Significant Improvement of Mechanical Properties in NiAl-Cr(Mo)/Hf Alloy by Suction Casting and Subsequent Hot Isostatic Pressing

The NiAl-28Cr-5.5Mo-0.5Hf eutectic alloy was prepared by the suction casting (SC) technique and subsequent hot isostatic pressing (HIP) treatment, and tested for compressive strength and fracture behavior in the temperature range of 300 to 1373 K. The microstructure of suction-cast alloy is characterized by fine interlamellar spacing, large area fraction of eutectic cell, and fine Heusler (Ni₂AlHf) phase distributed semicontinuously at the cell boundaries. After HIP treatment, Ni₂AlHf phase at the cell boundaries is transformed into Hf solid solution phase and distributed homogeneously within the NiAl matrix. Compared with the conventionally cast alloy, the room-temperature compressive strain and elevated temperature strength of suction-cast alloy are enhanced markedly after HIP treatment. The reason is that the HIP treatment causes Hf solid solution phase to distribute homogeneously and then strengthens the NiAl matrix.

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Rapid Solidification of Sm-Co Permanent Magnets

In this article, we report the microstructural evolution of rapidly solidified Sm-Co alloys from 4 to 16 at. pct Sm. We have observed a wide variety of phase formation and microstructures, ranging from primary Co dendrite formation to eutectic structures to the formation of the metastable SmCo₇ compound. In particular, we observed nonequilibrium formation of Co along with SmCo₇, whose presence caused a decrease in coercivity from ∼10 kOe to 500 Oe. Alloying elements reduced the scale of the microstructure, effectively offsetting the detrimental effects of the Co phase formation and leading to a recovery of the coercivity. The eutectic structure with Co rods surrounded by SmCo₇ provides a natural path to nanoscale hard/soft magnetic nanocomposites, where control of scale and phase content is critical. This article is based on a presentation made in the symposium entitled “Phase Transformations in Magnetic Materials”, which occurred during the TMS Annual Meeting, March 12–16, 2006, in San Antonio, Texas, under the auspices of the Joint TMS/MPMD and ASMI-MSCTS Phase Transformations Committee.

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