Strain-hardening behavior and microstructure development in polycrystalline as-cast Mg-Zn-Y alloys with LPSO phase subjected to cyclic loading

The strain-hardening behavior and microstructural development of polycrystalline as-cast Mg-Zn-Y alloys with various volume fractions of the long-period stacking ordered (LPSO) phase subjected to cyclic loading were experimentally evaluated. For all alloys, cyclic loading tests with a constant strain amplitude of 0.5% for up to 100 cycles showed asymmetric cyclic hardening behavior. That is, the absolute value of the compressive peak stress significantly increased during cyclic loading while the tensile peak stress slightly decreased. With increasing volume fraction of the LPSO phase, the stress amplitude significantly increased. Cyclic loading tests after compressive preloading up to 200 or 250\ua0MPa resulted in a significant increase in the stress amplitude, while a number of kink bands developed during preloading. For the cyclic hardening behavior, the contribution of the increase in kinematic hardening was significant in the alloys with a higher volume fraction of the LPSO phase. Transmission electron microscopy observation of the cyclically deformed MgZnY alloy indicated the formation of a deformation-induced band, where the crystal structure was transformed from 18R-LPSO to hcp-Mg with the exclusion of solute elements

Lip Formation and Ejecta from LPSO-type Magnesium Alloy Plates in Hypervelocity Impact

Long period stacking ordered (LPSO) type magnesium alloys have the low density, excellent mechanical strength and ignition resistance. LPSO-type magnesium alloys have a great potential as structural materials of satellites. Lip formation and ejecta size were examined when spherical projectiles strikes thin plates made of LPSO-type magnesium alloy at hypervelocities of 5 km/s. Witness plates were placed in front of and behind each target to determine the scattering area. After impact experiments, ejecta were collected from test chamber and lips near penetration hole was examined x-ray computed cosmography (CT) in detail. Results of LPSO-type magnesium alloy plates were compared with those of aluminum alloy (A6061-T6). Images of scattering ejecta taken by a high speed video camera were also discussed.11th International Symposium on Plasticity and Impact Mechanics(IMPLAST 2016), 11 - 14 December 2016, Indian Institute of Technology Delhi, New Delhi, Indi

Unified Constitutive Model Considering Equivalence of Back Stress Due to Plastic and Viscoplastic Deformation

This paper treats a unified constitutive model for the viscoplastic and creep deformations. First, experimental observations on the correlation between back stress due to plasticity and creep are carried out using Type 304 stainless steel at room temperature. The experimental results show that there is the equivalence of back stress due to plastic and creep deformation. Then considering the equivalence of back stress, a unified constitutive model for plasticity and viscoplasticity is proposed. To verify the applicability of this model, the numerical simulations are conducted by the model, and compared with the experiments.Key Engineering Materials, Volume 233-23

Constitutive Model for the Subsequent Time-Dependent Deformations of Type 304 Stainless Steel at Room Temperature

In this study, subsequent time-dependent deformation after cyclic preloading and a constitutive model is discussed. Stress-strain curves of 10th and 30th cycle of cyclic loading under the strain rate of 0.01%/sec with the strain amplitude of 0.5% using Type 304 stainless steel at room temperature are almost same. However, creep and stress relaxation after the two cyclic loading are different due to the number of cycle of the cyclic preloading. The experimental results are simulated by a unified constitutive model. The model is constructed referring to dislocation observations after 10th and 30th cycles of cyclic loading by using TEM (Transmission Electron Microscope). The constitutive model shows good agreements with the experimental results of the subsequent creep and the subsequent stress relaxation.Key Engineering Materials, Volume 274-27

Deformation behavior of a magnesium alloy sheet with random crystallographic orientations under various loading paths

This paper reports deformation behavior under various loading paths at room temperature of a magnesium alloy sheet with random crystallographic orientations both experimentally and numerically. The flow stress under uniaxial tension was larger than that of uniaxial compression. When the sheet was subjected to reverse loading, a slightly sigmoidal curve occurred under compression followed by tension, whereas it did not occur under tension followed by compression. These results demonstrated that the sample showed a tension-compression asymmetry and a strain-path dependency as in the case of rolled Magnesium alloy sheets with strong basal texture. Results of crystal plasticity finite-element simulation depicted that these characteristics occurred because of a tension-compression asymmetry in twinning activity even in the sheet with random crystallographic orientations. These results exhibited that a tension-compression asymmetry and a strain-path dependency were inevitable in the deformation of magnesium alloys at room temperature irrespective of initial textures. An evolution of contour of plastic work was also investigated numerically to support this conclusion