Improvement of Wear Resistance in Alumina Matrix Composites Reinforced with Carbon Nanotubes

Alumina matrix composites reinforced with carbon nanotubes (CNTs) fabricated by CNT purification, mixing, compaction, and sintering processes, and the effects of the CNT addition on wear resistance were investigated in relation to the relative density, hardness, and fracture toughness. Wear resistance and fracture toughness were measured by the dry sliding wear test method and the indentation fracture test method, respectively. Zero to ~3 vol pct of CNTs were homogeneously distributed in the composites, although some pores existed. The wear resistance and fracture toughness increased with an increasing CNT fraction, but the composite specimen containing 3.0 vol pct of CNTs hardly showed an increase over the specimen containing 2.25 vol pct of CNTs. Observations of worn surfaces revealed that the wear mechanism involved both the abrasive and delamination wear modes in the specimens containing 0 to ~0.75 vol pct of CNTs, whereas the surface was worn largely in an abrasive wear mode in the specimens containing 1.5 to ~3.0 vol pct of CNTs. This was because CNTs helped to change the delamination wear mode to the abrasive wear mode by preventing crack initiation and propagation at alumina grains. The fracture toughness increase provided beneficial effects in the resistance to crack initiation and propagation, the reduction in delamination wear on the worn surface, and the consequent improvement in wear resistance. Because the effect of the porosity increase due to the CNT addition unfavorably affected the improvement of wear resistance and fracture toughness in the specimen containing 3.0 vol pct of CNTs, the appropriate level of CNT fraction was 1.5 to ~2.25 vol pct.     

Correlation of microstructure and fracture toughness in high-chromium white iron hardfacing alloys

A correlation is made of microstructure and fracture toughness in hypereutectic high-chromium white iron hardfacing alloys. In order to investigate the matrix effect of these alloys, in particular, four different matrices such as pearlite, austenite, and a mixture of pearlite and austenite were employed by changing the ratio of Mn/Si, while the total volume fraction of carbides was fixed. The hardfacing alloys were deposited twice on a mild steel plate by the self-shielding flux-cored arc-welding method. Fracture toughness was increased by increasing the volume fraction of austenite in the matrix, whereas hardness and abrasion resistance were nearly constant.In situ observation of the fracture process showed that cracks initiated at large primary carbides tended to be blocked at the austenitic matrix. This suggested that fracture toughness was controlled mainly by the amount of austenite in the matrix, thereby yielding the better toughness in the hardfacing alloy having the austenitic matrix. Considering both abrasion resistance and fracture toughness, therefore, the austenitic matrix was preferred for the high-chromium white iron hardfacing alloys.     

Fracture mechanisms of a 2124 aluminum

This study was aimed at investigating the effects of microstructure on the fracture behavior of a 2124 aluminum composite reinforced with SiC whiskers. Particular emphasis was placed on the role of matrix intermetallic particles, inhomogeneous distribution of whiskers, and whisker breakage in the fracture process. Various tests were conducted on the composite to identify the micromechanical processes that were involved in microvoid or microcrack formation. Detailed microstructural analyses showed that the aluminum matrix contained a significant amount of coarse manganese-containing particles of various sizes which could have been formed during composite processing.In situ scanning electron microscope (SEM) fracture study of the crack initiation and propagation processes clearly showed that these coarse particles fractured prior to matrix/whisker decohesion or whisker breakage, suggesting that the manganese-containing par- ticles significantly accelerated crack initiation in the 2124 Al-SiC_w composite. For a better ma- trix alloy for use in the composite, it is suggested that microalloying elements must be monitored to prevent the formation of the coarse intermetallic particles.     

Fabrication of multilayered titanium aluminide sheets by self-propagating high-temperature synthesis reaction using hot rolling and heat treatment

This study is concerned with the fabrication of multilayered and bulk Ti aluminide sheets by self-propagating high-temperature synthesis (SHS) reaction using hot rolling and heat treatment. A multilayered Ti/Al sheet was prepared by stacking thin Ti and Al sheets alternatively. When this sheet was hot-rolled and heat-treated at 1000°C, a multilayered sheet composed of Ti₃Al and TiAl was made through the process of formation and growth of intermetallic phases at Ti/Al interfaces and porosity reduction. A bulk Ti aluminide sheet having a lamellar structure of TiAl and Ti₃Al was also fabricated successfully by heat treatment at 1400°C.     

Effects of C and Si on strain aging of strain-based API X60 pipeline steels

Four types of strain-based API X60 pipeline steels were fabricated by varying the C and Si contents, and the effects of C and Si on strain aging were investigated. The 0.05 wt% C steels consisted mainly of polygonal ferrite (PF), whereas the 0.08 wt% C steels consisted of acicular ferrite (AF). The volume fraction of AF increased with increasing C content because C is an austenite stabilizer element. The volume fractions of bainitic ferrite (BF) of the 0.15 wt% Si steels were higher than those of the 0.25 wt% Si steels, whereas the volume fractions of the secondary phases were lower. From the tensile properties before and after the aging process of the strainbased API X60 pipeline steels, the yield strength increased and the uniform and total elongation decreased, which is the strain aging effect. The strain aging effect in the strain-based API X60 pipeline steels was minimized when the volume fraction of AF was increased and secondary phases were distributed uniformly. On the other hand, an excessively high C content formed fine precipitates, and the strain aging effect occurred because of the interactions among dislocations and fine precipitates.