Anovel two-phase alloy of composition Fe- 20Ni-35Mn-15Al (all compositions in atomic percent) was produced by arc melting. Disk samples of 3 mm diameter were electro-polished in a mixture of 20% nitric acid, 10% butoxyethanol, and 70% methanol by volume at 253K (–20°C, –4°F).
The resulting thin foils were examined in a Technai F20 field emission gun TEM operated at 200 kV. Selected area diffraction showed that the phases present were B2 (ordered body centered cubic) and face-centered cubic. The alloy consisted of alternating ~200 nm wide B2 and ~500 nm wide FCC lamellae, as shown in Fig. 1. The compositions of the B2 and FCC phases were determined by EDS to be Ni-28Al-18Mn-7Fe and Fe-37Mn-7Ni-6Al, respectively.
Fig. 1 — Brightfield TEM image showing the as-cast
microstructure of Fe-20Ni-35Mn-15Al, which consists of
alternating lamellae of B2 (dark) and FCC (light) phases.
The as-cast alloy was subsequently heated in argon to 1573K (1300°C, 2370°F), which is above the eutectoid transformation temperature, and quenched into water. Very fine features were formed in the quenched specimen. These features consisted of 50 to 70 nm wide B2, and 80 to 150 nm wide FCC lamellae, as shown in Fig. 2. Asubstantial Vickers hardness increase from the as-cast hardness of HV310 to HV380 occurred due to this microstructural refinement.
Fig. 2 — Brightfield TEM image showing the refinement in
microstructure of Fe-20Ni-35Mn-15Al after quenching,
which consists of alternating finely-spaced B2 (dark) and
FCC (light) phases.
For more information: Prof. Ian Baker, Thayer School of Engineering, Dartmouth College, Hanover, NH 03755; tel: 603/646-2184; ian.baker@dartmouth.edu;
www.dartmouth.edu.
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