The nano-sized SiCp/Al–Cu composites were successfully fabricated by combining semisolid stirring with ball milling technology. Microstructures were examined by an olympus optical microscope (OM), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Tensile properties were studied at room temperature. The results show that the α-Al dendrites of the composites were strongly refined, especially in the composite with 3 wt. % nano-sized SiCp, of which the morphology of the α-Al changes from 200 μm dendritic crystal to 90 μm much finer equiaxial grain. The strength and ductility of the composites are improved synchronously with the addition of nano-sized SiCp particles. The as-cast 3 wt. % nano-sized SiCp/Al–Cu composite displays the best tensile properties, i.e., the yield strength, ultimate tensile strength (UTS) and fracture strain increase from 175 MPa, 310 MPa and 4.1% of the as-cast Al–Cu alloy to 220 MPa, 410 MPa and 6.3%, respectively. The significant improvement in the tensile properties of the composites is mainly due to the refinement of the α-Al dendrites, nano-sized SiCp strengthening, and good interface combination between the SiCp and Al–Cu alloys.

Bainite transformation is a kinetic process that involves complex solid diffusion and phase structure evolution. This research systematically studies the bainite transformation of austempered 4140 steel in a wide range of isothermal temperatures, in which four bainite phases structures were generated: upper bainite; mixed upper bainite and lower bainite; lower bainite and mixed lower bainite and martensite. The kinetics of bainite transformation has been described with a linear trend using an Avrami n-value. It was found that the bainitic ferrite sheaves grow with widthwise preference. The sheaves are stable when half-grown and are variable in length, due to austenite size limit or soft/hard impingement, or autocatalytic nucleation, or these conditions combined. The full-grown upper/lower bainite sheaves were found to be 1.9 μm/1.2 μm in width under the conditions of this study. Each individual bainite sheave is lath-like instead of wedge-like. The upper bainite sheaves mostly appear as broad-short-coarse lath, while the lower bainite sheaves appear as narrow-long-fine lath. The overall bainite transformation activation energy ranges from 50–167 kJ/mol.

In this research, an austempering heat treatment was applied on gray cast iron using various austempering temperatures ranging from 232 °C to 371 °C and holding times ranging from 1 min to 120 min. The microstructure and hardness were examined using optical microscopy and a Rockwell hardness tester. Rotational ball-on-disk sliding wear tests were carried out to investigate the wear behavior of austempered gray cast iron samples and to compare with conventional quenched and tempered gray cast iron samples under equivalent hardness. For the austempered samples, it was found that acicular ferrite and carbon saturated austenite were formed in the matrix. The ferritic platelets became coarse when increasing the austempering temperature or extending the holding time. Hardness decreased due to a decreasing amount of martensite in the matrix. In wear tests, austempered gray cast iron samples showed slightly higher wear resistance than quenched and tempered samples under similar hardness while using the austempering temperatures of 232 °C, 260 °C, 288 °C, and 316 °C and distinctly better wear resistance while using the austempering temperatures of 343 °C and 371 °C. After analyzing the worn surface, abrasive wear and fatigue wear with the presence of pits, spalls, voids, long cracks, and wear debris were the main mechanisms for austempered gray cast iron with a low austempering temperature. However, only small pits and short cracks were observed on the wear track of austempered gray cast iron with high austempering temperature. Furthermore, the graphite flakes were exposed and ground by the counterpart surface during wear tests. Then, the graphite particles would form a tribo-layer to protect the contact surface.