Fe—Cr—Mn（W,V）合金焊接热影响区的辐照损伤及诱发的晶界元素偏析

采用电子束辐照研究裂变－聚变混合反应堆预选结构材料Ｆｅ－Ｃｒ－Ｍｎ（Ｗ,Ｖ）合金的焊接热影响区（ＨＡＺ）组织损伤,测定损伤组织和晶界处的合金元素浓度变化。实验结果表明：低温下剂达１０ｄｐａ,合金奥氏体组织稳定,未发现空洞形成和第二相析出;然而在高温下,剂量达４．８ｄｐａ就明显出现低密度微小空洞,并出现晶界移植现象。其次,在低温下,晶界附近大尺寸溶质原子Ｃｒ和Ｍｎ贫化,而小尺寸原子Ｓｉ,Ｖ,Ｎｉ,Ｗ     

Effect of grain boundary orientation on radiation-induced segregation in a Fe–15.2 at.% Cr alloy

Ferritic chromium steels are important structural materials for future nuclear fission and fusion reactors due to their advantages over traditional austenitic steels, such as higher thermal fatigue resistance, lower thermal expansion coefficients and reduced swelling. However radiation-induced segregation or depletion (RIS/RID) of solute atoms at grain boundaries in these materials is a concern because these phenomena could adversely affect their mechanical properties. In an effort to develop a full mechanistic understanding of RIS/RID, a systematic approach combining orientation imaging, site-specific specimen preparation and three-dimensional atomic-scale analysis has been developed to characterize the behaviour of Cr and C at grain boundaries during irradiation. This methodology has been applied to a Fe–15.2 at.% Cr alloy to investigate the effects of grain boundary misorientation, irradiation depth and impurities. Systematic differences in Cr segregation are reported as a function of grain boundary character and irradiation conditions. The similar properties demonstrated by grain boundaries of similar type means that it should be possible to apply relatively simple models to predict the long-term behaviour of these materials under irradiation conditions.     

Al-Zn-Mg-Cu合金在时效过程中η相沿小角晶界的析出序列

采用高分辨透射电子显微镜和高角环形暗场扫描透射电子显微镜结合X射线能谱仪,研究Al-Zn-Mg-Cu合金叩相沿小角晶界的析出序列。试样在135℃分别时效5min到6h。结果表明,η相在小角晶界的析出序列是：SSS→VRC→GPⅡ区→η’→η。基于非平衡晶界偏析和非平衡晶界共偏析理论,在Al-Zn-Mg-Cu合金时效过程中,通过溶质一空位对的扩散,大量的沉淀形成元素偏析到晶界。这种晶界偏析在VRC、GPⅡ区、η’相和η相的形核和生长中起重要作用。     

Grain coarsening inhibited by solute segregation

It will be shown that grain boundaries are in a metastable thermodynamic equilibrium in the presence of solute atoms and, therefore, grain coarsening is stopped as there is no driving force. This is in contradiction to a generally accepted interpretation, where solute drag, i.e. zero mobility of the boundaries, stops grain coarsening. Based on the empirical relation between terminal solubility of a solute and its grain boundary segregation it can be shown that a two-phase mixture with solute atoms agglomerating in a precipitated phase will be the stable thermodynamic equilibrium state. However, if precipitation is kinetically hindered, the metastable equilibrium with a certain grain boundary area and a zero grain boundary energy is attained. Changes in this grain boundary area or grain size respectively are calculated as a function of temperature and compared with experimental findings.     

NON-EQUILIBRIUM SEGREGATION OF SOLUTE TO GRAIN BOUNDARIES

In consideration of the local equilibrium among vacancies,solute atoms and vacancy-soluteatom complexes and the influence of equilibrium grain boundary segregation,theoretical dy-namic formulas for non-equilibrium grain boundary segregation of solute have been derivedon the basis of the vacancy-dragging mechanism.Theoretical calculation by computer hasbeen carried out for the non-equilibrium segregation of boron to austenitic grain boundariesduring isothermal holding and continuous cooling after heating at high temperatures.The re-suits agree well with those obtained from experiments.     

THE NON-EQUILIBRIUM SEGREGATION OF BORON TO AUSTENITE GRAIN BOUNDARIES

采用高分辨率的径迹显微照相技术,研究了淬火硼钢中硼向奥氏体晶界偏聚的规律,定量地测定出跨过奥氏体晶界的硼的成分剖面图以及非平衡晶界偏聚的特征参量(晶界贫硼区宽度、晶界富集程度和富集带宽度)。试验表明,这种偏聚具有如下特征: 在偏聚晶界的两侧存在有一定宽度的贫硼区,晶界偏聚的硼是在冷却过程中由该区富集而来;这种偏聚对冷却速度很敏感,急速冷却可以抑制这种偏聚。冷却速度降低,晶界偏聚由连续的偏聚带,逐步发展为不连续的聚集直至明显地析出硼相,贫硼区宽度与冷却速度的平方根成反比;它的温度关系与晶界平衡偏聚预言相反,随淬火温度升高,晶界偏聚程度与贫硼区宽度增加。 通过试验,论证了淬火钢中硼向奥氏体晶界的偏聚,是在冷却过程中发生的一种非平衡的晶界偏聚现象。     

热矫正次数对7N01铝合金应力腐蚀开裂的影响

采用慢应变速率拉伸试验(SSRT)和透射电镜(TEM)测试分析方法研究了热矫正次数对7N01-T5铝合金应力腐蚀敏感性的影响。结果表明:随着热矫正次数的增加,7N01-T5铝合金应力腐蚀敏感性增加。原因主要在于晶界析出相的转变以及溶质元素Zn和Mg从基体向晶界的不断扩散。基于非平衡偏聚理论可知,三次热矫正的等效恒温时间小于临界时间t_c。因此,随着热矫正次数的增加,晶界处Zn和Mg元素含量逐渐增加,增大了晶界与基体间的腐蚀电位差,使7N01-T5铝合金应力腐蚀敏感性增加。     

Solute segregation transition and drag force on grain boundaries

We investigate solute segregation and transition at grain boundaries and the corresponding drag effect on grain boundary migration. A continuum model of grain boundary segregation based on gradient thermodynamics and its discrete counterpart (discrete lattice model) are formulated. The model differs from much previous work because it takes into account several physically distinctive terms, including concentration gradient, spatial variation of gradient-energy coefficient and concentration dependence of solute–grain boundary interactions. Their effects on the equilibrium and steady-state solute concentration profiles across the grain boundary, the segregation transition temperature and the corresponding drag forces are characterized for a prototype planar grain boundary in a regular solution. It is found that omission of these terms could result in a significant overestimate or underestimate (depending on the boundary velocity) of the enhancement of solute segregation and drag force for systems of a positive mixing energy. Without considering these terms, much higher transition temperatures are predicted and the critical point is displaced towards much higher bulk solute concentration and temperature. The model predicts a sharp transition of grain boundary mobility as a function of temperature, which is related to the sharp transition of solute concentration of grain boundary as a function of temperature. The transition temperatures obtained during heating and cooling are different from each other, leading to a hysteresis loop in both the concentration–temperature plot and the mobility–temperature plot. These predictions agree well with experimental observations.     

微量溶质元素在金属晶界的偏聚

溶质元素在金属晶界的偏聚可以分为两类，即平衡偏聚和非平衡偏聚。本文回顾了这两类偏聚现象及其机理，着重讨论分析了有重要工程用途的三类典型的晶界偏聚：硼在奥氏体晶界的两类偏聚的实验规律及对硼钢淬透性影响；磷在奥氏体晶界的平衡偏聚及对钢回火脆性的作用，钢中的其他合金元素对磷的偏聚的影响；硼在多种金属间化合物的偏聚及对这些金属间化合物塑性的影响及其机理。     

BEHAVIOUR OF BORON SEGREGATION AT GRAIN BOUNDARIES IN BCC Fe-3% Si ALLOY

The behaviour of baron segregation at grain boundaries in Fe-3% Si has been studied bymeans of particle tracking autoradiography(PTA).The results indicate that the tendency ofequilibrium segregation of B at grain boundaries in Fe-3% Si is higher than that in austeniticalloys.No observable nonequilibrium segregation of B at grain boundaries was revealed inFe-3% Si alloy during cooling and isothermal holding.The binding energy between boron at-oms and grain boundaries is 55.7±1.7 kJ/mol.Based on the complex mechanism of B segre-gation of quenching-induce-nonequilibrium segregation,different behaviours of B segrega-tion in γ-Fe and bcc Fe-3% Si alloy were discussed.     

The role of segregation in diffusion-induced grain boundary migration

The problem of grain boundary motion in the diffusion field of a solute is formulated for the case of infinitely fast diffusion along a straight boundary. The steady state solution suggests that (de)alloying occurs by two different modes, namely: the solute diffusion through the stationary boundary to the bulk, or by diffusion-induced grain boundary migration (DIGM). The transition from one mode to another depends on the grain boundary segregation coefficient. The result enables an assessment of the relative importance of different possible driving forces. When the equilibrium concentrations of the bulk solute with the external gas is low, the entropy of mixing is the leading driving force. DIGM does not occur in isotope solution because the solute atom does not segregate to the boundary. Based on this theory, we construct the phase diagram in the plane of the (gas/bulk) equilibrium concentration vs the segregation coefficient, representing the transition from DIGM to alloying via stationary boundaries.     

电子辐照Fe-Cr-Ni奥氏体合金的空洞体胀与诱起晶界偏析的研究

本文用超高压电镜(HVEM)作为电子束源,研究了Fe-15％Cr-30％Ni合金电子辐照后,合金空洞体胀和诱起晶界偏析.实验结果证明:723K,最大剂量15dpa条件下,体胀量为0.62％.在晶界发生Cr贫化、Ni富化.随着辐照温度提高,晶界偏析程度增加,723K时Cr,Ni在晶界出现最大偏析峰,相应空洞体胀量也最大.结果表明,空洞形成和晶界偏析密切相关,本文提出了辐照诱起晶界偏析模型.     

Sc,La对Al-Mg焊料中杂质元素偏聚的调控作用

采用金相显微镜、扫描电镜、能谱分析和X射线衍射仪,研究了添加微量钪、镧对铝镁焊料中杂质元素偏聚的调控作用. 结果表明,在铝镁焊料合金凝固过程中,铁、硅等杂质元素在晶界有偏聚效应,添加微量钪、镧可以降低铁、硅杂质在晶界的偏聚浓度. 钪与镧减轻杂质元素在晶界偏聚的效果存在差别,添加钪的合金比添加镧的合金晶界处杂质元素偏聚强度明显减弱. 造成这种差别的原因是,钪、镧对杂质元素偏聚的调控机制不同.     

Heterogeneous dislocation formation and solute redistribution near grain boundaries in austenitic stainless steel under electron irradiation

We investigated the radiation-induced segregation (RIS) near grain boundaries in austenitic stainless steel under electron irradiation, taking into account the simultaneous evolution of faulted dislocation loops and network dislocations. The formation of a dislocation-free zone (DLFZ) in the vicinity of the grain boundary due to in situ irradiation was observed using a high-voltage electron microscope (HVEM). Predicted temperature and dose dependencies of DLFZ formation agreed with the experimental results. The relationship between RIS and DLFZ formation near a grain boundary has been substantially clarified.     

固溶态不锈钢在尿素生产介质中的晶间腐蚀与磷(硅)的晶界富集

用扫描电镜、透射电镜能谱和俄歇能谱等研究了尿素厂汽提塔内实际腐蚀部件的腐蚀形态和机理。结果发现:固溶态的316L 不锈钢发生了晶间腐蚀,并伴生全面腐蚀和晶粒脱落,因而晶间腐蚀不深;其晶界区并不贫铬但富集磷、硅,晶界磷含量比钢中平均含量高二个数量级;而同样介质条件下未遭受晶间腐蚀的固溶态00Cr25Ni22Mo2N 高纯不锈钢却未见磷、硅的晶界富集。故认为磷(硅)的晶界富集是固溶态316L 不锈钢在尿素生产介质中产生晶间腐蚀的重要原因之一.     

Grain growth in dilute alloys of copper

An experimental study of grain growth in dilute binary alloys of copper was performed. It was found that the grain growth law D = Kt ⁿ fit most of the data. The grain growth exponent n decreased with solute content until a saturation value was obtained at slightly under 0.2 for both silicon and aluminum solutes. It is concluded that the grain growth exponent n depends upon solute adsorption at the grain boundaries. An estimate of the activation energy was made and it was found that the activation energy decreased with temperature. The activation energies for grain boundary migration compare well with values obtained from internal friction studies on identical alloys. This contributes strong experimental support for the two-step mechanism for complete grain boundary stress relaxation.     

Thermodynamic aspects of the grain boundary segregation in Cu(Bi) alloys

The grain boundary segregation of Bi in dilute polycrystalline Cu–Bi alloys was systematically studied as a function of temperature and composition. The temperature dependencies of the Gibbsian excess of Bi at the grain boundaries exhibited discontinuous changes at the temperatures close to, but different from the bulk solidus temperatures. The observed segregational phase transition was interpreted in terms of prewetting model.     

Hot Deformation Behavior and Microstructural Characteristics of Ti–46Al–8Nb Alloy

Hot deformation behavior,microstructural evolution and flow softening mechanism were investigated in Ti–46Al–8Nb alloy via isothermal compression approach.The true stress–strain curves exhibited typical work hardening and flow softening,in which the dependence of the peak stress on temperature and strain rate was obtained by hyperbolic sine equation with Zener–Hollomon(Z)parameter,and the activation energy was calculated to be 446.9 k J/mol.The microstructural analysis shows that the alternate dark and light deformed ribbons of Al-rich and Nb-rich regions appeared and were associated with local flow involving solute segregation.The Al segregation promoted flow softening mainly arising from the recrystallization of γ phase with low stacking fault energy.The coarse recrystallized γ and several massive γ phase were observed at grain boundaries.While in the case of Nb segregation,β/B2 phase harmonized bending of lamellae,combined with the growth of recrystallized γ grains and α+β+γ→α+γ transition under conditions of temperature and stress,leading to the breakdown of α_2/γ lamellar colony.During the hot compression process,gliding and dissociation of dislocations occurred in γ phase that acted as the main softening mechanism,leading to extensive c twins and cross twins in α/γ lamellae and at grain boundaries.In general,homogeneous microstructure during the hot deformation process can be obtained in Ti Al alloy with high Nb addition and low Al segregation.The deformation substructures intrinsically promote the formability of Ti–46Al–8Nb alloy.     

Discontinuous precipitation at the deformation band in copper alloy

The Cu-Ni-Si alloy is known as a precipitation hardening alloy, where the Ni₂Si intermetallic compound is precipitated in the matrix during aging. There are two types of precipitation of Ni₂Si: continuous and discontinuous cellular. The discontinuous cellular precipitation is generally initiated at interfaces especially grain boundaries in the matrix. To observe the grain boundary effect on the discontinuous precipitation, a large-grained Cu-Ni-Si-Ti alloy was intentionally fabricated by unidirectional solidification and plastically deformed by groove rolling. While discontinuous cellular precipitation has been generally known to occur only at the high angled grain boundaries in the alloys, we found that it was also generated inside the grains, at the deformation bands formed by plastic deformation.     

Influence of interface energies on solute partitioning mechanisms in doped aluminas

The experiments described in this paper have been designed to understand how particular dopants in alumina (Ca, Mg, Si, and Y) affect microstructural development through the energetics of their associated precipitates. Specifically, the role of the interphase boundary energy and precipitation activation energy are considered to be in competition with grain boundary complexion (disorder) transitions for partitioning excess solute. The results reveal a relationship between the relative precipitation activation energy and the temperature at which grain boundary complexion transitions occur. The large differences in activation energy primarily derive from the interphase boundary energy. Precipitates that form lower interphase boundary energies tend to suppress complexion transitions, while systems that contain precipitates with high interphase boundary energies are more susceptible. Based on the findings, a new criterion for additive selection to control complexion transitions and abnormal grain growth is proposed that is based on interfacial energies between the host and precipitate.