The Mechanical Properties of Near-equiatomic B2/f.c.c. FeNiMnAl Alloys
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The Mechanical Properties of Near-equiatomic B2/f.c.c. FeNiMnAl Alloys Xiaolan Wu1, Ian Baker1, Hong Wu2 and Paul R. Munroe3 1 Thayer School of Engineering, Dartmouth College, Hanover NH 03755, U.S.A 2 State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, P.R. China 3 Electron Microscope Unit, University of New South Wales, Sydney NSW 2052, Australia ABSTRACT Two types of as-cast microstructures have been observed in a series of near-equiatomic FeNiMnAl alloys: 1) an ultrafine microstructure in Fe30Ni20Mn20Al30 [1] and Fe25Ni25Mn20Al30, which consists of (Fe, Mn)-rich B2-ordered (ordered b.c.c.) and (Ni, Al)-rich L21-ordered (Heusler) phases aligned along ; and 2) a fine two-phase microstructure in Fe30Ni20Mn30Al20 and Fe25Ni25Mn30Al20, which consists of alternating (Fe, Mn)-rich f.c.c. and (Ni, Al)-rich B2-ordered platelets with an orientation relationship close to f.c.c (002) // B2 (002); f.c.c. [011] // B2 [001] [2]. The phases in Fe25Ni25Mn20Al30 coarsened upon annealing with no significant change in the chemical partitioning. The hardness behavior was studied as a function of the annealing time at 823 K. AnL21-to-B2 transition, which occurred at 573-623K, was observed using in-situ heating in a TEM. After annealing at 973 K for 100 h, needle-shaped clusters of (Fe, Mn)-rich precipitates were observed along the grain boundaries and in the matrix. The temperature dependence of the yield strength of as-cast Fe25Ni25Mn20Al30 was also studied. INTRODUCTION Several two-phase alloys have been recently discovered in the FeNiMnAl system [1-3]. Because of their combination of attractive mechanical properties, especially high room temperature strength, inexpensive elemental components, and potentially high corrosion resistance due to the aluminum content, these FeNiMnAl alloys have been of great interest. The primary phases observed in the as-cast alloys exhibit b.c.c., B2, L21 or f.c.c structures. The microstructure of two alloys with a higher Mn concentration and a lower Al concentration, i.e. Fe30Ni20Mn30Al20 and Fe25Ni25Mn30Al20, consists of alternating (Fe, Mn)-rich f.c.c and (Ni, Al)-rich B2 platelets with an orientation relationship close to f.c.c (002) // B2 (002); f.c.c. [011] // B2 [001] [2]. On the other hand, alloys with a lower Mn concentration and a higher Al concentration, such as Fe30Ni20Mn30Al30 show a very fine B2/L21 two-phase microstructure with interphase interfaces aligned along . The microstructure and mechanical properties of Fe30Ni20Mn20Al30 have been studied in some detail [1]. An L21-to-B2 transition occurred at 750 ± 25 K, resulting in a microstructure consisting of two B2 phases [1]. The hardness of as-cast Fe30Ni20Mn20Al30 was 514 ± 7 VPN [1]. Apart from a ~6% increase, the hardness showed no significant change upon subsequent annealing up to 72 h, although the microstructure coarsened considerably [1]. The temperature dependence of the compressive strength of as-cast Fe30Ni20Mn20Al30 showed three distinct regions: 1) fracture by transgranular cleavage before y
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