Papers by Author: Yoshihito Kawamura

Abstract: Mg alloys are lightweight structural alloys that normally have a good castability and machinability as well as an excellent specific strength and rigidity. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of industrial applications is limited. Recently, Mg–Zn–Y alloy has been found to show a high tensile yield strength with a good elongation. The alloy has a long-period stacking order (LPSO) phase as the secondary phase in an α-Mg phase. In general, the tensile yield strengths of LPSO-type Mg alloy are known to be markedly enhanced by the formation of kink bands in the LPSO phase and by microstructural refinement of the α-Mg phase during plastic deformation. The separate roles of the LPSO phase and the α-Mg phase in relation to the mechanical properties of high-strength LPSO-type Mg alloy were investigated at ambient and high temperatures. For high strengths at ambient and high temperatures, it was important that the α-Mg phase consisted of a fine-grain region and a nonrecrystallized region, and that the LPSO phase remained as a block-type phase. On the other hands, it was necessary to change the LPSO phase from a block-type phase into a plate-type phase by heat treatment before tensile testing to improve the ductility of the alloy while maintaining its tensile yield strength. Microstructural control of the LPSO phase and the α-Mg phase is necessary to obtained Mg–Zn–Y alloy with superior mechanical properties at ambient-to-high temperatures.

Abstract: This paper deals with the influence of fourth element addition to the high strength Mg-Zn-Y extruded alloys with long period stacking ordered (LPSO) phase on corrosion resistance and surface film formation. Salt water immersion test, SEM observation, and EDX analysis are performed on Mg97Zn1Y2(at.%), Mg97Zn1Y1.9La0.1 and Mg97Zn1Y1.9Al0.1 alloys. The Al or La addition to the extruded Mg97Zn1Y2 alloy is effective for the formation of thin, dense and uniform surface layer, resulting in improvement of corrosion resistance.

Abstract: The composition dependence of yield strength and elongation of the Mg-Zn-Y ternary alloy system were investigated. Yield strength was found to increase with the increasing Zn and Y contents, while elongation decreased in Zn- and Y- rich- regions in the system. The composition ratio of Zn to Y, both of which shows large elongation and comparatively high yield strength, was 1:1 in the Mg-rich region of the alloy system. Mg98Zn1Y1, Mg96Zn2Y2, and Mg94Zn3Y3 (at.%) cast alloys exhibited yield strength of 107, 129, and 173 MPa, and elongations of 17, 8, and 12 %, respectively. These alloys consist of Mg-, LPO-, and Mg3Zn3Y2- phases. We also prepared Mg-Zn-Y alloy sheets by hot rolling using cast alloys, and we investigated their microstructure and mechanical properties. The Mg98Zn1Y1, Mg96Zn2Y2, and Mg94Zn3Y3, alloy sheets exhibited yield strength of 276, 319, and 379 MPa, and elongations of 12, 11, and 8 %, respectively. After annealing at 673 K for 3 h, large elongations above 20 % were achieved both of the Mg98Zn1Y1 and Mg96Zn2Y2 alloy sheets.

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: Magnesium alloys are very attractive in such applications as automotive and aerospace. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and this limits their range of applications. In the present study, effects of annealing on the mechanical properties and the thermal stability of the microstructure in LPSO Mg-Zn-Y extruded alloys were investigated. Even when elongation showed 10% with having maintained high strength to annealing temperature at 573 K for 3.6 ks, and this alloy extended annealing time for 1.0 kh, LPSO phase did bent and was able to confirm kink bands. On the other hand, α-Mg phase was fine structure without pronounced grain growth. In elevated temperature deformation of as extruded and annealed materials, LPSO phase did finely dispersed to α-Mg phase by grain boundary sliding of α-Mg phase, but fracture did not occurred in the interface between LPSO and α-Mg phases. Kink bands introduced in LPSO phase during plastic deformation and fine-grained α-Mg phase were important to obtained of high strength and ductility.

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: Interests in magnesium alloys increase as eco-material for its lightweight, and many investigations have been carried out on the development of manufacturing processes and alloy development. In 2001, Mg97Zn1Y2 (at%) alloy with a tensile yield strength of 610 MPa and an elongation of more than 5 % has been developed by rapidly solidified powder metallurgy (RS P/M) processing. The developed alloy was characterized by a novel phase with long period stacking ordered (LPSO) structure. Recently, we have investigated new compositions for LPSO RS P/M Mg-Zn-Y-X alloys in order to improve the corrosion resistance of the RS P/M Mg97Zn1Y2 alloy with maintaining the superior mechanical properties. Consequently, we have developed a RS P/M Mg96.7Zn0.85Y2Al0.45 alloy with high strength and high corrosion resistance. The RS P/M Mg96.7Zn0.85Y2Al0.45 alloy contained the LPSO phase and exhibited a tensile strength of 525 MPa, an elongation of 9 % and a fatigue strength of 325 MPa, which were similar to those of the RS P/M Mg97Zn1Y2 alloy. However, the corrosion resistance of the RS P/M Mg96.7Zn0.85Y2Al0.45 alloy was 1.5 times that of the RS P/M Mg97Zn1Y2 alloy. The specific tensile yield strength, the specific fatigue strength and the corrosion resistance of the RS P/M Mg96.7Zn0.85Y2Al0.45 alloy were about 1.7 times, 1.8 times, and twice those of extra-super-duralumin (7075-T6 or 7075-T73), respectively.

Abstract: Lightweight Mg alloys are being widely adopted in electronic devices and automotive parts. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of applications is limited. Recently, Mg-Zn-Y alloys have been found to exhibit superior mechanical properties to other Mg alloys. However, for use as industrial materials, it is necessary to improve their ductility while retaining their strength. We studied the mechanical properties of extruded Mg96Zn2Y2 alloy with long-period stacking order (LPSO) phases, in addition to the thermal stability of its structure during annealing. The elongation of the alloy improved from 5 % to 10 % with slightly loss of strength up to an annealing temperature of 623 K. Even at this temperature, it was possible to maintain good mechanical properties and a thermally stable microstructure.

Abstract: Recently, a number of amorphous alloys that possess high glass-forming ability and a wide supercooled liquid region before crystallization have been discovered. Especially, bulk metallic glasses, which are made in bulk form with a thickness of ~10 mm at slow cooling rates of the order of 1~100 K/s, have been noted as an industrial application. Hence the welding of bulk metallic glasses to other materials is very important. Explosive welding of most popular Zr41.2Ti13.8Cu10Ni12.5Be22.5 bulk metallic glass to crystalline pure Ti and SUS304 plates is investigated in this paper. The BMGs was found to retain the amorphous structure and the original mechanical properties. The sound bonding with other materials is expected to push forward the application of bulk metallic glass for industrial usage.

Abstract: Microstructures developed by warm extrusion for Mg97Zn1Y2 alloy including long-period stacking order (LPSO) phase have been investigated using SEM and TEM. The extruded magnesium alloy with LPSO phase exhibits high strength and sufficient ductility. Such superior mechanical properties appear by warm extrusion around the temperature of 623K. The microstructure of the extruded alloy consists of matrix of fine-grained hcp phase and elongated grains with fine-lamellae including LPSO phase. The grain size of hcp matrix was about 1μm, indicating that remarkable grain refinement was occurred by extrusion since the grain size of as-cast alloy was about 500μm. Special attention has been paid on the enrichment of solutes at stacking faults and grain boundaries in the fine-grained matrix, which would contribute not only to the strengthening but also to the stability of fine-grained structure because of its role of an inhibiter against grain coarsening.