Relation between grain boundary segregation and grain boundary character in FCC alloys

An analytical model of segregation at grain boundaries, which takes into account all five macroscopic parameters of grain boundary character, has been developed. The model is based on a combination of previous bond energy treatments of grain boundary energy and of segregation to free surfaces. It is tested by comparing its predictions against previous computations of segregation to symmetrical twist grain boundaries in simple fcc alloys obtained by Monte Carlo simulations in conjunction with embedded atom method potentials. The comparison shows good agreement with the previous computer simulations. Examples of model predictions in the case of asymmetric grain boundaries are also provided.     

Effect on the grain interior and grain boundary reactions by Cr addition to low gold content alloys

The effect on grain interior and grain boundary reactions by Cr addition to low gold content alloys were investigated by electric resistivity measurements, hardness tests, optical microscopic observations, and TEM observations. The grain interior reactions were accelerated by the chromium addition while grain boundary reactions were retarded. The formation of AuCu l type ordered phase in the grain interior was accelerated, and the T-T-T curve of AuCu l type ordered phase shifted to higher temperatures and shorter times with increasing chromium content.     

Effect of solute interaction on interfacial segregation and grain boundary embrittlement in binary alloys

The effect of solute interaction on interfacial segregation and intergranular embrittlement is modeled on the basis of the combined Fowler and Rice–Wang approaches in a binary system using the chosen values of standard thermodynamic parameters of interfacial segregation and varied values of the binary interaction coefficients. It is clearly shown that attractive interaction strengthens interfacial segregation and substantially enhances intergranular embrittlement, while repulsive interaction exhibits an opposite effect. This finding is demonstrated in the available literature data.     

Retardation of grain boundary reactions in Ag-Pd-Cu alloys by additions of small amounts of other elements

The grain-boundary reactions in Ag-25 mass% Pd-10 mass% Cu alloys with 1 mass% aluminium, cobalt, tin, chromium or indium, were investigated by optical microscopy, electrical resistivity, and hardness tests. Addition of tin and chromium retarded grain-boundary reactions. The growth rate of nodules with tin addition was 1/18 of the alloy without additons. Grain-interior reactions are accelerated with chromium, tin and indium addition, with chromium the most effective. The activation energies for the grain-interior and grain-boundary reactions are 192 to 196 and 144 to 208 kJ mol^–1, respectively. Additions retard the growth but not the nucleation at the grain boundary. It is concluded that elements which form stable precipitates in the grains retard the grain-boundary reaction.     

合金元素对Cu-Te-Cr合金抗氧化性能的影响

利用氧化增重法,结合SEM、XRD等不同测试手段,研究了不同碲含量的Cu-Te-Cr合金在300～600℃的抗氧化性能.实验结果表明,在纯铜基体中加入Te、Cr元素,合金发生了内氧化,提高了氧化膜和合金基体的结合强度和氧化膜的致密度,增强了氧化膜的粘附性,从而提高了铜合金的抗氧化性能;且随着温度的升高,合金的氧化趋势由抛物线规律向直线规律转变.     

Effect of titanium on the structure and mechanical properties of Cu-Al-Ti alloys

The structure and mechanical properties of Cu10 wt% Al base alloys with 0–2.5 wt% Ti additions were investigated using transmission electron microscopy, optical microscopy and tensile tests. Addition of titanium has a decreasing effect on the grain size after quenching fromα + β region and causes significant strengthening of alloys. Alloy containing 1 wt%Ti quenched from 900° C shows mixture ofα, retainedβ (DO₃), disorderedβ′ (3R) and orderedβ′ ₁ (18R) martensites. Alloy with 2.5 wt% Ti addition after quenching containsα, retainedβ (DO₃), ordered T₁ phase of L2₁ superlattice and orderedβ′ ₁ martensite with either R18 or L1₀ structure indicating different stacking of ordered planes as the effect of titanium addition.     

冷变形对IF钢晶界特征分布及连通性的影响

为了研究冷轧工艺对IF钢显微组织的影响.本文借助电子背散射衍射技术,研究了20%到75%冷轧压下率的IF钢退火后的微观结构、晶界特征分布和晶界连通性.结果表明:随着冷轧压下率的增加,冷轧态晶粒逐渐由等轴状变为纤维状,退火态晶粒逐渐细化并变得均匀,小角晶界的出现频率呈下降趋势;随机晶界的出现频率呈明显上升趋势,且随机晶界占绝对优势;而CSL晶界在冷轧压下率40%以下时,变化较小,40%以上时,含量明显增加.大变形量轧制后的IF钢CSL晶界主要由Σ3晶界组成,而小变形量轧制后主要是Σ3和Σ13b晶界.对于75%压下率,还含有较多的Σ7,Σ9和Σ11等CSL晶界.通过增加冷轧压下率,0-CSL三叉晶界的含量减少,1-CSL三叉晶界的含量增加.因此,通过改变冷轧工艺,可以优化IF钢板的晶界特征分布及连通性.     

Effect of grain boundary faceting on kinetics of grain growth and microstructure evolution

The Von-Neumann-Mullins relationship for two-dimensional grain growth is modified for the case of grain boundary faceting. It is shown that the anisotropy of grain boundary energy alone slows down the rate of normal grain growth. For highly mobile facets, however, the acceleration of the growth process is possible, accompanied by development of anisotropic microstructure. It is shown that the mean-field approach to the problem of grain growth in highly anisotropic polycrystal results in parabolic growth law similar to that for isotropic systems, with the facet mobility and maximal torque substituting the grain boundary mobility and grain boundary energy in isotropic systems.     

S32750双相不锈钢相界与晶界特征对其力学性能和耐蚀性能的影响EI北大核心CSCD

通过固溶处理获得不同初始组织状态的S32750双相不锈钢样品,然后进行厚度压下量80%的冷轧变形和1050℃的退火处理,采用SEM-EBSD和XRD技术研究合金相界与晶界特征以及相组成分布情况,并利用拉伸实验、纳米压痕和双环电化学动电位再活化法(DL-EPR)分析不同初始状态样品的组织对力学性能与耐晶间腐蚀性能的影响规律。结果表明:高温固溶处理的合金样品经冷轧退火后晶粒细小均匀,两相分布接近1∶1,且相界占内界面(晶界+相界)比例较高,同相晶粒团簇程度最低,表现出优异的综合力学性能。合金样品经敏化处理后,σ相易沿α相晶界析出,高温固溶并经轧制退火后的样品中,由于α晶界比例较少且满足K-S取向关系的相界比例较高则又表现出良好的晶间腐蚀抗力。因此,通过适当的工艺来调控合金的相界与晶界分布可以实现材料强度和晶间腐蚀抗力的同步改善。     

Surface and grain boundary complexions in transition metal – Bismuth alloys

This article reviews experimental and theoretical work related to grain boundary complexions in transition metals, especially bismuth alloy complexions on nickel and copper surfaces and grain boundaries. One-, three-, five-, and seven-layer bismuth complexions are observed on the Ni(1 1 1) surface. Recent experiments suggest that Bi impurities segregate to form bilayer complexions on Ni and Cu grain boundaries which could possibly explain liquid metal embrittlement. Density functional theory calculations of Bi films on transition metal grain boundaries confirm that Bi bilayer complexions (actually a pair of monolayers bound to the metal surfaces) are thermodynamically stable. Meanwhile, complexion transitions have been demonstrated with molecular dynamics and Monte Carlo simulations and are supported by analytical thermodynamic models.     

Effect of alloying elements on microstructure,mechanical and damping properties of Cr-Mn-Fe-V-Cu high-entropy alloys

A series of new Cr-Mn-Fe-V-Cu high-entropy alloys were prepared by arc melting and suction casting. It is found that with the addition of Cu, the structure of the alloys evolved from BCC?+?BCC1 phases to BCC?+?FCC phases. With increase of Cu, the volume fraction of the Cu-Mn-rich FCC phase increased, and the morphology of the FCC phase transformed from granular particles to long strips and blocks. Compared with other reported HEAs, the Cr-Mn-Fe-V-Cu HEAs exhibit a good balance between strength and ductility. The CrMn0.3FeVCu0.06 alloy with granular FCC particles exhibits the highest compressive yield strength (1273?MPa) and excellent ductility (ε_f?=?50.7%). Quantitative calculations for different strengthening mechanisms demonstrate that dislocation and precipitate strengthening are responsible for high strength of the CrMn0.3FeVCu0.06 alloy, while the solid solution strengthening effect is very low because of its small atomic-size difference. In addition, the CrMn0.3FeVCu0.06 alloy exhibits good damping capacity due to its high dislocation and interface damping effects. Therefore, the dislocation density and distribution of FCC phase are the crucial factors for improvement of both mechanical properties and damping capacity of the HEAs.     

Influence of vanadium on grain boundary segregation of phosphorus in iron and iron-carbon alloys

The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.     

The effect of solutes on grain boundary mobility during recrystallization and grain growth in some single-phase aluminium alloys

The effect of solutes (Si, Mn, Mg) in quantities typical of commercial aluminium alloys, on grain boundary mobility in aluminium, has been investigated with in situ annealing and electron backscattered diffraction in the SEM, and grain growth experiments. The in situ experiments provided information on the migration of the high mobility tilt boundaries of misorientations close to 40°〈1 1 1〉. Grain growth experiments were used to investigate boundary migration in alloys of high solute content (1-5wt%Mg), and a comparison between the in situ and bulk experiments is made. The relationship between boundary velocity and driving pressure was found to be linear in all cases, and the activation energies for boundary migration were higher than those controlled by lattice diffusion of the solutes at higher solute concentrations.     

Influence of grain size on tensile properties of Al-Mg alloys

Abstract

Grain size refinement is an important strengthening mechanism in Al-Mg 5000 series alloys, which have a relatively large Hall-Petch slope compared with other Al alloys. In addition, the high work hardening rate exhibited by Al-Mg alloys provides excellent formability. This paper investigates the influence of grain size on the flow stress over a range of strains, and in several different Al-Mg alloys. It is found that the Hall-Petch slope decreases after yield, indicating that the large grain size effect is primarily associated with initiating plasticity in these alloys. Beyond yield the slope decreases to a value equivalent to other, non-Mg containing alloys, and shows no clear dependence on strain. The intercept stress from the Hall-Petch plots at different strains is non-linear with ? 1/2 for alloys containing up to 3 wt-%Mg, which indicates that the free slip distance is strain dependent in these alloys. In an Al-5 wt-%Mg alloy the intercept stress is linear with ? 1/2, indicating that solute atoms are controlling the free slip distance. If Mn is added to the Al-5 wt-%Mg alloy, as it is in commercial alloys, it has little influence on the grain size dependence, but it does increase the frictional stress at the highest Mn level of 0.7 wt-%.     

Effect of cooling rate on structure and mechanical properties of monotectoid zinc-aluminium alloys

The cooling curves of a number of monotectoid zinc-aluminium allows have been obtained during casting at different pouring and mould temperatures using thermocouples and an analogue-digital converter in a computer. The average cooling rate for each ingot of the allows was determined by taking the derivative of the cooling curves. The effect of cooling rate on the structure and properties of the alloys was investigated. It was observed that as the cooling rate increased the secondary dendrite arm spacing of the alloys decreased but their hardness, tensile strength and percentage elongation increased. Correlation of the experimental results showed that the cooling rate of the alloys could be related to their secondary dendrite arm spacing, hardness, tensile strength and percentage elongation using first and second degree mathematical equations. In addition, the Skenazi equation was found to be applicable to these alloys. As a result of this work, the optimum range of cooling rate for the alloys was found to be between 4·40?4·56 K s^-1.