Papers by Author: Hiromoto Kitahara

Abstract: In this study, tensile tests of rolled Mg-Li alloy and Mg-Ce alloy sheets were carried out at room temperature to investigate effects of alloying additions on the relationship between mechanical properties and activities of slip systems in magnesium polycrystals. In Mg-Li alloy, ductility increased while strength decreased by lithium addition. Frequency of non-basal slips increased with increasing lithium content. In Mg-Ce alloy, strength and ductility were similar pure magnesium, and non-basal slips were hardly activated. Since critical resolved shear stress of non-basal slips were decreased by lithium addition, ductility of magnesium was increased while its strength was decreased.

Abstract: Tensile tests of Mg-Y single crystals with different yttrium concentrations: 0.07 and 0.3 at.% were carried out to investigate effects of yttrium on pyramidal <c+a> slip system. In Mg-0.07at%Y alloy single crystals, {11 2}< 23> second order pyramidal <c+a> slip was activated and yield stress increased, compared to pure Mg single crystals. On other hand, in Mg-0.3at%Y alloy single crystals, {10 1}< 23> first order pyramidal <c+a> slip was activated and yield stress increased, compared to Mg-0.07at%Y alloy single crystals. The change in slip system by yttrium addition would be caused by increasing critical resolved shear stress (CRSS) for second order pyramidal slip.

Abstract: In this study, to investigate effects of yttrium and other elements for non-basal slips, magnesium alloy single crystals were stretched parallel to basal plane in various temperatures, and polycrystalline magnesium alloys were also tested to estimate contribution of non-basal slips to their tensile deformation behaviour. In pure magnesium single crystals, second order pyramidal (c+a) slip (SPCS) was observed at 298K. Above room temperature, first order pyramidal (c+a) slip (FPCS) was active. In the Mg - (0.6-0.9) Y alloy single crystals, FPCS was observed at 77K to 298K, while yield stress of the Mg-Y alloy single crystals was higher than that of pure magnesium. In tensile test of polycrystalline specimen, slips lines of non-basal slip systems such as SPCS, FPCS and prismatic slip were observed even at yielding in addition to basal slip lines. Among the non-basal slips, activities of FPCS and prismatic slips were increased with increasing strain in Mg - Y alloy polycrystals. Our study suggested that active non-basal slip system in tension parallel to basal plane is (c+a) pyramidal slip and enhanced ductility of magnesium - yttrium alloy would be caused from increased activity of FPCS by yttrium addition.

Abstract: It is well known that magnesium (Mg) shows anisotropic fatigue behavior. However, the fatigue mechanisms have yet to be elucidated. The relationships between crystal orientations and crack initiation behavior in Mg single crystals were investigated by uniaxial tension-compression fatigue tests. Three types of round-bar specimens were prepared. The lording direction of AD, BC and EF specimen were [110], [100] and [0001], respectively. Fatigue tests were carried out with the stress ratio R=-1 and the frequency of 10Hz at room temperature in laboratory air. At stress amplitude (σ_a) over 40 MPa, fatigue lives of BC specimen and EF specimen were the longest and shortest. However, at σ_a =20 MPa, the fatigue life of all specimens were almost the same. It was found that fatigue lives of Mg single crystals strongly depend on crystal orientations and stress.

Abstract: According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip and alloying effect for the non-basal slip, pure magnesium and Mg-Al-Zn single crystals were stretched in the [110] direction. While {112}<23> second order pyramidal slip was activated at room temperature in pure magnesium, {101}<23> first order pyramidal slip became active slip at higher temperature. In Mg-Al-Zn alloy single crystal, {101} twin also activated by adding aluminum. These results indicate that active non-basal slip systems and twin in magnesium strongly depend on deformation temperature and alloying elements.

Abstract: Zn and Al are major alloying elements of Mg alloys. Main slip system of Mg is a basal slip and the CRSS increases with Zn or Al content. According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip, Mg-Zn and Mg-Al single crystals were stretched in the [110] direction and Mg-Zn single crystals were compressed in the [0001] direction. {112}<23> second order pyramidal slip was activated in Mg-0.1at%Zn and Mg-0.5at%Al. On the other hand, {101} twin was mainly activated in Mg-1.0at%Al alloy. Yield stress due to the pyramidal slip of magnesium decreased by 0.1at%Zn addition, however they increased with addition of aluminum..

Abstract: Fatigue crack propagation behaviors of ultrafine grained (UFG) Al sheets fabricated by the accumulative roll bonding (ARB) process were investigated. The ARB process was carried out up to 6 cycles (equivalent strain, eq.=4.8). The ARB processed sheet had lamellar boundary structure elongated to rolling direction of the sheet. The mean spacing of the boundaries was 182 nm. The tensile strength of the starting Al sheet increased after the 6-cycle of the ARB. Fatigue crack growth tests were performed to clarify the crack growth rate and threshold stress intensity factor range for crack growth (Kth). The fatigue crack profile in the ARB processed sheet differs from that in the starting Al sheet. The Kth of the ARB processed sheet was smaller than that of the starting sheet. The Kth of Al would decreased with decreasing the crack closure phenomena after the 6-cycle of the ARB. The fatigue crack growth rate test shows that the critical load for starting to propagate the fatigue crack and the fatigue crack growth rate decreased by ultrafine grain refinement.

Abstract: {10 2} twin is common in pure hcp metals. In this study, initiation and development of {10 2} twin in hcp metal was simulated by molecular dynamics (MD) method. Two types of model crystals were stretched along the y-axis by applying displacement of 0.005a (a is lattice constant of the basal plane) every 1000 MD steps and relaxed atoms by molecular dynamics method. The Y-boundary of the model was applied fixed boundary condition and the X and the Z-boundary were free boundary condition. The Lennard-Jones type interatomic potential was employed in this simulation. In the single crystal, {10 1} pyramidal slip initiated at the crack tip and the pyramidal slip was stopped at Y-boundary atoms. After that, a {10 2} twin was initiated at a front of the slip. With increasing external strain, the twin grew with increasing external strain. In the bi-crystal, {10 0} prismatic slip occurred at the crack tip in Crystal 1 and a-dislocation of the slip stopped at a grain boundary between the Crystal 1 and 2. With increasing applied strain, {10 2} twin was initiated in the Crystal 2 at the grain boundary. From the simulation results, the ‘shuffling’ process of twin deformation was estimated.

Abstract: In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals by non-basal slips and twins, c-axis compression and a-axis tension tests were performed in the range of 77K-573K. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression was bigger than that of a-axis tensile. In compression, fracture surface were (11 4) under 293K and were {30 4} above 373K, and fracture strain was smaller than the case of tension test. {10 1}-{10 2} double twin were activated at higher temperature and the crystal, therefore, fractured along the twin interface.

Abstract: In this study, effects of high pulse current (current density :>103 A/cm2, applied time: 15 s) on microstructure of magnesium alloys were investigated. A mild steel was also employed as comparison material. Refinement of microstructure by electropulsing was observed in Mg-3B2O3 and mild steel and micro-Vickers hardness of these alloys were increased by modification of microstructure.