Tomoyuki Homma

Development of an extruded Mg–Zn–Ca-based alloy: new insight on the role of Mn addition in precipitation

An age hardenable Mg–2.5Zn–0.1Ca–0.1Mn (mol%) alloy has been developed. The extruded sample followed by a T5 treatment reveals 290 and 269 MPa of tensile and compressive proof stresses, respectively. Elongation to failure in tension and compression were 22 and 13%, respectively. The stresses were comparable to those in an as-extruded Mg–2.4Zn–0.1Ca–0.1Zr (mol%) and a T6-treated 6063 Al alloys. and phases appeared in the extruded samples, and these precipitates enhanced the proof stresses. Mn was detected in the fine precipitates, which may lead to dense distribution of the fine precipitations. The precipitates pinned the grain growth of dynamic recrystallisation and, therefore, fine grain size could be obtained. The fine precipitates suppressed the occurrence of tensile twinning in unrecrystallised regions, resulting in a high value associated with yield anisotropy.

Application of the Bons–Azuma method and determination of grain growth mechanism in rolled Ti–Zr alloys

Zr-containing Ti alloys have widely been developed owing to the infinite solid solubility of Zr in Ti and its avirulence, leading respectively to high strength and good biocompatibility. It is known that the Zr addition gives rise to grain refinement when rolled Ti–Zr alloys are annealed; nevertheless, the governing mechanism by which Zr addition in Ti can reduce grain size is not fully understood. In this study, the grain growth behaviour of rolled Zr-free and Zr-containing (Ti–10Zr, wt.%) alloys is analysed using analytical transmission electron microscopy and the classical and Bons–Azuma methods by evaluating the grain growth exponent. Irrespective of the evaluation technique and Zr content, the grain growth exponent is found to be close to ~0.3, indicating the occurrence of normal grain growth in the Zr-free alloy and solute drag mechanism in the Zr-containing alloy. It is found that the grain size and grain growth rate are significantly reduced by Zr segregation near grain boundaries, resulting from the solute drag mechanism.

Dispersion of nanoscale oxides in MnSi1.73 fabricated by solid state reaction and pulsed electric current sintering

We have investigated a relationship between the fabrication process and the resultant crystal structure of a MnSi1.73 thermoelectric material in order to understand the effect of microstructures on the thermoelectric properties. Local crystal structure of the matrix phase is identified as a modulated Mn15Si26 phase based on the Ye and Amelinckx method [J. Solid State Chem. 61, 8 (1986)]. The Mn15Si26 phase is a degenerate semiconductor showing metal like behavior. In addition, nanoscale SiO2 particles are dispersed. The crystal structure of SiO2 is amorphous, and the SiO2 particles reduce thermal conductivity because of its inherent characteristic. A relatively high figure of merit of 0.38 is obtained, resulting from the multiple microstructures of Mn15Si26 and SiO2.

Effect of Mn Addition on Creep Property in Mg-2Al-2Ca Systems

Planar Al2Ca phase forms on the basal plane in the as-cast AX22 and AXM220 alloys. In the AXM220 alloy, Mn is detected in the Al2Ca phase. The number density of the Al2Ca phase does not alter, irrespective of the creep deformation and Mn addition. Fine and planar precipitates appear during the creep deformation. The number density increases by the addition of Mn. The size of the precipitate is reduced by the Mn addition. The Mn addition can improve the creep property.

Enhancement in age hardenability of sintered Ti–5Fe alloy by Zr addition processed by pulsed electric current sintering

ABSTRACT The age-hardenable Ti–5Fe–5Zr (wt. %, 5Zr) alloy has been consolidated by pulsed electric current sintering, following a β solution treatment, and the results are compared with a Ti–5Fe (0Zr) alloy. The precipitation sequence measured at 640°C ageing is β + athermal ω → β + isothermal ω → β + α. At the peak hardness isothermal ω phase forms at 20 s of ageing. The Zr addition retards the precipitation kinetics of the α phase; as a result, the α phase nucleates at latest at 300 s ageing in the overaged state. Fe is partitioned into β, while it is depleted from the α phases. There is Zr enrichment near the α/β interface when the α phase precipitates due to a solute drag effect; the growth rate of the α phase in the 5Zr alloy is significantly reduced compared with that in the 0Zr alloy.

Effect of Zr Addition on Recrystallization Behavior in Rolled Ti‐Zr Alloys

As-rolled and annealed microstructures of Zr-free and Zr-containing (2 and 10 wt. % Zr) αTi alloys have been evaluated by optical microscopy and analytical transmission electron microscopy. In the as-rolled samples, grain refinement by the Zr addition is accomplished, and 0.2% proof stress of 833 MPa in the 10Zr alloy is obtained. The mechanical properties are enhanced as the Zr content increases. When annealing is applied to the as-rolled samples at 750 °C, then rapid grain growth of recrystallized grains occurs in the Zr-free alloy. However, it is found that the grain growth is sufficiently suppressed by the Zr addition due to a solute drag effect, and this is obvious when the Zr content increases. The grain refinement by the Zr addition leads to high strength in the Zr-containing alloys as similar to the as-rolled samples following the Hall-Petch rule.

Improvement of the mechanical properties of Mg-Gd-Y-Zn alloy castings by grain refinement

The effects of the addition of grain refining elements and a high cooling rate on the mechanical properties of cast Mg-Gd-Y-Zn alloys are investigated. The relationship between the average grain size and hardness may be correlated with the Hall-Petch equation. Tensile properties obtained at 250°C in Mn- or Zr-containing alloys are much higher than that those of a T6-treated A4032 alloy. Grain refinement is a more effective means of enhancing the mechanical properties.