Crystallographic and Thermodynamic Study on the Martensitic Transformations in Nanometer-sized Particles of Alloys

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At the first stage of the present study, we have discussed in thermodynamic points of view the depressions of the martensitic transformation temperatures in nanoparticles previously observed for Au-Cd and Fe-Ni alloys, compared with those for the corresponding bulk, where T_0 temperature is reduced, while it remains almost unchanged, respectively ; and proposed thermodynamic conditions for the occurrences : (Δs^<P→M>)_<bulk> < (Δs^<P→M>)_<particle> for Au-Cd, and (Δs^<P→M>)_<bulk> 【approximately equal】 (Δs^<P→M>)_<particle> and (Δh^<P→M>)_<bulk> 【approximately equal】 (Δh^<P→M>)_<particle> for Fe-Ni.Next, we have tried to prepare nanoparticles of Fe-x at.%Pt(x【less than or equal】25) alloys following a chemical synthesis method by which Sun et.al. succeeded in making monodispersed nanoparticles of Fe-Pt alloys near equiatomic composition (Science, 2000). At the beginning, following their method, iron pentacarbonyl (Fe(CO)_5) and platinum acetylacetonate (Pt(acac)_2) were used as raw mate … More rials for the component metals, and at the second step, instead of the toxic Fe(CO)_5, iron acetylacetonate (Fe(acac)_3) was adopted. Identification of the product phases in as-synthesized and subsequently heat-treated specimens were made by means of CTEM, HRTEM, ED and analytical TEM.As-synthesized specimens were confirmed to consist of fairly monodispersed particle-like products with nanometer size. Unexpectedly, however, the products were not of alloys with nominal compositions, but were the mixture of Pt-rich γ phase Pt-Fe alloys near pure Pt and an Fe oxide Fe_3O_4, their relative amounts depending on nominal composition. This indicates that alloying of Fe and Pt hardly occurs at the reflux condition near 560KAlloying of Fe and Pt proceeded upon heating up to 1073K in TEM and the products at room temperature after heating were particles of only Fe-rich γ phase or a mixture of α and Fe-rich γ phases. The size of the α and γ phase alloy particles were increased up to about 15 nm.To be noticed was that the γ phase alloy particles with compositions less than 10at.%Pt remained untransformed at room temperature and even at 103K.Therefore it is experimentally made clear that the γ phase of the Fe-Pt alloy is by more than 500K stabilized in the nanoparticles, compared with the corresponding bulk. Less