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

在本研究的第一阶段，我们从热力学的角度讨论了以前观察到的Au-Cd和Fe-Ni合金纳米颗粒的马氏体相变温度与相应块体合金相比的降低，其中T_0温度分别降低，但几乎保持不变；并提出了发生这种现象的热力学条件：(ΔS^&lt；P→M&gt；)_&lt；(ΔS^&lt；P→M&gt；)_&lt；颗粒&gt；对于Au-Cd，和(ΔS^&lt；P→M&gt；)_&lt；Bulk&gt；[近似相等](ΔS^&lt；P→M&gt；)_&lt；颗粒&gt；和(Δh^&lt；P→M&gt；)_&lt；Bulk&gt；[近似相等](Δh^&lt；P→M&gt；)_&lt；颗粒&gt；对于Fe-Ni，我们尝试用化学合成的方法制备Fe-x at.%铂(x[小于或等于]25)合金的纳米颗粒。成功地制造了接近等原子组成的铁铂合金的单分散纳米颗粒(《科学》，2000)。首先，按照他们的方法，用五甲基铁(Fe(CO)_5)和铂乙酰丙酮(Pt(AcAc)_2)作为原料…第二步，用乙酰丙酮铁(Fe(Acac)_3)代替有毒的Fe(CO)_5。用CTEM、HRTEM、ED和分析电子显微镜对合成样品和后续热处理样品的物相进行了鉴定，证实合成样品是由纳米级的均匀分散的颗粒状产物组成。然而，出乎意料的是，产物并不是名义成分的合金，而是接近纯铂的富铂γ相铂-铁合金和铁氧化物Fe_3O_4的混合物，它们的相对含量取决于名义成分。这表明在560K附近的回流条件下，Fe和Pt几乎不发生合金化。在透射电子显微镜下，加热到1073K时，Fe和Pt的合金化继续进行，加热后的室温产物仅为富Fe的γ相或α和富Fe的γ相的混合物。α和γ相合金颗粒的尺寸增大到约15 nm，值得注意的是，在室温和103K下，铂含量小于10at.%的γ相合金颗粒保持不变。因此，实验表明，与相应的体相相比，Fe-Pt合金的γ相在纳米颗粒中稳定了500K以上。较少

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