Development of High-Strength High-Temperature Cast Al-Ni-Cr Alloys Through Evolution of a Novel Composite Eutectic Struc
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RECENT technological advancements in the aviation and automobile industries demand high-performance lightweight alloys with stable microstructure that are easily castable and exhibit superior room- and high-temperature (> 150 C) strength and plasticity. The lower density together with good strength gives high specific strength that increases fuel efficiency.[1,2] Among all the light alloys, Al-based alloys have attracted significant attention for structural applications. The densities of other structural alloys, namely, Ti alloys and Fe alloys, are 2 and 3 times greater, respectively, than Al alloys. Thus, the latter exhibits superior materials index (strength-to-density ratio). Moreover, aluminum alloys are also known for their good oxidation and corrosion resistance due to formation of stable, adherent, and protective a-Al2O3 layer.[3–7] However, most of the commercially available Al alloys suffer from a major drawback of drop in strength at elevated temperature (> 150 C). Some of the aluminum alloys containing Si have reported adequate strength around 200 C and a P. PANDEY, C.S. TIWARY, and K. CHATTOPADHYAY are with the Department of Materials Engineering, Indian Institute of Science, Bangalore, Karnataka, 560012, India. Contact email: [email protected] S. KASHYAP is with the Department of Materials Engineering, Indian Institute of Science, and also with the School of Engineering and Technology, BML Munjal University, 67th KM Stone NH-8, Gurgaon, 122413, India. Manuscript submitted May 3, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
drop in strength beyond that.[8–10] A possible strategy to develop Al alloys for service at high temperature is by in-situ reinforcement of hard intermetallic phases in metal matrix through eutectic solidification, where coupled growth of two or more phases occurs simultaneously. Among the binary eutectics, some of the most extensively studied eutectics are Al-Al3Fe, Al-Al2Cu, and Al-Al3Ni.[11–17] Al-Al3Fe and Al-Al3Ni eutectics exhibit rod morphology with fibers of intermetallic phases embedded in Al matrix. On the other hand, the growth morphologies for Al-Al2Cu eutectic are different and consist of an alternate lamella of Al2Cu in Al matrix. These eutectics show good mechanical properties due to the presence of hard intermetallic phases that impede the dislocation motion and, hence, impart strength. However, the high volume fraction of intermetallic phases often compromises the ductility of the alloys.[18–20] The strength and ductility of the eutectics are dependent on interlayer spacing, phase fraction, and morphology. The deformation behavior of rod eutectic is well reported by Misra et al.[21,22] They have shown that the metal rods oriented perpendicular to the crack can lead to crack bridging and, hence, development of plasticity.[21,22] Cracks oriented parallel to the rods lead to smaller bridging of cracks and result in a smaller increment of improvement in toughness. In recent years, it has been shown that careful manipulation of microstructure by rapid solidificat
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