Papers by Author: Koji Hagihara

Abstract: The powder-bed additive manufacturing (AM) process offers advantages in terms of reduced material waste, ability to create complex shape and a decrease in the lead time from design to manufacturing. Recently, custom-made implant of Ti alloys is being developed by selective laser melting (SLM) in additive manufacturing (AM) process. However, the difficulty in the fabrication of titanium alloys due to their pre-alloyed powder cost, resulting in a limited usage of titanium alloys. To overcome this disadvantage, it is effective to fabricate the Ti alloys by SLM from mixture of pure elemental powders. In this case, it is avoided the preparing of the pre-alloyed powders. Therefore, the purpose of the present study is the trying to the fabrication of the Ti-20at.%X (X = Cr, Nb) alloys from the mixture of pure elemental powders by SLM.

Abstract: The effect of alloying element (such as Cr, Zr, and Ir) addition on the high-temperature creep deformation behavior of C40/C11_b lamellar-structured (Mo_0.85Nb_0.15)Si₂ silicide crystals was examined. The results indicated that these additions all lead to a decrease in the steady-state creep strain rate (SSCR) when the applied stress is parallel to the lamellar interface. To clarify the origin of this, the dependence of the creep deformation behavior on the microstructure was determined in detail. As a result, it was found that the C40 phase acts as a strengthening phase during the deformation of the C40/C11_b duplex-phase crystals. The variant-1-type C11_b phase grains, whose loading orientation is parallel to [001], also acts as an effective strengthening component. The decrease in SSCR by Cr or Zr addition is attributed to the increase in volume fraction of those C40 phase and C11_b-V1 grains. The refinement of microstructure by Ir addition was also found to result in a modest decrease in the SSCR.

Abstract: NbSi₂/MoSi₂ duplex silicide crystals are potentially a new-class of ultra-high temperature structural materials. Improvement in the thermal stability of their lamellar microstructure was accomplished by the addition of a minute amount of either Cr or Zr. The mechanical properties of the duplex silicide, such as fracture toughness and high temperature strength, show strong orientation dependence, thereby suggesting the importance of the control of microstructure to improve their properties.

Abstract: Co-Cr-Mo based alloys have been widely employed as heat resistant materials and as biomaterials for implants because of their high strength and superior wear resistance. In general, the alloys exhibit a very complicated composition-dependent microstructure containing stacking faults and related mechanical properties. Thus, the essential properties must be clarified by using not only polycrystals but also single crystals. To our knowledge, single crystals and related properties have not been reported elsewhere. Thus, Co-Cr-Mo single crystals were grown and used to analyze the microstructure and the related properties. Single crystals with a composition Co-27 mass% Cr-6 mass% Mo alloy defined by ASTM F75 were grown by two single crystal apparatuses: the optical floating zone and the Bridgman methods. The single crystals with the smooth-surface shape were successfully obtained in the Bridgman method under an Ar gas atmosphere at a crystal growth rate of 5.0 or 2.5 mm/h. A portion of the crystals contain Al as Al₂O₃ precipitates from the crucible. Since the Al₂O₃ precipitate induces martensitic phase transformation from fcc (γ) phase to hcp (ε) phase, the single crystals were separated into two parts (a) containing Al₂O₃ precipitate and (b) in the absence of the clear precipitate. The microstructure was significantly altered by the martensitic phase transformation from the γ to ε phase induced by stress field or heating. In addition, variant formation of ε phase has a large influence on the mechanical functions of these Co-Cr-Mo alloys. Novel findings were preliminary obtained in the single crystals.

Abstract: Microstructural factors that govern the plastic deformation of the long-period stacking ordered (LPSO) phase were clarified. The decrease in length of the long-axis for the plate-like shape of LPSO-phase grains increases the yield stress of the alloy in which basal slip is predominant in deformation. On the other hand, the yield stress tended to increase as the thickness of the plate-like shapes of the grains decreased for the alloy in which the formation of deformation kinks carried the strain.

Abstract: High strength and ductile Mg96Zn2Y2 (at%) alloys with multi-modal microstructure are developed. Microstructure of the extruded Mg96Zn2Y2 alloy consists of three regions; the dynamically recrystallized -Mg fine-grains region, the hot-worked -Mg coarse-grains region elongated along extrusion direction, and the long-period stacking ordered (LPSO) phase region with kink deformation bands. Bimodal microstructure evolution in -Mg matrix is influenced by the morphology of the LPSO phase in the as-cast state, therefore, the effect of secondary dendrite arm spacing in cast state on the microstructure evolution and mechanical properties of the extruded Mg-Zn-Y alloy is investigated. An increase in the dynamically recrystallized grains improves ductility of the extruded alloys; the effective dispersion of the LPSO phase enhances strengthening of the alloy.

Abstract: Deformation mechanisms of Mg89Zn4Y7 (at.%) extruded alloy, which is mostly composed of LPSO-phase, was investigated focusing on their temperature dependence. The yield stress of as-extruded alloy showed extremely high value of ~480 MPa at RT, but it largely decreased to ~130 MPa at 300 °C. The decreasing rate of the yield stress could be significantly reduced, however, by the annealing of specimen at 400 °C, by suppressing the microyielding which is considered to occur related by the grain boundary sliding in restricted regions. The yield stress of the annealed specimens with random textures could be estimated by the Hall-Petch relationship by regarding the length of long-axis of plate-like grains as a grain size between RT and 300 °C. The yield stress of the annealed specimens maintained high values even at 200°C, but it also showed large decreases at 300 °C.

Abstract: D022-Ni3V contains three variants whose [001] c-axes are mutually perpendicular. In addition, due to the order-disorder transition the anti-phase domains (APD) are also introduced under some heat-treatment conditions. In this study, the influence of such microstructures; the size of variants and APDs, on the plastic deformation behavior of Ni3V was investigated by using single crystals, in which the microstructure was variously controlled by heat-treatments.

Abstract: Plastic deformation behavior of Ni3V with D022 structure was examined using the single crystals containing two dominant variants of three. At [557] loading orientation, {111}1/6<112] twinning is dominantly operative at low temperatures, but {111}1/2<112] slip is activated at high temperatures accompanied by a rapid drop of yield stress.

Abstract: A porous Ti-48.0at.%Al (Ti-rich TiAl) crystal, in which lotus-type long cylindrical pores were aligned and (γ/α2) two-phase lamellar structure was simultaneously developed, was fabricated by floating zone method under the pressure of hydrogen and helium mixed gas. Plastic deformation behavior and microstructure of the Ti-rich TiAl crystal with lotus-type aligned pores were investigated by focusing on the elongated pore direction. The as-grown and annealed crystals show a well-developed lamellar structure and no texture accompanied by 52% porosity and a mean pore diameter of 380 μm. Yield stress strongly depends on the loading direction against the elongated pore. When loading directions are parallel and perpendicular to the pore direction, yield stresses obey K=1 and 2.5, respectively, in equation of σ=σ0(1-p)K, where σ is the yield stress with pores, σ0 is the yield stress without pores and p is porosity. This reflects macroscopically homogeneous and locally heterogeneous plastic deformation between pores, respectively.