Microstructure and Mechanical Properties of Fe-Ni-Mn-Al Alloys
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Microstructure and Mechanical Properties of Fe-Ni-Mn-Al Alloys M.W. Wittmann I. Baker, J.A. Hanna, Thayer School of Engineering, Dartmouth College, Hanover, NH 03755-8000, USA, P. R. Munroe, Electron Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia Abstract In an attempt to produce a two-phase alloy consisting of a L21–structured (Fe,Ni)2MnAlbased phase in either a B2 or b.c.c. matrix, seven Fe-Ni-Mn-Al alloys were cast. Transmission electron microscopy (TEM) of the as-cast alloys revealed a range of microstructures including single phase L21, a f.c.c./B2 eutectic, and alternating, coherent 10-60 nm wide ordered and disordered b.c.c. rods aligned along . A description of the phases, including chemical compositions and hardnesses is presented. Introduction Incorporating L21 precipitates into a B2 matrix has been shown to be an effective method for producing an alloy with excellent high temperature strength. For example, an alloy consisting of L21-structured Ni2AlTi precipitates in a B2 NiTi matrix has far superior creep properties relative to the matrix phase alone [1], and Ni2AlTi particles in a NiAl matrix results in better creep properties than either of the constituent phases [2-4]. The drawback with these strongly-ordered nickel-based alloys is that they are quite brittle. By comparison, there has been only limited work on the less strongly ordered Fe2AlX compounds. A recent investigation of the mechanical properties of Fe2AlMn single crystals revealed that they could exhibit some room temperature tensile ductility (εf~6%) and an increasing yield strength with increasing temperature to 800 K [5]. Since Ni2MnAl also adopts the L21 structure, we explored the possibility of forming a multi-phase alloy containing an L21 phase by substituting Ni for Fe in Fe-rich Fe2MnAl. To this end, seven alloys were cast with concentrations of Fe, Ni, Mn, and Al ranging from 15-35 at. %. One alloy of interest, Fe-15Ni-25Mn-25Al, was also given a 115 hour anneal at 823 K in an attempt to increase the L21 ordering. Transmission electron microscopy (TEM) including energy dispersive x-ray spectrometry (EDS) was used to characterize the resulting microstructure in the as cast, and annealed condition, and room temperature hardness measurements were performed to survey the mechanical properties. Experimental Seven alloys, see Table 1 for compositions, were arc melted in a water-cooled copper mold under argon from constituent elements that were of 99.9% purity or better. Ingots were flipped and melted 4 times to ensure mixing. Slices were cut from the as-cast samples from which 3 mm dia. disks were cut using an EDM. The disks were mechanically polished using 200 grit SiC paper to a thickness of 200 µm and subsequently electropolished in 30% nitric acid in methanol at -20oC using a Tenupol 5, at 12V and 100 mA. TEM analysis was performed using either a Tecnai FEG F20 or Philips CM200 operated at 200kV, both equipped with EDS. Hardness measurements were performed at room temperature using a Leitz MINIload
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