Nanocrystalline magnesium alloys having high tensile strength, high elevated-temperature tensile strength, high-strain-rate superplasticity and high thermal stability have been developed in Mg 97 Zn 1 Y 2 (at% I alloy by rapidly solidified powder metallurgy (RS P/M) processing. The tensile yield strength and elongation that were dependent on the consolidation temperature were in the ranges of 480 to 610MPa and 5 to 16%, respectively. Young's modulus of the RS P/M alloy was 45 GPa. The specific tensile yield strength was four times as high as that of a commercial AZ91-T6 alloy, and was higher than those of conventional titanium (Ti-6Al-4V) and aluminum (7075-T6) alloys. The RS P/M alloys exhibited excellent elevated-temperature yield strength that was 510 MPa at 423 K. The RS P/M alloy also exhibited high-strain-rate superplasticity at a wide strain-rate range from I × 10 - to I × 10 0 s -1 and at a low temperature of 623 K. It is expected that the Mg 97 Zn 1 Y 2 RS P/M alloy can he applied in some fields that requires simultaneously the high specific strength at ambient and elevated temperatures and high workability.

Rapidly Solidified Powder Metallurgy Mg97Zn1Y2Alloys with Excellent Tensile Yield Strength above 600 MPa

Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)-based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass-transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF-based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.

Processing of Bulk Metallic Glass

Amorphous, nanoquasicrystalline and nanocrystalline alloys in Al-based systems

Bulk metallic glasses (BMGs) are strong, highly elastic, and resistant to wear but still find limited utility due to their macroscopic brittle nature, high costs, and difficulty of processing, particularly when complex shapes are desired. These drawbacks can be mitigated when BMGs are used in miniature parts (< 1 cm), an application which takes advantage of BMGs' enhanced plasticity at small length scales as well the insignificant material cost associated with such parts. As an alternative to traditional metal processing techniques, thermoplastic forming (TPF)-based microfabrication methods have been developed which can process some BMGs like plastics. In this article, we discuss the properties and fabrication of BMGs on minuscule length scales to explore their prospective application in small-scale devices.

Bulk Metallic Glass: The Smaller the Better

The major thrust underlying the processing of Fe-based composites has been directed towards improving the wear resistance of steel or castiron by incorporating some reinforcing phase, e.g., carbides, oxides, etc. The present article provides a review on the various synthesis routes of TiC reinforced Fe-based composites, i.e., powder metallurgy, conventional melting and casting, carbothermic reduction, combustion synthesis, aluminothermic reduction, electron beam radiation, laser surface melting, and plasma spray synthesis, highlighting the advantages and disadvantages associated with the different routes of synthesis.

A Review on the various synthesis routes of TiC reinforced ferrous based composites

Particulate TiC-reinforced ferrous matrix composites were processed by solidification of (1) an FeTiC melt of appropriate composition from which particulate titanium carbide precipitates and (2) an FeC melt in which particulate titanium carbide is dispersed through electromagnetic stirring. The microstructure of composites processed by precipitating titanium carbide was controlled by controlling melt composition and homogeneity, as well as cooling rate. That of composites processed by dispersing titanium carbide powder in a steel or cast iron melt was controlled by controlling the original melt composition, volume fraction and particle size of titanium carbide added, mixing temperature and time, and cooling rate. The specific wear rate and the friction coefficient between a diamond stylus and the polished surface of composite specimens decreased with increasing volume fraction of titanium carbide and decreasing carbide particle size and spacing. They also decreased with increasing steel matrix microhardness, achieved through heat treatment and martensitic transformation.

Solidification processing and tribological behavior of particulate TiC-Ferrous Matrix composites

The newly developed “clean process” enabled safe atomization and consolidation of fine Mg-based alloy powders. An Mg 85 Cu 5 Y 10 powder atomized at 923 K exhibited an amorphous phase in the particle size range smaller than 25 μm. A fully dense bulk amorphous Mg 85 Cu 5 Y 10 alloy in the cylindrical form 6 mm in diameter was obtained by extrusion at 373 K below T x . Amorphous Mg 87.5 Cu 5 Y 7.5 (smaller than 25 μm) extruded at 673 K above T x had a mixed structure consisting of Mg 24 Y 5 and Mg 2 Cu dispersed in an h.c.p. Mg matrix. The tensile strength for the bulk Mg 87.5 Cu 5 Y 7.5 alloy is 740 MPa at 298 K which is about twice as high as the highest value for conventional Mg alloys. The high strength is presumably due to the formation of the fine microstructure consisting of nanoscale intermetallic compound particulates homogeneously embedded in the Mg matrix with a fine grain size. The oxygen content of this alloy bulk produced by the “clean process” is 0.085 mass% which is half as low as the value for the bulk produced by the ordinary process in which the powders are exposed once in air. The tensile strength of the bulk extruded at an area reduction of 60% by the “clean process” is also about 43% as high as that for the bulk produced by the ordinary process. These data allow us to expect that the newly developed Mg-based alloys will proceed hereafter as a new type of high specific strength material.

T. Suganuma

Papers

Solidification processing and tribological behavior of particulate TiC-Ferrous Matrix composites

Consolidation and mechanical properties of atomized Mg-based amorphous powders