Early stages of decomposition of alloys (spinodal or nucleation)

Progress in theory and experiment on the early stage of decomposition of alloys is reviewed. In the first half of this paper, theoretical treatments on decomposition of alloys by spinodal and nucleation mechanism and also continuum theories and the discrete lattice theories are comparatively reviewed. In the latter half of this paper, experimental results on Al-Zn, Ni-Al, Cu-Co and Al-Ag alloys are discussed.     

Genetic alloy design based on thermodynamics and kinetics

A theory-guided computational approach for alloy design is presented. Aimed at optimising the desired properties, the microstructure is designed and an alloy composition optimised accordingly, combining criteria based on thermodynamic, thermokinetic and mechanical principles. A genetic algorithm is employed as the optimisation scheme. The approach is applied to the design of ultra-high strength stainless steels. Three composition scenarios, utilising different strengthening precipitates (carbides, Cu and NiAl/Ni₃Ti), are followed. The results are compared to a variety of existing commercial high-end engineering steels, showing that the design strategy presented here may lead to significant improvements in strength beyond current levels.     

Properties of the Ce–Pd–Pt(111) overlayer system

The Pd–Pt(111) and the Ce–Pd–Pt(111) overlayer systems were studied by X-ray photoelectron spectroscopy and low-energy electron diffraction (LEED). Variations in the work function were measured by ultraviolet photoelectron spectroscopy. The Pd overlayer thicknesses were in the range of 1 to 4 monolayers (MLs). The Ce overlayer thicknesses were in the range is of 0.5 to 1.5 MLs. The interfaces were studied for annealing temperatures up to 700°C. Surface alloying or intermixing of Ce, Pd and Pt was observed. Upon deposition at ambient temperatures, Ce forms an overlayer. During annealing, both Pd and Ce were found to dissolve into the Pt substrate. For a Pd coverage of about 1 ML, the Ce–Pd–Pt(111) system was found to be Pt, terminated after annealing to 700°C. For Pd coverage above 1 ML, both Pd and Pt were found to be present at the surface. Cerium was found to be absent from the top surface layer in both cases.     

Unusual nucleation and growth of γ′ iron nitride upon nitriding Fe–4.75 at.% Al alloy

The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe₄N_{1? x} ) was investigated upon nitriding in NH₃/H₂ gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe₂N_{1? z} ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite.     

Reverse α–α´ phase separation in Fe-20Cr-6Al alloy

The reverse α–α′ phase separation in Fe-20Cr-6Al alloys has been monitored by differential scanning calorimetry in as-hot rolled and recrystallized samples previously aged at 435?°C. The splitting in the position of the minima, and the increasing enthalpy change involved, might be explained in terms of dissolution of α′ particles with different compositions and volume fractions. These results are fully consistent with the α–α′ phase separation kinetics determined by atom probe tomography during ageing at 435?°C. The monitoring of activation energy for α′ dissolution indicates that it is faster in as-hot rolled samples. Finally, the activation energy evolution indicates that there is first a dissolution of β′ Fe(AlTi) phase followed by α′ dissolution.     

On the absence of shear cracking and grain boundary cavitation in secondary tensile regions of Ti–6Al–4V–0.1B alloy during hot (α + β)-compression

Deformation instabilities, such as shear cracking and grain boundary cavitation, which are observed in the secondary tensile region of Ti–6Al–4V alloy during compressive deformation in the (α?+?β)-phase field, do not form in Ti–6Al–4V–0.1B alloy when processed under the same conditions. This has been attributed to the microstructural modifications, e.g. the absence of grain boundary α and adjacent grain boundary retained β layers and a lower proportion of 90^o-misoriented α-colonies that occur with boron addition.     

Investigation of radiation defects in Fe-Cr alloy

Abstract

The kinetics of radiation defect accumulation and subsequent recovery during/after electron irradiation below 273 K, at 323 K and 373 K were investigated for the Fe-15.7 at.% Cr using positron annihilation measurements at room temperature. Formation of vacancy clusters was observed at all of the irradiation temperatures. The formation of clusters and kinetics of their accumulation point to mobility of vacancies at least at room temperature. The cluster rearrangement and variations in the cluster configuration take place during annealing.     

激光喷丸改善TC6钛合金组织和力学性能

为了使激光冲击强化技术能较好地应用于TC6钛合金的发动机叶片,对TC6钛合金进行试验研究。通过X射线衍射仪、透射电子显微镜等测试技术分析了不同参数下TC6钛合金的微观组织变化,用显微硬度计和残余应力测试仪分别表征表层硬度和残余应力变化,并测试材料冲击后的振动高周疲劳性能。试验结果表明:激光冲击材料后表面组织得到明显细化,随着冲击次数的增加,先后出现了高密度位错、位错胞、亚晶和纳米晶。性能方面,表面硬度在冲击一次即可提高19%,硬度影响深度达到700 m;与此同时表面残余应力最高达到-608.5 MPa,在500 m深度上仍具有-100 MPa左右的应力存在。经三次冲击后,标准疲劳试片的疲劳极限提高近20%。     

Mössbauer study of the annealing effect on low‐alloyed steels

Structural analyses of Fe-Cr and Fe-Cr-Ni low‐alloyed steels before and after annealing at 600^○C using Mössbauer spectrometry, X‐ray diffraction and scanning electron microscopy are reported. At the received state, the two steels present a fine bainitic microstructure with equal hardness. From Mössbauer analysis, two iron sites are identified as substitutional by Cr, Mo, Ni atoms and as insertional by carbon in bainitic ferrite. Both sites are locally deformed by residual stresses. The presence of small quantities of retained austenite and ε carbide has been observed. Annealing for one hour at 600^○C causes a decrease in hardness for both steels with a decrease of retained austenite. After longer time of annealing, precipitation of (Fe,Ni)₂₃C₆ occurs in the Fe-Cr-Ni steel and increases hardness.