On the behaviour of body-centred cubic metals to one-dimensional shock loading

The response of metallic materials to shock loading, like all loading regimes, is controlled largely by factors operating at the microscopic or atomic levels. Over the past few years, face-centred cubic (fcc) metals have received a level of attention where the role of features such as stacking fault energy and precipitation hardening have been investigated. We now turn our attention to body-centred cubic (bcc) metals. In the past, only tantalum, tungsten, and their alloys have received significant attention at high strain-rate conditions due to their use by the ordnance community. In particular, this investigation examines the shear strength of these materials at shock loading conditions. Previous results on tantalum, tungsten, and a tungsten heavy alloy are reviewed, and more recent experiments on niobium, molybdenum, and Ta–2.5 wt% W presented. Results are discussed in terms of known deformation mechanisms and variations of Peierl’s stress.     

Stability and structures of the CFCC-TmC phases: A first-principles study

The η-M₆C, γ-M₂₃C₆, and π-M₁₁C₂ phases (M = Cr, Mn and Fe) have complex cubic lattices with lattice parameters of approximately 1.0 nm. They belong to the CFCC-TmC family (complex face-centered cubic transition metal carbides), display a rich variety of crystal structures, and play in important role in iron alloys and steels. Here we show that first-principles calculations predict high stability for the γ-M₂₃C₆ and η-M₆C phases, and instability for the π-M₁₁C₂ phases, taking into account various compositional and structural possibilities. The calculations also show a wide variety in magnetic properties. The Cr-containing phases were found to be non-magnetic and the Fe-phases to be ferromagnetic, while the Mn-containing phases were found to be either ferrimagnetic or non-magnetic. Details of the local atomic structures, and the formation and stability of these precipitates in alloys are discussed.     

Constitutive models for bcc engineering iron alloys exposed to thermal–mechanical fatigue

The progress in designing high temperature components relies on more accurate viscoplastic constitutive models. The capability of various models under high temperature and variable temperature conditions is investigated for two body centred cubic alloys, cast iron and ferritic stainless steel. Improvements are shown to overcome problems encountered by standard viscoplastic models. Firstly a physically based modified flow equation predicts reliably the behaviour of cast iron under thermal–mechanical loading. Secondly further improvement is proposed drawing on dislocation models to describe static recovery effects in stainless steels. Good agreement is thus obtained between experiment and model prediction under various thermal mechanical loading path.     

Laves-phase formation criterion for high-entropy alloys

In this study, we have analysed Laves-phase formation in high-entropy alloys (HEAs). For that purpose, the AlCr_xNbTiV and Al_xCrNbTiVZr (x?=?0, 0.5, 1, 1.5) alloys were produced and examined. It was found that the AlNbTiV and AlCr_0.5NbTiV alloys had single-phase body-centred cubic structure, while the other alloys contained Laves phase. Analysis has demonstrated that Laves-phase formation in the produced and in the other HEAs, which are predominantly composed of Al and the elements of 4–6 groups and tend to form body-centred cubic solid solutions, can be predicted by the atomic size mismatch, δ_r, and the Allen electronegativity difference, Δχ_Allen, parameters. It was shown that Laves-phase formation is observed when δ_r?>?5.0% and Δχ_Allen?>?7.0%.     

Intergranular crack paths during fatigue in interstitial-free steels

Interstitial-free (IF) steels are known to exhibit intergranular (IG) faceting during fatigue under certain conditions. The presence of IG facets is often ascribed to either environmental effects or to grain boundary embrittlement. In many cases this attribution to environment or embrittlement is erroneous with the IG faceting actually arising from the intrinsic slip characteristics of body-centred cubic (bcc) alloys. This paper summarises the background to slip-induced intergranular fatigue and explores two issues of importance to users of IF steels; alloy conditions in which IG fatigue occurs and whether the fatigue performance in the presence of an IG crack path is lower than similar IF steels which exhibit a transgranular crack path. To explore this latter issue fatigue performance is presented as a function of yield strength.     

Interatomic potentials for materials with interacting electrons

Evidence for the significant part played by magnetism in the picture of interatomic interactions in iron and iron-based alloys has recently emerged from density functional studies of the structure of radiation induced defects. In this paper we examine the range of validity of the currently available model interatomic potentials for magnetic iron, investigate the effect of electron–electron interaction on the strength of chemical bonding between atoms, follow the link between the multi-band Hubbard and the Stoner models, and review the concepts underlying the recent development of a semi-empirical magnetic interatomic potential.     

Fatigue life predictions for irradiated stainless steels considering void swellings effects

The objective of this study is to estimate fatigue life of irradiated austenitic stainless steels types 304, 304L, and 316, which are extensively used as structural alloys in the internal elements of nuclear reactors. These reactor components are typically subjected to a long-term exposure to irradiation at elevated temperature along with repeated loadings during operation. Additionally, it is known that neutron irradiation can cause the formation and growth of microscopic defects or swellings in the materials, which may have a potential to deteriorate the mechanical properties of the materials. In this study, uniaxial fatigue models were used to predict fatigue properties based only on simple monotonic properties including ultimate tensile strength and Brinell hardness. Two existing models, the Bäumel–Seeger uniform material law and the Roessle–Fatemi hardness method, were employed and extended to include the effects of test temperature, neutron irradiation fluence, irradiation-induced helium and irradiation-induced swellings on fatigue life of austenitic stainless steels. The proposed models provided reasonable fatigue life predictions compared with the experimental data for all selected materials.     

Special features of the effect of oversized impurities on the cascade development in α-iron alloys containing special carbides

We consider the behavior of an α-iron alloy containing special molybdenum and tungsten carbides (MoC and WC) and oversized impurity atoms (Mo and W) in the solid solution. The effects of molybdenum and tungsten on the development of ballistic (cascade) processes in the alloy under reactor irradiation conditions have been studied by computer simulation methods to assess the possibility of substituting W for Mo in the alloy to obtain low-activated steels.     

核聚变堆包层结构材料研究进展及展望

随着人类对能源需求的增加,核聚变能的发展越来越受到人们的关注。材料问题是目前限制聚变能发展的一个重要因素。包层是实现能量转换、氚自持及辐照屏蔽的主要部件,满足包层结构材料苛刻环境要求的结构材料的开发及性能检测成为目前研究的热点。以低活化铁素体马氏体(RAFM)钢为代表的包层结构材料已发展多年,然而依据中国聚变能发展路线图,CFETR一期包层结构材料的中子辐照水平可达到约10dpa,在二期达到约50dpa,目前没有材料能满足包括抗辐照损伤在内的苛刻环境要求并能满足工程建设需求。低活化铁素体马氏体钢是目前包层结构材料的首选候选结构材料,国内外已开发了多个牌号的低活化品种并具备了丰富的材料基础数据库,然而低活化钢的工作温度区间严重受限,高温蠕变及抗辐照能力无法满足CFETR二期及未来聚变堆的要求。为解决传统RAFM钢的不足,提出了两条思路:一种是添加氧化物弥散相以有效提高高温蠕变强度,其中又以制备过程中是否涉及机械合金化可进行进一步的区分;另一种思路是基于热力学模拟计算,优化RAFM钢化学成分并进行多轮热机械处理以增加MX相密度。其中,机械合金化制作的氧化物弥散强化钢(ODS钢)的性能最佳,但受限于机械合金化法,成本高且效率低。非机械合金化ODS钢与优化的RAFM钢的性能接近机械合金化ODS钢,成本远远低于机械合金化ODS钢且制备效率高,大批量制备技术相对容易。除了铁基材料外,钒合金及碳化硅复合材料在多方面展现了优势,长期以来都是研究人员关注的热点。钒合金的热蠕变和氦脆导致温度上限低并且与氢同位素兼容性不好,碳化硅复合材料的规模化生产及连接技术仍存在困难,这些缺陷限制了钒合金与碳化硅复合材料的发展,使之在现阶段无法满足应用需求。面向更高辐照水平的示范堆及商用堆,目前已有的包层结构材料可能无法满足需求。根据目前很有限的研究数据,非晶材料及高熵合金的工程应用还非常遥远:一方面需要借助材料设计和制备的新理念、新方法不断挖掘现有材料的性能潜力,另一方面应重视具有潜在优势的复合块状非晶材料及低活化高熵合金等新型材料的研发。本文依据中国磁约束聚变材料路线图草稿,对RAFM钢、机械合金化制备的ODS钢,钒合金以及碳化硅复合材料的发展进行了综述,对最近几年兴起的改良RAFM钢、非机械合金化制备的ODS钢等新型候选结构材料进行了介绍,并对具有更佳性能的先进结构材料种类进行了展望。     

Microstructure and mechanical properties of CoCrNi-Mo medium entropy alloys:Experiments and first-principle calculations

The effect of Mo additions on the microstructures and mechanical properties of CoCrNi alloys was investigated,meanwhile,ab initio calculations are performed to quantitatively evaluate the lattice distortion and stacking fault energy(SFE).The yield strength,ultimate tensile strength,and elongation of(CoCrNi)₉₇Mo₃alloy are 475 MPa,983 MPa and 69%,respectively.The yield strength is increased by~30%and high ductility is maintained,in comparison with CoCrNi alloy.Besides the nano-twins and dislocations,the higher density of stacking faults is induced during the tensile deformation for(CoCrNi)₉₇Mo₃alloy.Ab initio calculation results indicate the mean square atomic displacement(MSAD)and SFE value of(CoCrNi)₉₇Mo₃alloy is 42.6 pm²and-40.4 mJ/m²at 0 K,respectively.The relationship between mechanical properties and MSAD,SFE for various multiple principal element alloys is discussed.     

氦离子辐照下钨纳米丝的自保护行为

采用低能量(200 eV)大流强的He⁺辐照多晶钨材料,辐照温度为1023 K和1373 K,辐照剂量为1.0×10²⁵~1.0×10²⁶ ions/m²。用称重、扫描电子显微镜、导电原子力显微镜等手段分析辐照后钨材料的质量损失、表面形貌和内表面缺陷分布,研究了刻蚀速率与表面形貌的关系。结果表明,具有粗糙钨纳米丝表面的钨样品刻蚀速率只有平滑表面的30%。其原因是,在大流强He⁺辐照下钨表面纳米丝的形成阻碍钨原子的溅射。这也意味着,钨纳米丝表面的形成可作为钨材料的自保护结构层,抑制ITER相关辐照下的强刻蚀。     

Magnetic origin of nano-clustering and point defect interaction in Fe–Cr alloys: an ab-initio study

To understand the behaviour of point defects generated in irradiated FeCr ferritic/martensitic steels and to identify the kinetic pathways of micro-structural evolution of binary model Fe–Cr alloys, we use a combination of density functional theory (DFT) with statistical approaches involving cluster expansion and Monte Carlo simulations. This makes it possible to generate in a systematic way the low-energy configurations required for the subsequent DFT study of intrinsic defects (vacancies, interstitials) and impurity-defect interactions over the entire range of Fe–Cr alloy compositions. In the limit of low Cr concentration, DFT calculations predict that an intermetallic compound Fe15Cr has the lowest negative heat of formation. At higher Cr concentrations, simulations performed using a 4 × 4 × 4 super-cell show that magnetism is responsible for the nano-segregation of the ferromagnetic Fe15Cr and anti-ferromagnetic (α′-Cr) phases giving rise to the formation of clusters characterised by a very low positive heat of formation. We perform a systematic investigation of formation energies of point defects and their energies of interaction with Cr atoms. Further investigation of interaction of interstitial and vacancy defects with impurities (V, Nb, Ta, Mo, W, Al, Si, P, S) also shows a complex picture of interplay between magnetism and short-range ordering that affects the interaction between defects and impurities in the presence of chromium in Fe-rich alloys.     

ODS钢的抗辐照设计及纳米第二相粒子表征的研究进展

先进裂变反应堆及聚变堆要求材料在高温高压、强中子辐照、长服役周期等苛刻服役环境下具有卓越的结构和性能稳定性。氧化物弥散强化(ODS)钢由于具有优异的耐高温及耐辐照性能成为第四代反应堆包壳及核聚变包层最有希望的候选材料。基于材料的中子辐照损伤特性,主要介绍了ODS钢的抗辐照设计及纳米第二相粒子的表征方面的研究进展。     

低能大流强氢离子辐照对钨的刻蚀行为

在聚变相关的钨(W)偏滤器辐照下,研究了低能大流强氢(H)离子辐照对多晶钨材料的刻蚀行为。使用扫描电子显微镜(SEM)、导电原子力显微镜和基于SEM的电子背散射衍射等手段研究了大流强(~10²² ions/m²·s)、剂量为1.0×10²⁶ ions/m²、能量为5~200 eV的氢离子辐照对多晶W材料表面刻蚀行为的影响。结果表明,随着H离子辐照能量的增加W的溅射率迅速提高,W表面发生刻蚀后产生条纹状结构,而且同一晶粒上条纹的方向具有一致性,条纹两侧的缺陷分布明显不同,意味着W表面的刻蚀优先沿某一特定晶面方向进行。     

不同辐照粒子下钨及钨合金辐照损伤行为的研究进展

研究核聚变、准稳态等离子体下面向等离子体材料的辐照行为,发展适合于先进实验超导托卡马克(EAST)、国际热核聚变实验堆(ITER)和中国聚变工程实验堆(CFETR)长脉冲高参数运行乃至未来聚变反应堆稳态运行的高性能面向等离子体材料是当前核聚变研究一项艰巨而又紧迫的任务。钨因具有高熔点、高导热率、低溅射腐蚀速率、高自溅射阀值以及低蒸气压和低氚滞留等优异性能,被认为是聚变装置最具有前景的面向等离子体材料。综合评述了钨及钨合金在不同辐照粒子下损伤行为的最新研究进展。粒子辐照造成的微观缺陷在钨及钨合金内部累积,辐照造成缺陷的形成和数量与钨基材料颗粒微观结构、第二相成分等密切相关,辐照缺陷情况各异。同时,辐照粒子种类、能量、剂量和温度等辐照条件都会对钨材料辐照后的形貌特征和缺陷产生重要影响。     

The energetic and structural properties of bcc NiCu,FeCu alloys: A first-principles study

Using special quasirandom structures (SQS’s), we perform first-principles calculations studying the metastable bcc NiCu and FeCu alloys which occur in Fe–Cu–Ni alloy steels as precipitated second phase. The mixing enthalpies, density of states, and equilibrium lattice parameters of these alloys are reported. The results show that quasi-chemical approach and vegard rule can satisfactorily predict the energetic and structural properties of FeCu alloys but fail to accurately yield that of NiCu. The reason rests with the difference of bond energy variation with composition between NiCu and FeCu alloys induced by competition between ferromagnetic and paramagnetic state. Furthermore, the results show that the energetic and structural properties of these alloys can explain the element distribution of the Cu-rich precipitates in ferrite steels.     

Building Crystals from Clusters

Itisoftenusefultoregardas0lidcrystallinemate-rialascomp0sed0felementarystructuralunits,per-hapsclusters,fromwhichthecrystalmaybeassem-blediftheunitsarearrangedappropriately;theunitsare,sot0speak,brickswithwhichthecrystalcanbebuilt.Theseunitshavevari0usdegreesofreal-ity.Inthecaseofmolecularcrystalstheunitsarem0leculeswhicharewelldefinedinthegasphaseandretaintheiridentityinthes0lidbeingheldtogetherweaklyinthecrystalby1hnder1Yhalsf0rces.Insomecasesthestructuralunitsaremerelyc0nvenientandformp0lyh…     

Microstructures of binary Cr–xNi alloys (0 ≤ Ni ≤ 50 wt.%) in their as-cast state and after high temperature exposure

Metallic materials designed for use at very high temperature must be based on elements with high melting points. In this work, several binary alloys, chosen in the Cr-rich part of the Cr–Ni diagram, were elaborated by foundry and characterised by metallography in the as-cast condition and after exposure at 1200 °C. Many of the obtained alloys are composed of imbricated body-centred cubic chromium phase saturated in Ni and face-centred cubic nickel phase saturated in Cr. These structures may evolve more or less at high temperature. High values of hardness were obtained for some of these alloys, suggesting high strength at elevated temperature. The hardness evolution vs. the Cr content was well represented by a law of mixture of the volume fractions and the hardness of the separated phases. Already intrinsically resistant to oxidation at high temperature, most alloys are also not sensitive to internal nitridation, contrary to pure Cr. Such Cr–Ni alloys may be considered as possible bases for heat-resistant alloys.