Strong Magnetic-Field Treatment Strengthens NiAl-Cr(Mo)-Hf Alloy
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RESEARCH/RESEARCHERS
Strong Magnetic-Field Treatment Strengthens NiAl-Cr(Mo)-Hf Alloy NiAl-Cr(Mo)-Hf alloys offer significant potential for use as high-temperature, lightweight structural materials. They show good strength at high temperatures, good oxidation resistance, and high wear resistance. These alloys, however, are very brittle and have low tensile strengths, particularly at room temperature. Various efforts to improve the ductility of these alloys have been marginally successful. A method has now been reported to significantly improve the ductility and bending strength of the alloy. By subjecting a NiAlCr(Mo)-Hf alloy to a high magnetic field, G.J. Ma and W.L. Zhou of the Dalian University of Technology, J.T. Guo of the Shengyang Institute of Materials Research, and their co-workers have increased the bending strength of the alloy by 75%. As reported in the February 2005 issue of the Journal of Materials Research (p. 295; DOI: 10.1557/JMR.2005.0047), rectangular
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Figure 1. Electron probe microanalysis of the microstructures (a) without magnetic-field treatment, in which N-2 represents the NiAl phase and N-3 represents the Cr(Mo) phase; and (b) after magnetic-field treatment, in which T-2 represents the NiAl phase and T-3 represents the Cr(Mo) phase. Reproduced with permission from the Journal of Materials Research 20 (2) (February 2005), p. 295; DOI: 10.1557/JMR.2005.0047. © 2005 Materials Research Society.
cross-sectional NiAl-Cr(Mo)-Hf alloy specimens, formed from a nominal composition of Ni-33Al-28Cr-5.5Mo-0.5Hf, were heated to 900ºC under a 10 T homo-
geneous high magnetic field. Upon cooling to room temperature, the bending strength of the specimens was characterized using the three-point bending
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MRS BULLETIN • VOLUME 30 • APRIL 2005
RESEARCH/RESEARCHERS
method. While the average bending strength of the untreated alloy was 245 MPa, the average bending strength of the alloy treated with the magnetic field was 430 MPa, roughly a 75% increase. The elastic modulus remained unchanged. Scanning electron microscopy observations of the fracture surfaces of the treated specimens showed typical ductile fracture characteristics. The fracture surfaces of the untreated specimens clearly showed brittle characteristics with microcracks, the researchers reported. In order to understand the underlying mechanism, C.W. Wu in the Department of Mechanical Engineering at Dalian said that the research group performed electron probe microanalysis on the untreated and treated samples. The untreated alloy consists of NiAl, Cr(Mo), and Heusler phases. These do not change when the alloy is heated to 900ºC. In the magneticfield-treated samples, the Heusler phases at the NiAl/Cr(Mo) grain boundaries were partly dissolved into the NiAl and Cr(Mo) phases. In addition, small Heusler phase particles at the Cr(Mo) and matrix phase interfaces partially dissolved into the NiAl matrix (see Figure 1). The researchers speculated that the dissolution of the Heusler phase
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