不锈钢热塑性降低的非平衡偏聚机理

Mintz于1997年报道了一个有趣的试验现象:随着拉伸应变速率的增加,奥氏体钢韧性降低,铁素体钢韧性反而提高。其机制未得到解释。通过计算试验钢中P原子的非平衡晶界偏聚临界时间,结果发现奥氏体钢拉伸前热过程的等效时间短于其临界时间,而铁素体钢的等效时间长于其临界时间。由于奥氏体钢和铁素体钢分别在850和800℃等效时间最接近临界时间,韧性最低,即试验钢的热塑性降低都是由于非平衡晶界偏聚的临界时间造成的。应变速率降低,弹性应力作用时间增加。晶界偏聚量改变,热塑性降低的程度也随之改变。即热塑性降低的程度随应变速率的改变是由应力引起的非平衡晶界偏聚决定。     

The grain boundary segregation of boron during isothermal holding

Boron segregation to grain boundaries was investigated by means of particle tracking autoradiography (PTA) in a low carbon and two Mo-bearing steels and in an Fe-30% Ni alloy. Non-equilibrium segregation took place after rapid cooling to a degree which increased as the temperature difference between the austenitization and subsequent holding temperatures was increased. The amount of deformation during isothermal holding had a similar effect. The maximum segregation produced in this way can be many times higher than that associated with equilibrium segregation. The observations support the view that boron atoms are transported to the boundaries by forming complexes with vacancies which migrate to the boundaries prior to annihilation. During holding at different temperatures, the boron intensity curves produced by the PTA method exhibit three types of time dependence. Two of these involve the appearance of a segregation peak, after which there is either complete or partial disappearance of the segregation. The former is associated with the back diffusion of boron into the depleted zone, a process that takes place at the highest holding temperatures and in the absence of precipitation. The latter involves the conversion of temporary segregation into grain boundary precipitates and is observed at intermediate temperatures. The third type is observed at the lowest temperatures; in this case the development of non-equilibrium segregation is converted into precipitation prior to the appearance of segregation peak. The temperature range associated with each type of curve depends on the relation between the kinetics of non-equilibrium segregation on the one hand and that of precipitation on the other.     

Surface segregation in austenitic stainless steel

We report in this paper a study of surface segregation in austenitic stainless steel. Auger electron spectroscopy was used to measure segregation as a function of time and temperature. We have found that P, N, S, Cr, and Ni will all segregate to the surface. However, their presence on the surface often depends on the competitive and attractive interactions between the various elements. We show that thermodynamic data on ternary liquid iron alloys are quite valuable in predicting these interactions. We also discuss possible applications of these studies.     

Duplex grain boundary precipitation in austenitic stainless steels containing aluminium and titanium

This paper considers the grain boundary precipitation reactions which may occur during the ageing of stainless steels based on the composition Fe-20%Cr-25%Ni which contain additions of aluminium and titanium. It is shown that the formation of the equilibrium precipitate phase, associated with the composition of the alloy, causes the subsequent formation of adjacent regions of chromium-rich ferrite. In addition, it is shown that this ferrite is unstable, and may decompose to a more stable intermetallic phase. The choice of final intermetallic phase, together with the multiphase crystallographic orientation relationships obeyed during the precipitation reaction and the local precipitate phase compositions, are discussed with respect to the overall reaction sequence.     

The effect of the surface on grain boundary migration in austenitic stainless steel weld metal

The direction of grain boundary (GB) migration that occurs after solidification in the original surface of the weld of an austenitic stainless steel was compared to that in the interior (the depth 100 μm from the original surface). The results are as follows. In the interior, the direction of GB migration was decided by the moving direction of the triple junctions that bring about their equilibrium. This equilibrium results from the GB approaching a cross angle of 120 deg with another GB in the vicinity of the triple junction. Meanwhile, in the original surface, the direction of GB migration was decided by the direction that makes the GB plane, which is just at solidification, being perpendicular to the surface. That is, the GB plane migrates to be perpendicular to the original surface. This GB migration was often observed when the angle between the GB plane and the original surface had been less than 40 deg. Furthermore, the amount of the GB migration had rapidly increased with the decrease in the angle. The validity of these results is examined with a discussion of the reduction of the interfacial energy.     

On the chemistry of grain boundary segregation and grain boundary fracture

This paper considers the problem of impurity segregation in metals and the effect of these impurities on grain boundary cohesion. The primary goal of this paper is to provide a physical model that will allow us to think about these two processes. We describe both of them in chemical terms. Segregation is treated as a distribution of a solute between two phases. In this way, it is a typical example of heterogeneous equilibrium. We also consider the various driving forces for solute segregation and find that the correlation between decreased solubility and increased segregation, first proposed by Hondros and Seah,^[9] is still an adequate one. We introduce the discussion of grain boundary fracture by pointing out that as the impurity enters the boundary, it establishes chemical bonds with the structural units of the boundary. The segregated boundary can then be thought of as a string of molecular units with bonds of different types. Some of these bonds will be weaker than others, and they will be the ones that eventually fracture when a stress is applied. We consider the cause of these weak bonds and suggest that the primary reason for them is the transfer of electronic charge from the metal atoms to the impurity, as proposed in previous work.^[3] However, some of the ideas in the earlier models should be amended based on new results obtained from the quantum mechanical analysis of bonding in metals presented by McAdon and Goddard.^[10,11] We also suggest that intergranular brittleness of intermetallic compounds such as Ni₃Al, which occurs in the absence of impurity segregation, can be explained by the charge distribution present at the grain boundary. Finally, we provide a critique of other models that have been used to describe grain boundary fracture and segregation. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Sources of variability in grain boundary segregation

This paper considers the sources of variability in segregation to random high angle grain boundaries. The variation in segregation to most grain boundaries is within ±30% of an average value. However, the composition of some boundaries is even farther away from the average. Although deviation in fracture path, bulk chemical inhomogeneities, and a lack of equilibrium can contribute to the variation, analysis of the data clearly shows that differences in grain boundary structure must play a role. The cause of the relationship between structure and segregation is discussed in terms of chemical bonds that are formed between the segregant and the metal atoms comprising the boundary. Finally, it is shown that variation in segregation in a single boundary is not as great as the variation among grain boundaries.     

Grain boundary segregation of boron in INCONEL 718

The segregation behavior of boron at grain boundaries in two INCONEL 718⁺ based alloys with different B concentrations was studied. The alloys, one containing 11 ppm of B and the other 43 ppm, were homogenized at 1200 °C for 2 hours followed by water quenching and air cooling. A strong segregation of boron at grain boundaries was observed using secondary ion mass spectrometry after the heat treatment in both the alloys. The segregation was found mainly to be of nonequilibrium type. The homogenized samples were also annealed at 1050 °C for various lengths of time. During annealing, boride particles were observed to first form at grain boundaries and then to dissolve on continued annealing at 1050 °C. The mechanisms of segregation and desegregation of B are discussed.     

奥氏体不锈钢物相的精细结构表征

以热轧态316L不锈钢为研究对象,首先利用X射线衍射仪(XRD)对其主要物相进行标定,进而利用透射电镜(TEM)的选区电子衍射和高分辨像功能确认了χ相的存在。然后利用透射电子背散射衍射技术(t-EBSD)对所有物相的分布进行表征,与常规电子背散射衍射(EBSD)的结果进行比较,准确显示了χ相的分布特征。结果表明,组织中以奥氏体基体相和带状组织为主,带状组织中主要含有σ相,还有少量的铁素体相和奥氏体相,χ相不仅存在于带状组织中,还存在于奥氏体基体中,且χ相和σ相中都富含Fe、Cr、Mo元素。     

Grain boundary segregation in austenitic stainless steels and its effect on intergranular corrosion and stress corrosion cracking

This paper reports a study of grain boundary segregation, intergranular corrosion, and intergranular stress corrosion cracking in austenitic stainless steels. The results show that phosphorus, nitrogen, and sulfur all segregate to grain boundaries in these materials and that they can affect one another's segregation through site compctition. In particular, the results demonstrate that phosphorus segregation can be lowered by the presence of nitrogen and sulfur in the steel. Also, if manganese is present in the steel, sulfur segregation will be greatly decreased as a result of formation of manganese sulfides. Phosphorus, sulfur, and nitrogen will not initiate intergranular corrosion in the modified Strauss test, although if corrosion is initiated by chromium depletion, these elements might enhance the corrosion process. Phosphorus segregation does enhance corrosion in the Huey test, even in steels that have not undergone grain boundary chromium depletion, although there does not appear to be a precise correlation between the depth of corrosion penetration and phosphorus segregation. Intergranular stress corrosion cracking in 288 °C water at a pH of 2.5 and electrochemical potential of OV_SHE can occur in these steels even in the absence of chromium depletion if sulfur is present on the grain boundaries. Phosphorus segregation appears to have very little effect.     

Role of grain boundary segregation in diffusional creep

The high temperature deformation of polycrystalline materials by the stress directed flow of vacancies is now a well established creep mechanism which operates in two temperature regimes: high temperature, or Nabarro-Herring creep, in which lattice diffusion is rate determining, and low temperature, or Coble creep, in which grain boundary diffusion predominates. Basic studies have been conducted mostly with pure metals for which there exists in general a good correspondence between predicted and observed behavior. Multicomponent engineering alloys will normally experience, as part of their processing history or service lives, the segregation enrichment of interfaces such as grain boundaries by species present in solid solution. The aim of this paper is to evaluate the experimental information and to explore the manner in which this segregation affects the principal forms of diffusional creep. Cases of retarded Herring-Nabarro creep are analyzed in terms of the efficacy of grain boundaries as sources and sinks for vacancies: strongly bound segregant atoms at grain boundaries affect the mobility of defects and hence control the operation of vacancy sources. Recently, observations have been made on the effect of strongly segregating solutes on grain boundary diffusivity. Such behavior influences Coble creep rates, producing in general a retardation. Here we assess the magnitude of the effect induced by various surface active species on grain boundary diffusivity and consequently on Coble creep; predictions show that in general, small amounts of highly surface active impurities induce a remarkable inhibition of this form of creep.     

Grain boundary segregation and grain growth inhibition in silicon iron: The effect of boron and nitrogen

The degree of grain growth inhibition in iron-3.1 pct silicon alloys with small additions of boron, nitrogen and sulfur has been observed to correlate strongly with the degree of nitrogen segregation to the grain boundaries. Grain growth was seen to increase monotonically with decreasing nitrogen segregation at 950°C, the temperature at which significant grain growth was first observed to occur. Boron affected the retention of nitrogen in the material at high temperatures and in this way had an indirect effect on grain growth inhibition. Sulfur acted to enhance the effectiveness of nitrogen as a grain growth inhibitor. It is suggested that nitrogen, even at very low grain boundary concentrations affects grain boundary migration by poisoning sites at the grain boundaries which are particularly efficient in attaching atoms to the growing grain surface. This paper is based on a presentation made at a symposium on “Recovery, Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Grain boundary diffusion and grain boundary segregation of tellurium in silver

Using the radiotracers ¹²¹Te and ¹²³Te with high specific activity, grain boundary (GB) diffusion of Te in Ag has been studied in a wide temperature interval of about 600 K. At low temperatures (378–504 K) Harrison's type-C regime has been realized, which has enabled us to perform a number of direct measurements of the GB diffusion (GBD) coefficient D_GB. The activation energy and the preexponential factor of GBD are equal to H_GB = 86.75 kJ/mol and D_{GB = 1.01 × 10⁻⁴m²/s}, respectively. Combining the obtained D_GB-values with the P = sD_GB°-values measured at higher temperatures 650–970 K) in the B-regime, the GB-segregation factor s has been estimated taking the GB width of δ = 0.5 nm. The resultant s-values predict an enthalpy of GB-segregation of H_s ≅ −43.29 kJ/mol. This is the first study with C-regime measurements of impurity GBD and accurate estimations of the GB-segregation factor from the GBD-data. Comparing the present results with the results of our recent C-regime measurements of GB self-diffusion in silver [J. appl. Phys.72, 2758 (1992)], a possibility of 2D-phase formation during Te GBD in Ag is discussed.     

Metastable grain boundary segregation in copper containing tellurium

Evidence for metastable grain boundary segregation has been experimentally obtained in Cu alloyed with 25 to 100 ppm Te. The solution-treated material fractured in a ductile intergranular mode. After aging for a short time; i.e., one day at 540°C, the fracture is intergranular and is attributed to the observed intergranular segregation of Te. The magnitude of the tellurium segregation and its local nature are established using Auger electron spectroscopy. Further aging results in precipitation of a stable phase throughout the grains and grain boundary fracture no longer takes place.     

Surface and grain boundary segregation of deuterium in nickel

The segregation of deuterium to surfaces and grain boundaries in nickel has been studied using SIMS (secondary ion mass spectroscopy) techniques. Conditions for optimizing the detection of deuterium are discussed. Segregation of deuterium to external surfaces and grain boundaries was shown to occur at temperatures near 300 K The segregation of sulfur to the external surfaces was also studied and co-segregation effects for sulfur and deuterium were examined. In the presence of sulfur, segregation of deuterium to surfaces was shown to occur at temperatures as high as 845 K.     

The influence of segregation on grain boundary cohesion

Based upon a detailed thermodynamic analysis where a grain gives up one or several dilute solutes to the grain boundary, we have devised two models which adequately describe the energetics of grain boundary segregation induced intergranular fracture. In the first model the cohesive energy of the precipitate is much smaller than the original material giving rise to easy separation of the boundary region. In the second model a highly cohesive directionally bonded glassy segregate forms at the boundary. The ternary glassy segregate bonds well with itself but not with the surrounding lattice giving rise to a weak interface between the grain boundary segregate and the lattice. This paper is based on a presentation made at a symposium on “Fundamentals of Grain Boundary Segregation“ held at the Niagara Falls Meeting of The Metallurgical Society of AIME, September 21, 1976, under the sponsorship of the Physical Metallurgy Committee.     

溶质原子晶界偏聚动力学过程的数值模拟

建立了溶质原子在晶界的平衡偏聚、非平衡偏聚、晶界偏聚溶质向沉淀析出转化以及冷却速度等因素的晶界偏聚物理模型和数学模型.模型考虑了晶界及晶界附近扩展畸变区对溶质的吸附作用和吸附能力.对含硼0.0010%的Fe-40%Ni-B合金体系从1150℃连续冷却到640℃的过程中硼的晶界偏聚状态进行了模拟计算.计算表明,晶界区域硼富集因子在降温初期增加较快,随后增幅变缓,模拟数据显示过程中有晶界区域硼原子向晶内的反向扩散;当晶界上偏聚的硼转化为析出物时,晶界区域富集因子的增加再次变快.模拟计算结果与已发表的实验结果吻合较好.     

Surface and grain boundary segregation on and in iron

The equilibrium segregation A (dissolved) = A (segregated), where A = C, Si, Sn, N, P, O, S … is studied for binary systems Fe-A in the stability range of the α-solid solution using surface-analytical methods. On the surfaces ordered structures were observed by LEED and surface concentrations were determined by AES – in dependence on bulk concentrations and temperature. The chemical binding state was characterized by XPS. In grain boundaries the segregation was determined by AES after intergranular fracture of samples. A survey is given of the existing results.