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The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (c) up to 6 pct Si and, beyond that, becomes two-phase c + d ferrite. The Fe 5 Ni 3 Si 2 type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the c phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of d-ferrite. The shape memory effect (SME) in these alloys is essentially due to the c to stress-induced e martensite transformation, and the extent of recovery is proportional to the amount of stress-induced e martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced a¢ martensite through c-e-a¢ transformation and the large volume fraction of d-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced e martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal e martensite formed during cooling is also observed to decrease with the increase in Si.

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The solution of hydrogen and hydride formation in Pd/Cr2O3 composites has been investigated. Internal oxidation of Pd−Cr alloys is employed to prepare the composites consisting of nanosized Cr2O3 precipitates within Pd matrices. The dissolved H segregates to the internal Pd/Cr2O3 interfaces. Plots of p H2 1/2 versus H content for internally oxidized Pd−Cr alloys exhibit small positive intercepts along the H/Pd axis which can be attributed to trapping of H by Pd/Cr2O3 internal interfaces. The amount of trapping is found to be directly proportional to the atom fraction Cr in the alloys after internal oxidation at the same temperature. In addition to the enhanced solubility noted from these intercepts, H2 solubilities in the dilute phase of these internally oxidized alloys are larger than in pure Pd or in other internally oxidized Pd-rich alloys and these solubilities also increase with atom fraction Cr, X Cr. In contrast with internally oxidized Pd−Al alloys studied earlier, internally oxidized Pd−Cr alloys with X Cr ≥ 0.02 have an initial plateau p H2 lower than those of Pd−H. The H2 isotherms for internally oxidized alloys with X Cr ≥ 0.05 exhibit gradual phase transitions to the two phase coexistence region rather than an abrupt one as for annealed Pd.

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