Extrusion, Properties, and Failure of Spray-Formed Hypereutectic Al-Si Alloys Based on the Optimization of Fe-Bearing Ph
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HYPEREUTECTIC Al-Si (hyper-AS) alloys that are light weight and have good wear resistance have been used as automobile materials, such as coatings to the inner surface of the cylinder bores or engine wear-resistant parts.[1,2] Generally, the Si phase and other intermetallics with higher hardness and elastic modulus should be the most important contribution to the good wear resistance of the hyper-AS alloys. To some heat-resistant parts such as cylinder liners, the high stiffness should be also the key property required by these hyper-AS alloys. This means that the higher mechanical properties at room or elevated temperatures should be obtained from these alloys. Thus, the size, morphology, distribution of the Si phase and other intermetallics will be mainly in charge of the mechanical properties as well as wear resistance.[3] The traditional processing, such as grain refinement, electromagnetic stirring, thermomechanical treatments, and L.G. HOU, Research Associate, Y.H. CAI, Engineer, and L.Z. ZHUANG and J.S. ZHANG, Professors, are with the State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, P.R. China. Contact e-mail: [email protected] H. YU, Engineer, is with the Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, P.R. China. H. CUI, Senior Engineer, is with the School of Materials Science and Engineering, University of Science and Technology Beijing. Manuscript submitted June 4, 2012. Article published online November 10, 2012 1814—VOLUME 44A, APRIL 2013
others,[4–10] were widely employed to control the size and morphology of the Si phase and intermetallics. Therefore to some extent, they can improve the properties. However, when they are used as engine parts, the traditional processed hyper-AS alloys cannot satisfy the property requirements of the advanced engine techniques with much high inner pressures and temperatures. As a result, the high strength, heat resistance, and good machinability of these alloys should be gained simultaneously for the wear-resistant parts. The rapid solidification techniques (for example, spray forming) were introduced to prepare hyper-AS alloys within recent decades[2,11–16] for the refined and uniform microstructures as well as good properties. Although two classic alloys, Al-20Si-4Cu-1Mg and Al20Si-5Fe-2Ni (wt pct), developed by PEAK Werkstoff GmbH (Velbert, Germany),[17] can contain fine Si particles and some reinforcements (e.g., Al2CuMg, Al9FeNi, Al2Cu, and Mg2Si, etc.), to some extent, their high-temperature properties and thermal stability cannot meet the actual requirements because of the fast diffusion of the alloying elements, such as Cu, Mg, and Ni.[18,19] Compared to Fe/Ni, the transition elements Mn/Cr possess much lower diffusion coefficients and high activation energy for their diffusion to Al matrix,[19,20] resulting in the amelioration of the heat resistance of the aluminum alloys. In addition, the introduction of Mn/Cr can form the fine and stable granular a-Al
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