AZ91 Mg-Al合金中β-(Mg_(17)Al_(12))析出相的形态及其晶体学特征

应用透射电镜研究了时效ＡＺ９１ Ｍｇ－Ａｌ合金连续析出相的显微组织形态及晶体学特征。发现四种不同形态和晶体学特征的β－（Ｍｇ１７Ａｌ１２）析出相，其中占绝大多数的是板条状析出相沿基面（０００１）α析出的板条析出相与基体保持Ｂｕｒｇｅｒｓ位向关系，生长方向为［７， ２，９，０］；其晶体学特征与由不变线应变原理预测的完全一致。第二类析出相是以其轴线与基面垂直的六棱柱体，六个棱柱面都是｛１１００｝α／／｛１１０｝β，与基体保持对称性好的Ｃｒａｗｌｅｙ位向关系。另有少量短棒状析出相与基体保持Ｐｏｒｔｅｒ位向关系。实验中还观察到一种未见报道过的位向关系。讨论了形成不同形态和晶体学特征的机制。     

铁硅合金轧制-退火过程中立方取向晶粒的形核与长大

利用金相显微镜和EBSD技术分析研究了Fe-3.2%Si合金二次冷轧织构、(100)[001]立方取向晶粒形核、初次再结晶以及二次再结晶后立方织构的形成。结果表明,二次冷轧之后的织构主要为{111}<112>和{111}<011>,并存在少量的{112}<110>,同时在变形晶粒内部存在有接近{100}<001>取向的微区。冷变形晶粒内部各微区取向连续变化,并且逐渐向近立方取向靠近。冷变形晶粒内部立方取向的微区作为形核的核心,在退火过程中利用(100)晶粒低表面能和γ→α相变最终发展成为具有集中立方织构的柱状晶组织。     

贝氏体相变的形核与长大——贝氏体相变新机制(一)

研究贝氏体形核长大具有重要理论价值。实验观察了贝氏体的形核,从块状贝氏体形成机理研究起,延伸到各种碳含量钢的贝氏体相变机制,指出在超低碳钢中是γ→BF相变,在其他钢中是贫碳区(γ)→BF相变,本质上相同,仅仅组织形貌不同。奥氏体中存在贫碳区,依靠涨落形成贫碳区,贝氏体铁素体在贫碳区中形核。形核地点优先选择奥氏体晶界,仅下贝氏体可在晶内形核。计算了临界晶核尺寸和形核功。贝氏体形核-长大是界面上原子非协同热激活跃迁过程。超低碳贝氏体可呈块状、条片状,增加碳含量时,则以亚单元方式形成贝氏体铁素体(BF),形貌从块状向条片状演化。     

TiAl_3对TiC粒子在铝基体中分布及α(Al)晶粒形核的影响

研究TiC和TiAl_3细化工业纯铝时TiAl_3的存在对TiC在铝基体中分布及α(Al)晶粒形核的影响,分析Al-Ti-C晶粒细化机制.结果表明:TiC单独作为工业纯铝的晶粒细化剂时,大量TiC被α(Al)晶粒推向树枝晶的晶界处,从而限制了TiC的异质形核作用;当TiC和TiAl_3共同作为晶粒细化剂时,在α(Al)晶粒内部出现了大量TiC粒子,大量的TiC粒子成为了α(Al)的结晶核心,并且在TiC颗粒和铝基体的界面处存在"富Ti过渡区";TiAl_3在铝熔体中分解释放出Ti原子并向TiC粒子周围偏聚,形成的"TiC/铝熔体界面富Ti过渡区" 改善了TiC与α(Al)的结构适应性,降低了TiC粒子的表面张力,促进了TiC粒子在铝熔体中的均匀分布,提高了其形核能力.     

A crystallographic model of fcc/bcc martensitic nucleation and growth

A crystallographic model of face-centered cubic/body-centered cubic martensitic nucleation and growth has been developed in which the glissile martensitic interface is described in terms of discrete transformation and misfit dislocations within the framework of the Frank–Bilby equation. In the present model heterogeneous nucleation of a martensitic embryo occurs spontaneously by dissociation of an existing perfect dislocation under certain conditions, such as the catalysis on grain boundaries. Autocatalytic dislocation is proposed to be responsible for embryo growth, then nucleation and subsequent growth of a single embryo can be regarded as an autocatalytic process. As a natural result the crystallographic features of grown martensite are consistent with the phenomenological theory of martensite crystallography. Martensitic nucleation in this model agrees with many experimental observations.     

马氏体相变切变机制的实验和理论研究——评刘宗昌等的马氏体相变非切变机制

刘宗昌等学者近几年相继在国内杂志发表了有关马氏体相变非切变机制的论文,并将这些论文作为他们2012年出版的《马氏体相变》一书中的主要创新内容予以强调。他们否定已被国际公认的马氏体相变切变机制的主要依据有两条:1)马氏体相变切变机制缺乏实验基础,即缺乏现代实验技术,如透射电镜(TEM),原子力显微镜(AFM)的实验证明;2)根据他们的计算,马氏体切变能太大,相变驱动力无法克服切变能。本文作者首先结合马氏体晶体学表象理论(PTMC)给出论证马氏体相变切变机制的TEM和AFM实验;随后介绍徐祖耀提出的计算马氏体相变切变能的方法;最后评论刘宗昌等的论断和切变能的计算方法。     

Concurrent nucleation, formation and growth of two intermetallic compounds (Cu6Sn5 and Cu3Sn) during the early stages of lead-free soldering

This study investigates the concurrent nucleation, formation and growth of two intermetallic compounds (IMCs), Cu₆Sn₅ (η) and Cu₃Sn (ε), during the early stages of soldering in the Cu-Sn system. The nucleation, formation and growth of the IMC layers is simulated through a multiphase-field model [1] and [2] in which the concurrent nucleation of both IMC phases is considered to be a stochastic Poisson process with nucleation rates calculated from classical nucleation theory [3]. CALPHAD thermodynamic models are used to calculate the local contributions to the free energy of the system and the driving forces for precipitation of the IMC phases. The nucleation parameters of the η phase are estimated from experimental results [4] and those of the ε phase are assumed to be similar. A parametric investigation of the effects of model parameters (e.g. grain boundary (GB) diffusion rates, interfacial and GB energies) on morphological evolution and IMC layer growth rate is presented and compared with previous works in which nucleation was ignored [5]. In addition, the resulting growth rates are compared with the available literature and it is found that, for a certain range in the model parameters, the agreement is quite satisfactory. This work provides valuable insight into the dominant mechanisms for mass transport as well as morphological evolution and growth of IMC layers during early stages of Pb-free soldering.     

Nucleation and growth of the γ′(AlAg2) precipitate in Al–Ag(–Cu) alloys

Precipitation of the γ′(AlAg₂) phase was investigated in Al–Ag(–Cu) alloys using high-resolution transmission electron microscopy and scanning transmission electron microscopy. Precipitation commenced with segregation of Ag to stacking faults, followed by thickening in steps corresponding to single unit cell height ledges. In conjunction with gradual segregation of Ag and Al into ordered layers, this yielded γ′ phase platelets with a thickness of either 2 or 3 × the AlAg₂c-lattice parameter. Plates with a thickness of 2c(AlAg₂) could not achieve self-accommodation of the shape strain for transformation. Further thickening of the precipitates was slow, despite considerable Ag segregation around the precipitates. Growth by the addition of single unit cell height ledges is expected to lead to an additional shear strain energy barrier to ledge nucleation and this may contribute to a process of nucleation-limited growth.     

Stability of Retained Austenite Through a Combined Intercritical Annealing and Quenching and Partitioning(IAQP) Treatment

Intercritical annealing(IA) at various temperatures followed by quenching and partitioning(IAQP) treatments was conducted on a cold-rolled Fe-0.2C-1.42Si-l.87Mn(wt%) sheet steel.Optimized microstructure and enhanced mechanical properties were achieved through appropriate adjustment of IA temperatures.The steel which was annealed at1,033 K for 600 s,then quenched to 573 K and partitioned at 693 K for 20 min,designated as 1033 QP steel,exhibits maximum 16.3 vol% retained austenite(RA) with good mechanical properties(ultimate tensile strength 886 MPa and total elongation 27%).It was found that the thermal and mechanical stabilities of RA are mainly influenced by the combined effect of its average carbon content and amount of adjacent martensite.Furthermore,the transformation-induced plasticity effect increased the peak n-values observed at the second stage of the work hardening of IAQP steels.     

Mg2+对氧化亚铁硫杆菌生长活性的影响

研究了在常用的细菌浸出条件下（ｔ＝３０℃,ｐＨ２．０）,Ｍｇ＾２＋对氧化亚铁硫杆菌（Ｔ．ｆ）５生长活性的影响。结果表明,实验所用菌株具有一定的耐Ｍｇ＾２＋能力;当培养基中ｐ（Ｍｇ＾２＋）≤１０．５ｇ／Ｌ时,Ｍｇ＾２＋对细菌的生长活性没有不利的影响;ｐ（Ｍｇ＾２＋）１５．５ｇ／Ｌ时对细菌生长活性的影响较明显;Ｐ（Ｍｇ＾２＋）２０．５ｇ／Ｌ时细菌的生长完全受到掏。经驯化的珠该菌株耐Ｍｇ＾２＋性明显提高     

Structure and bonding orientation of favorable growth unit Al6（OH）18（H2O）6 of gibbsite

1INTRODUCTION Thecrystalgrowthcharacteristicsofgibbsite insupersaturatedsodiumaluminatesolutionwere extensivelyinvestigated[15],butthestructureand formofthegrowthunitwerenotrelated.Firstly,Vanstratenetal[6]indicatedthatthegrowthunit transitionstateofaluminateacidradicalexistedin thecrystallizationofgibbsitefromalkalinesodiumaluminatesolution.Parkinsonetal[7]figuredout thatAl(OH)-4,Al(OH)2-5andAl(OH)3-6would belikelytobecomegrowthunitsofgibbsitefromtheviewpointofcrystalsurfacebindingene…     

Statistical model for atomic distances and site occupation in zinc-blende diluted magnetic semiconductors (DMSs)

The statistical strained-tetrahedra model is applied to describe the behaviour of zinc-blende ternary diluted magnetic semiconductors (DMSs) with manganese. Validation is confirmed by comparing calculated and published extended X-ray absorption fine structure (EXAFS) measurements relative to average nearest-neighbour and next nearest-neighbour inter-ion distances in zinc blende DMSs at different Mn contents. The sizes, shapes and site occupation preferences (SOPs) with respect to random, of elemental-configuration tetrahedra, are fully defined for ZnMnSe, ZnMnS and ZnMnTe. For CdMnTe and HgMnTe, all the tetrahedra are defined, with the exception of the configuration with a central Te ion and two cations of each kind at the vertices, which remains inaccessible due to lack of relative experimental data. It is shown that SOPs can normally be derived from EXAFS observation of the variation either of distance or of coordination-number measurements as a function of dilution, with both sets giving results in good agreement.     

Selective laser sintering/melting (SLS/SLM) of pure Al,Al–Mg,and Al–Si powders: Effect of processing conditions and powder properties

Selective laser sintering/melting (SLS/SLM) processing difficulties of aluminium powders had been attributed to issues associated with laser–materials interaction only while neglecting the role of powder properties. This study provides a wholistic understanding of factors that influence the development of SLS/SLM processing window, densification, and microstructure of pure Al, Al–Mg, and Al–Si powders, fabricated in single and multiple layer parts by exploring the roles of processing and material parameters. It was demonstrated that similarities existing in the SLS/SLM processing maps of the powders could be attributed to similarities in their packing densities with the alloying addition of magnesium and silicon having no predominant effect on their processing maps’ boundaries. Rather, alloying addition has significant effect on the nature of the evolved surface morphology of SLS/SLM processed aluminium powders in their processing windows. In addition, the flow and solidification behaviour of the melt pool of the powders during single layer scan was strongly influenced by the particle morphology and oxygen content of the powders as well as applied energy density. The energy density in the range of 12–16 J/mm² was found to be the threshold below which SLS was predominant and above which SLM occurred for the investigated powders. Moreover, successful oxide disruption phenomena which is necessary for inter-particulate coalescence in multi-layered SLS/SLM processed aluminium powders are found to be mainly controlled by the amount of oxide in the as-received powder, the degree of the uniformity of the distribution of the surface oxide film covering the aluminium particles, the nature of thermal mismatch existing between the oxide film and the parent aluminium particle which was dependent on the phase present in the oxide film. Al–12 wt% Si powder is hereby affirmed as a suitable candidate material for SLS/SLM process due to its low thermal expansion and uniform distribution of its surface oxide films as well as the mullite phase in its oxide film.     

A comprehensive tool-wear/tool-life performance model in the evaluation of NDM (near dry machining) for sustainable manufacturing

Traditionally, metal working fluids (MWF) are known to improve machining performance despite poor ecological and health side effects. A new sustainable process that has minimized the use and application of MWFs is NDM (near dry machining). Although there is much controversy on the effectiveness of NDM, it is agreed that a lack of science-based modeling prevents its widespread use. This paper presents a new method to predict tool-wear/tool-life performance in NDM by extending a Taylor speed-based dry machining equation. Experimental work and validation of the model was performed in an automotive production environment in the machining of steel wheel rims. Machining experiments and validation of the new equation reveal that tool-wear can be predicted within 10% when the effect of NDM is statistically different than dry machining. Tool-wear measurements obtained during the validation of the model showed that NDM can improve tool-wear/tool-life over four times compared to dry machining which underlines the need to develop sustainable models to match current practices.