액상가압공정으로 제조된 텅스텐 연속섬유 및 다공성폼 강화 Zr계 비정질 기지 복합재료의 압축연신율의 향상 연구

Abstract

Amorphous alloys have excellent properties such as strength, stiffness, hardness, and corrosion resistance because of their peculiar liquid-like structures, and thus have been accepted as new advanced materials after amorphous alloys having high glass forming ability have been developed by conventional casting methods. For wider applications of amorphous alloys, however, there remain problems to be solved, typical one of which is brittle fracture. This brittle fracture seriously limits applications to high-performance structural components such as electronic parts and sports goods, and works as an obstacle to long life and good reliability. In order to solve this problem, active studies on developing composites in which secondary phases or reinforcements are dispersed in an amorphous alloy matrix have been conducted.In this study, the method of a large improvement in compressive plastic strain of amorphous alloy was investigated. To avoid degradation of high strength of amorphous alloy, hard tungsten was used for reinforcement. Tungsten reinforcements are thermally stable, and the thermal stress due to the difference of the thermal expansion at the tungsten/matrix interface was considerably reduced. When fabricating cast amorphous matrix composites, it is important to control reactions of reinforcements with the amorphous melt. Liquid pressing process can be one of the answers. This process might be considered as a reliable fabrication method because the crystallization of the amorphous matrix can be prevented or minimized by rapid cooling of the amorphous melt.Zr-based amorphous alloy matrix composites reinforced with tungsten continuous fibers or porous foams were fabricated without pores or defects by liquid pressing process, and their microstructures and compressive properties were investigated. About 65~70 vol.% of tungsten reinforcements were homogeneously distributed inside the amorphous matrix. The compressive test results indicated that the tungsten-reinforced composites showed considerable plastic strain as the compressive load was sustained by fibers or foams. Particularly in the tungsten-porous-foam-reinforced composite, the compressive stress continued to increase according to the work hardening after the yielding, thereby leading to the maximum strength of 2764 MPa and the plastic strain of 39.4%. This dramatic increase in strength and ductility was attributed to the simultaneous and homogeneous deformation at tungsten foams and amorphous matrix since tungsten foams did not show anisotropy and tungsten/matrix interfaces were excellent. Dynamic deformation behaviors were also investigated by using a Split Hopkinson pressure bar. Under dynamic loading, fracture mechanisms of those composites quite different from quasi-static loading. The maximum stresses of the composites were increased but plastic strain decreased. This is because at the region which hit by projectile, stress concentration is much higher and cracks are easily initiated. Work softening of amorphous matrix was also observed.