Photon and Ion-Beam Induced Processes in Materials Studied by Hyperfine Interactions

Laser nitriding and laser cementation are investigated by Mössbauer spectroscopy and complementary methods. It is demonstrated how the backscattering versions of Conversion Electron and Conversion X-ray Mössbauer spectroscopy can contribute to the investigation of surface processes, like the laser-induced formation of nitrides and carbides. Additionally, the formation of semiconducting iron disilicide can be achieved by ion-beam mixing and pulsed laser irradiation of Fe/Si bilayers. The results of both processes are compared.

     

金刚石单晶合成效果与合成后铁基触媒组织的相关性研究

采用粉末冶金法制备铁基触媒片,在六面顶压机上高温高压合成金刚石单晶.利用扫描电子显微镜(SEM)、光学显微镜(OM)等表征了不同成分的触媒以及同一触媒在不同合成时间条件下金刚石单晶的合成质量和合成后的铁基触媒组织.结果表明:当金刚石单晶合成质量较好时,合成后铁基触媒组织特征表现为初生板条状渗碳体分布较均匀,呈平行生长的条束,渗碳体的板条两边缘较平直,而且数量较多.触媒成分和合成时间是影响铁基触媒组织中初生渗碳体的数量和形态的主要因素.     

Atomic structure of the Fe/Fe3C interface with the Isaichev orientation in pearlite

Abstract

The pronounced mechanical property of pearlitic steels highly correlates with the ferrite (bcc-Fe)/cementite (Fe₃C) boundaries inside. Unraveling the interface structure at an atomic level is essential for interpreting the material’s property. In the present study, using aberration-corrected scanning/transmission electron microscopy combined with density functional theory calculations, we reveal the atomic configuration as well as the electronic structure of the Fe/Fe₃C interfaces with the Isaichev orientation in pearlite. The interface with terminating layer Fe–C–Fe in cementite has the lowest energy due to the formation of interfacial Fe–C bonds. Terrace steps which are frequently observed at the interfaces would not break the lattice match between the two phases.     

Structure of several historic blades at nanoscale

Comparison of the structure of ancient Damascene steel blades at nanoscale with more recent ones – all made using crucible (wootz) technology and exhibiting ultra‐high carbon content – showed for the first time a common feature. Despite different microstructures, colonies of wire‐ and tube‐like particles with diameters of 40‐50 nm have been observed with the aid of high‐resolution transmission electron microscopy. Crystalline Fe₃C is the main phase forming those particles covered in numerous cases by a tube‐like layer. These tubes were also found in an empty or partly – covered filled variant. To assess the strengthening capacity of cementite various models were compared. Dispersion strengthening seems the most efficient. Cutting edge qualities may be related to surface corrugations due to nanoparticles. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A density‐functional study of the phase diagram of cementite‐type (Fe,Mn)3C at absolute zero temperature

The phase diagram of (Fe_1?x Mn_x)₃C has been investigated by means of density‐functional theory (DFT) calculations at absolute zero temperature. The atomic distributions of the metal atoms are not random‐like as previously proposed but we find three different, ordered regions within the phase range. The key role is played by the 8d metal site which forms, as a function of the composition, differing magnetic layers, and these dominate the physical properties. We calculated the magnetic moments, the volumes, the enthalpies of mixing and formation of 13 different compositions and explain the changes of the macroscopic properties with changes in the electronic and magnetic structures by means of bonding analyses using the Crystal Orbital Hamilton Population (COHP) technique. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010     

X-ray diffraction and Mössbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (～10¹⁵/m²) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ?1?1?1? screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.     

Nitrogen diffusion through cementite layers

Massive cementite layers with a time-dependent thickness were grown on ferrite substrates by nitrocarburising in a dedicated NH₃/H₂/CO/N₂-containing gas atmosphere at 783 K, 823 K and 843 K. Nitrogen diffusion through the cementite layer into the ferrite substrate took place in conjunction with growth of the cementite layer; a significant, i.e. measurable, solubility of nitrogen in cementite was not observed. The nitrogen concentration-depth profiles in the substrate, underneath the growing cementite layer, were quantitatively determined using a calibrated microhardness measurement technique. The nitrogen concentration-depth profiles were simulated on the basis of a model using an implicit finite-difference method. The simulation yielded values for the diffusivity of nitrogen through cementite, including its activation energy.     

On the nucleation of cementite on bainitic ferrite–austenite interphase boundaries

Among all possible variants of the Isaichev orientation relationship between cementite and ferrite, a single major cementite variant has been observed to appear in bainite. Interphase boundary nucleation of cementite on ferrite–austenite semi-coherent interfaces is considered a plausible reason for this observation. With the aid of known crystallographic relations and habit planes of the ferrite–cementite, ferrite–austenite and austenite–cementite phases, a model for cementite nucleation has been proposed. The interphase-boundary nucleus is assumed to form on a semi-coherent ferrite–austenite interface and to possess ferrite–cementite and austenite–cementite habits as two main facets of the nucleus. It is shown that interphase cementite nucleation will be viable if the energies of all facets of the nucleus are in the semi-coherent range.     

合金渗碳体稳定性研究

依据固体与分子经验电子理论,建立“实际晶胞模型”,采用统计法计算合金元素M（Cr、V、W、Mo、Mn）取代渗碳体（θ-Fe₃C）不同位置和数目的Fe¹、Fe²后的价电子结构.定义稳定性因子P并讨论分析不同位置、数目和类型的Fe原子被M取代后,P的变化规律.结果表明：晶格电子密度、原子键对称性和键能,对稳定性有重要影响;M取代Fe²比取代Fe¹稳定,Cr、Mo、W、V成对取代2、3或6、7位置的2个Fe²最稳定;合金渗碳体的稳定性按W、Cr、V、Mo、Mn的顺序递减.