitude of 0.25% shows both cyclic hardening and cyclic softening, while the cyclic loading tests with strain amplitudes of 0.75% and 1.0% show no cyclic softening. Secondly, the dislocation structures of the specimens after cyclic loading are observed by using a transmission electron microscope (TEM), and this observation reveals that the dislocation structure after cyclic loading test depends on the strain amplitude. Finally, a quantitative evaluation method of the dislocation structure is also proposed. The TEM images are converted into binary images and the resolution dependence of the generated binary image is used to visualize the characteristics of the dislocation structure. The relationship between strain amplitudes of cyclic plasticity and dislocation structure organization is clarified by the evaluation method. Finally, the heterogeneity of the dislocation structure is discussed.

Crystal Plasticity Analysis of Change in Active Slip Systems of α-Phase of Ti-6Al-4V Alloy under Cyclic Loading

In this paper, we investigated changes in active slip systems of α-phase of Ti-6Al-4V alloy under a cyclic plastic loading using a crystal plasticity finite element method. In the analyses, a bicrystal model was employed, and the crystallographic orientations were set so as that prismatic or basal slip system was the primary slip system in each grain. The results showed that there was a mechanism where the basal slip systems could reach the stage of activation under the cyclic plastic loading even though the condition was that the prismatic slips initially operate. The reason for the activity changes was due to the changes in the incompatibility between the grains by the work hardening, and the effect of the incompatibility on activities of slip systems appeared even in the perpendicular arrangements of the grains to the loading direction.

Influence of long period stacking ordered phase on non-uniform deformation in cast Mg-Zn-Y alloys

Influence of long period stacking ordered (LPSO) phase on non-uniform deformation in polycrystalline as-cast Mg-Zn-Y alloys was investigated by using a crystal plasticity finite element method. Material parameters for α-Mg phase and LPSO phase were identified by fitting to experimental stress-strain curves of Mg99.2Zn0.2Y0.6 and Mg85Zn6Y9 of which volume fractions of LPSO phase were 0% and 100%, respectively. The results of calculations showed that the increase in volume fraction of LPSO lead to the increased strain accumulation in α-Mg phase.

Surface Morphological Changes in CP-Ti and SUS316L under Pure Tensile Loading and Creep

The present paper investigates relationships between the macroscopic viscoplasticity and the surface morphological changes at room temperature for commercially pure titanium (CP-Ti) and austenitic stainless steel (SUS316L). Pure tensile test and tension-intermittent creep test are conducted. Both CP-Ti and SUS316L are deformed up to 16% of inelastic strain with a few unloading, and surface conditions are observed during pure tension test and tension-intermittent creep test. Qualitative surface observations and quantitative surface roughness measurements are made for the unloaded specimens. The surface roughness measurement shows that the curves plotted between surface roughness and inelastic strains are almost linear for all the present experiments. The slopes of curves depend, however, on material and type of tests, and this tendency agrees well with the qualitative surface observations by an optical microscope. The experimental results for CP-Ti suggest that different deformation mechanisms during tensile loading and creep contribute to different surface morphological changes.