Yo Kojima

Platform Science and Technology for Advanced Magnesium alloys

The four years project Platform Science and Technology for Advanced Magnesium Alloys started in September 1999 with financial support from the Ministry of Education, Science. Sports and Culture of Japan. The project members consisting mainly of Professors in Japanese Universities are going to make various efforts for the advancement of studies on magnesium alloys. The background and concept of the research project is described, and then the contents and practical activities of the group are briefly explained.

Realization of high strength and high ductility for AZ61 magnesium alloy by severe warm working

Abstract Extruded Mg–6%Al–1%Zn (AZ61) alloy bar was subjected to 4-pass Equal Channel Angular Extrusion (ECAE) processing at 448–573 K. At the processing temperature of 448 K, extremely fine grains with the average grain size of 0.5 mm are formed as a result of dynamic recrystallization originated by fine Mg17Al12 (b) phase particles having 50–100 nm diameter dynamically-precipitated during ECAE processing. The sizes of both α matrix and β phase decrease with decreasing processing temperatures. In tensile test at room temperature under the strain rate of 1×10—3 s—1, tensile strength increases with decreasing ECAE processing temperatures due to fine grains, fine precipitates and residual strain hardening. Especially, highest strength of 351 MPa was achieved in the specimen ECAE-processed at 448 K. In addition to such high strength, elongation reaches 33% in that specimen. This specimen exhibits clear strain rate dependencies of both flow stress and elongation even at room temperature. As a result, higher elongation of 67% is obtained under low strain rate of 1×10—5 s—1.In such specimen, non-basal slip and grain boundary sliding occur in addition to basal slip. Furthermore, there are grains with no dislocations, suggesting the occurrence of dynamic recovery. The contribution of all the deformation mechanisms would cause high ductility in fine-grained AZ61 alloy specimen with high strength.

Aging behavior of squeeze cast SiCw/AZ91 magnesium matrix composite

Abstract The aging behavior of SiC whisker reinforced AZ91 magnesium matrix composites was investigated with Vickers hardness measurement, differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The precipitation process observed in the monolithic alloy was not altered by the addition of SiC whiskers. The composite exhibited an accelerated hardening response compared with the unreinforced matrix alloy. However, the addition of SiC whiskers altered the distribution of the Mg17Al12 precipitates. Mg17Al12 precipitated preferentially at the SiCw–AZ91 interface, with a definite orientation relationship with SiC whisker: (110)Mg17Al12//(111)SiCw, [111]Mg17Al12//[01 1 ]SiCw. The preferential interfacial precipitate decreased the Al content in the matrix, resulting in a decrease in the amount of continuous precipitates. As a consequence, the age-hardening efficiency in the composite was lower.

Microstructure and mechanical properties of aluminum borate whisker-reinforced magnesium matrix composites

Squeeze casting technique was used to fabricate Al18B4O33w/MB15 and Al18B4O33w/MB8 magnesium matrix composites. The microstructure and mechanical properties of the composites were studied. The aluminum borate whisker is very effective in the improvement of the mechanical properties of the magnesium alloys. A continuously uniform MgO interfacial reaction layer of 50–80 nm was formed at the matrix–whisker interface in the two kinds of composites, which resulted form the reaction between Al18B4O33 whisker and Mg matrix. The interfacial reaction layer may effectively prevent the further reaction between aluminum borate whisker and magnesium alloys. D 2002 Elsevier Science B.V. All rights reserved.

Fabrication of NiTi intermetallic compound coating made by laser plasma hybrid spraying of mechanically alloyed powders

NiTi intermetallic compounds not only have shape memory effects but also high erosion resistance. Therefore, applying this material as a coating is an effective method for preventing erosion. In this study, a mixture of Ti and Ni powders was subjected to a mechanical alloying process. Then, the mechanical and structural properties of the coating fabricated by vacuum plasma spraying and laser plasma hybrid spraying were examined. The results of this study are summarized as follows: (1) Mechanically alloyed powders suitable for plasma spraying were obtained under the conditions of 2 mass% methyl alcohol as a process control agent and 72 ks alloying duration. (2) A NiTi intermetallic compound coating was directly fabricated by vacuum plasma spraying. This means that nickel and titanium were alloyed while they were passed through a plasma flame. The erosion resistance of this coating is more than 20 times as high as that of the coating made from a simple mixture of nickel and titanium powders. (3) The erosion resistance of the coating made by laser plasma hybrid spraying of mechanically alloyed powder was approximately 40 times as high as that of titanium alloy. This high erosion resistance was obtained at low relative fluence compared with that of the coating made from the simple mixture. This coating can prevent degradation of the interface, because a high fluence induces the formation of a brittle intermetallic compound at the interface.

Fundamental Magnesium Researches in Japan

In the current four-year term project in Japan, new platform science and technology is proposed as a core concept of research and development of advanced magnesium alloys together with understanding of their intrinsic characteristics. The research fields related to advanced super-light magnesium alloys for 21st Century have been focused to the selected three categories; ecomaterial design and processing, high qualification of mechanical performance, and high performance design and processing in functionality. On the basis of the obtained results, platform science and technology for environmentally benign and high performance magnesium alloys is constructed as an industrial base material for the next generation. As a result, numerous large-scale joint research and development projects on magnesium alloys based on partnerships between industries, academia and government has already started towards practical utilization since last year.

Improvement of Tensile Properties of Wrought Magnesium Alloys by Grain Refining

ECAE process was applied to magnesium alloys containing 3mass%Al or Zn, and the effect of isochronal annealing after ECAE process on the tensile properties of the alloys was evaluated in order to improve tensile properties of the magnesium alloys. When the investigated alloys are subjected to ECAE process, the grains of 1-pass specimens of all the investigated alloys are elongated towards 30° from the extrusion direction and 4-pass specimens of AZ31 and ZK31 alloys exhibit fine fiberous microstructure. In 1-pass specimens, recrystallization partly occurs by isochronal annealing at 300°C for lh. while complete recrystallization occurs by annealing at 400°C for lh. However, the recrystallized grains coarsen when annealing at 400°C for 1h. In 4-pass specimens, the temperatures at which recrystallization finishes become lower, and the grain sizes are finer than those of 1-pass specimens. As a result of the above-mentioned microstructural changes, particularly in grain size by annealing, elongation is improved remarkably and 0.2% proof stress decreases with an increase in annealing temperatures. As-ECAE and 300°C-annealed samples of 4-pass specimens of ZK31 alloy have tensile properties comparable to fully heat treated 6061 forging alloy due to grain refining by ECAE process.

Fabrication of Surface Modification Layer on Stainless Steel by Electrical Discharge Machining

Abstract A stainless steel (SUS 304) was machined by Electrical Discharge Machining (EDM) with a sialon and chromium electrode. According to the EDMed conditions, the deformed stainless steel surface turned to the coated surface with an electrode components. The following experimental factors were investigated to produce the modified surface: electrode polarity, work atmosphere, peak current, pulse duration and duty factor. In the suitable worked conditions, the modified layer had good corrosion and wear resistance.

Heat Resistance of Mg-Zn-Al-Ca Alloy Castings

New Mg-Zn-Al-Ca based alloys are evaluated for possible application at elevated temperatures. The as-cast microstructures of these alloys show that the amount of eutectic compounds increases with an increase in Zn+Al content, and the compounds are found to be mainly Mg 32 (Al,Zn) 49 or MgZn. High temperature tensile strength and 0.2% proof strength increase with an increase in the amount of compounds. The creep properties of the alloys are far superior to those of conventional die-cast AZ91D alloy. However, the presence of soft Mg 17 Al 12 compound deteriorates the creep properties of high Al containing alloys.

Development of magnesium alloys with high performance

In Mg-Al-Zn and Mg-Al-Mn alloys containing 2.0~6.0mass%Al and 0~1.5mass%Zn, grain refinement in the as-rolled (F) specimens containing large amount of Al and Zn are achieved by both dynamic recrystallization and dynamic precipitation during hot rolling and leads to high strength and high ductility at room temperature. At high temperatures, the tensile strength of the investigated alloys is almost the same, while the elongation of the F-specimens increases with increasing Al and Zn contents, leading to 150% in Mg-4.5%Al-1.5%Zn alloy. High Al and Zn contents alloys significantly accumulate large working strain in grain interiors, and involve large amounts of high angle grain boundaries and fine spherical precipitates, which can become the nucleation sites for recrystallization. Therefore, dynamic recrystallization in such alloys occurs at small strain region during tensile test. This dynamic recrystallization causes reduction of flow stress and large elongation by grain boundary sliding at high temperatures. Furthermore, .fine recrystallized grains contributes to deformation in normal direction, resulting in isotropic deformation behavior. Authors attempt to improve proof stress and its anisotropic property of Mg-Al-Zn wrought alloys by grain size and precipitates controls utilizing dynamic recrystallization and dynamic precipitation during hot extrusion. In the alloy specimens extruded at lower temperatures increasing Al and Zn contents enhance dynamic recrystallization and dynamic precipitation, resulting in grain refinement and large amount of Mg17Al12 precipitates. As a result, the extruded Mg-9%Al-1%Zn alloy specimen shows high tensile strength of 370MPa, 0.2% tensile proof stress of 240MPa and moderate elongation of 20%, which are almost same as standard values of tensile properties of T5-treated 6N01 Al extruded alloy. Furthermore, a ratio of compressive proof stress to tensile proof stress of the as-extruded specimen improves up to a higher ratio of 0.9 than that of Mg-3%Al-1%Zn alloy specimen with no precipitation, 0.5, due to prevention of tensile twin, which easily occurs during compressive deformation even under a low applied stress perpendicular to the extrusion direction, by dynamic precipitation of Mg17Al12 phase.