Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Alloy 617, a high-temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP), which will operate at temperatures exceeding 760 °C and a helium pressure of approximately 7 MPa. Observations of the crept microstructure using optical microscopy indicate creep stress does not significantly influence the creep void fraction at a given creep strain over the relatively narrow set of creep conditions studied. Void formation was found to occur only after significant creep in the tertiary regime (>5 pct total creep strain) had occurred. Also, orientation imaging microscopy (OIM) was used to characterize the grain boundaries in the vicinity of creep voids that develop during high-temperature creep tests (900 °C to 1000 °C at creep stresses ranging from 20 to 40 MPa) terminated at creep strains ranging from 5 to 40 pct. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular, or 45 deg to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids, while coincident site lattice (CSL)–related grain boundaries did not appear to be consistently associated with void development. Similar results were found in oxygen-free, high-conductivity (OFHC) copper, severely deformed using equal channel angular extrusion, and creep tested at 450 °C and 14 MPa.     

Ni-30Cr-1Cu合金晶粒度及晶界沉淀与脆性的关系

用晶界萃取复型技术研究了晶界一次相极少、二次相析出倾向很大的Ni-30Cr-1Cu合金的晶界沉淀规律.采用不同温度固溶后盐水快速冷却以抑制晶界二次相的析出,获得了晶界沉淀状态相近的不同尺寸晶粒.结果表明,随单纯晶粒长大,合金的室温冲击韧性、拉伸塑性均有不同程度的升高.于1180℃固溶后以不同速度冷却,结果显示随冷却速度的减慢,晶界M23C6显著增多、分布密度增大;其沉淀形态分别由空冷的精细薄片状向缓冷的颗粒状、棒状、薄膜状乃至枝晶状转变,使晶界在室温脆化,冲击韧性和拉伸塑性显著下降;断裂方式由以穿晶为主的混合型断裂向沿晶脆性断裂转变.     

稀土镧在Fe-La合金系中的晶界扩散行为

通过高频悬浮炉熔炼成实验用30Fe-70La合金,将该合金切成8 mm×8 mm×6 mm试样与同样尺寸纯铁试样扩散面对接,并借助Gleeble-1500D热模拟试验机在760℃和770℃ 30 min,10^-3 Pa真空和1 MPa压力下进行扩散试验;采用Axiovert 25型金相显微镜与QUANTA 400环境扫描电子显微镜,观察了稀土镧在Fe基体的扩散;利用相关扩散系数公式计算了镧在纯铁中的扩散系数。结果表明,稀土元素镧在纯铁中沿着α-Fe晶界进行扩散,镧在纯铁铁素体晶界中770℃时的扩散系数为Db=1.59×10^-17m²/s。     

Effect of Heat Treatment on Solute Concentration at Grain Boundaries in 7075 Aluminum Alloy

The effect of heat treatment on the relative amounts of magnesium, zinc, copper, iron and silicon in grain boundaries of aluminum 7075 alloy was investigated with Auger electron spectroscopy in a scanning Auger microprobe. It was observed that both aging and solution treatments affect the distribution of these elements at grain boundaries. If identical solution heat treatments were employed, the aging treatment determined the solute distribution at the grain boundaries, and for fixed final aging treatment, the temperature of solution anneal determined the solute distribution. Among specimens given a T6 final aging treatment (24 h at 394 K (250 °F)) with prior solution treatment temperatures in the 666 K (740 °F) to 800 K (980 °F) range, 711 K (820 °F) solution treated specimen was found to have the least solute concentration at grain boundaries. The observed behavior is explained on the basis of equilibrium and nonequilibrium segregation of solute elements to grain boundaries occurring on quenching and during aging.     

铈对Fe Cr Ni Nb Ti Al W合金持久寿命及晶界析出相的影响

 研究了稀土元素铈对铁基合金Fe Cr Ni Nb Ti Al W晶界析出相的数量、形貌、分布的影响及其与持久寿命的关系。结果表明：铈含量较低时，晶界析出相主要是大片状相及少量颗粒相；当铈含量增加时，晶界中颗粒相数量明显增加，片状相减少，且析出相分布较均匀，当铈含量过高时，晶界中析出大量片状相及颗粒相，且析出相总量增加。这些析出相的数量、形貌以及分布等特征的变化规律与合金持久寿命的变化规律一致。     

Ni-30Cr-1Cu合金晶粒尺寸及晶界沉淀与脆性的关系

用晶界萃取复型技术研究了晶界一次相极少、二次相析出倾向很大的Ni-30Cr-1Cu合金的晶界沉淀规律。采用不同温度固溶后盐水快速冷却以抑制晶界二次相的析出获得了晶界沉淀状态相近的不同尺寸晶粒，结果表明，随单纯晶粒长大，合金的室温冲击韧性、拉伸塑性均有不同程度的升高。于1180℃固溶后以不同速度冷却，结果显示了随冷却速度减慢，晶界M23C6显著增多、分布密度增大；其沉淀特征分别由空冷的精细薄片状向缓冷的颗粒状、棒状、薄膜状乃至枝晶状转变，使晶界在室温脆化，冲击韧性和拉伸塑性显著下降；断裂特征由以穿晶为主的混合型断裂向沿晶脆性断裂转化。     

Pinning of Austenite Grain Boundaries by

The growth behavior of austenite grains in the presence of A1N precipitates varies with the temperature and time of anneal. To study this behavior, two iron alloys, (in weight percent) a 0.1 carbon base chemistry with 0.03A1/0.01N and 0.09A1/0.04N, respectively, were annealed between 1000 °C and 1200 °C for times of up to 180 minutes. Using optical microscopy, as many as 1000 austenite grains per heat-treatment condition were measured. Conditions of sup- pressed, abnormal, and uniform grain growth were observed. Using an extraction replica tech- nique, the size, shape, and distribution of the A1N particles were determined using transmission electron microscopy (TEM). The largest grain boundary curvatures calculated, using the Hellman- Hillert pinning model, were in close agreement with independent calculations of curvatures using the grain size data. The largest grains in the lognormal size distribution of austenite grains were found to be the ones with the potential to grow to abnormally large sizes.     

Precipitate Redistribution during Creep of Alloy 617

Nickel-based superalloys are being considered for applications within advanced nuclear power generation systems due to their high-temperature strength and corrosion resistance. Alloy 617, a candidate for use in heat exchangers, derives its strength from both solid solution strengthening and the precipitation of carbide particles. However, during creep, carbides that are supposed to retard grain boundary motion are found to dissolve and reprecipitate on boundaries in tension. To quantify the redistribution, we have used electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS) to analyze the microstructure of 617 after creep testing at 900 °C and 1000 °C. The data were analyzed with respect to the location of the carbides (e.g., intergranular vs intragranular), grain boundary character, and precipitate type (i.e., Cr rich or Mo rich). We find that grain boundary character is the most important factor in carbide distribution; some evidence of preferential distribution to boundaries in tension is also observed at higher applied stresses. Finally, the results suggest that the observed redistribution is due to the migration of carbides to the boundaries and not the migration of boundaries to the precipitates.     

晶界在超塑性变形中的作用

超塑性材料所具有的微细晶粒组织,意味着材料的单位体积内存在着大量的晶界。本文结合作者及其合作者近几年来所进行的研究工作,详细地论述了晶界在超塑性变形中所起的重要作用。     

Effect of Grain Boundaries and Grain Orientation on Structure and Properties

The evolution of deformation microstructures in metals follows a universal pattern of grain subdivision. However, the structure in the grain boundary region may be different from that in the grain interior, although a characteristic region cannot be identified for polycrystals with medium to high stacking fault energy. In the grain interior, the dislocation structure is predominantly composed of almost planar boundaries (geometrically necessary boundaries) and cell boundaries (incidental dislocation boundaries) forming a cell block structure. For grains with grain sizes reaching down to about 4 μm deformed in tension and by rolling, a clear correlation has been established between the characteristics of the deformation structure and the orientation of the grain in which it evolves. A similar correlation is observed for single crystals of different orientations. Such correlations form the basis for a general analysis of active slip systems and for modeling of the flow stress and flow stress anisotropy of polycrystalline samples.     

Segregation at Melted Grain Boundaries in Steel

Abstract

Test samples from continuously cast steels have been fractured by grain boundary melting. The faceted fracture surfaces have been examined by Auger spectroscopy. Sulphur has been found to be segregated on the surface of all of the facets examined. The amount of segregation varies appreciably for different facets in a given sample; on average the segregation increases significantly with an increase in sulphur level from 0.014 to 0.024% S. No segregation of P was detected on the fractured surfaces.

Résumé

Des échantillons d'aciers de coulée continue ont été rupturés par fusion aux joints de grains. Les surfaces de rupture à facettes ont été examinées par spectroscopie Auger. On.a trouvé, sur toutes les surfaces des facettes examinées, une ségrégation de soufre. Le montant de la ségrégation varie appréciablement pour différentes facettes d'un même échantillon. En moyenne, l'étendue de la ségrégation augmente quand la concentration en soufre passe de 0.014 à 0.024%. On n'a pas détecté de ségrégation de P sur les surfaces de rupture.     

617合金的组织稳定性

 对先进超超临界燃煤发电机组锅炉高温部件候选材料617合金在750℃下时效1000,2000,3000和5000h后试样的组织稳定性进行了研究。通过OM,SEM,TEM和X射线衍射等方法对时效后晶界及晶粒内的析出物进行了分析。研究发现,时效后试验材料的晶界析出M23C6碳化物;晶内析出γ′相,且γ′相呈颗粒状弥散分布,γ′相尺寸随时效时间的延长而增长。经1000h时效后,试验材料的强度开始增加,并且随着时效时间的延长,强度开始呈现平稳趋势。随着时效时间的延长,试验材料的韧性呈下降趋势。     

Grain Boundaries and Mechanical Properties of Ultrafine-Grained Metals

The concept of grain boundary (GB) engineering of ultrafine-grained (UFG) metals and alloys is developed for enhancement of their properties by tailoring different GBs (low-angle and high-angle ones, special and random, or equilibrium and nonequilibrium) and formation of GB segregations and precipitations by severe plastic deformation (SPD) processing. In this article, using this approach and varying regimes and routes of SPD processing, we show for several light alloys (Al and Ti) the ability to produce UFG materials with different GBs, and this can have a dramatic effect on the mechanical behavior of the processed materials. This article demonstrates also several new examples of attaining superior strength and ductility as well as enhanced superplasticity at low temperatures and high strain rates in various UFG metals and alloys.     

Calculating Dislocation Spacings in Pile-Ups at Grain Boundaries

Dislocation pile-ups near planar grain boundaries have been analyzed employing full anisotropic elastic solutions, single-crystal anisotropic approximations, and isotropic approximations. The calculations were performed for a series of iron-silicon bicrystal configurations and results from the various methods compared. Three of the bicrystal/slip system combinations exhibited repulsive image forces, allowing elastically self-consistent position calculations. Based on these results, either linear elastic or nonlinear effects can dominate the behavior of these pile-ups, depending on specific boundary conditions.     

硼在奥氏体晶界的两种偏聚形式

用PTA方法研究了Fe-30%Ni合金中硼在晶界的偏聚行为。实验表明在550-1200℃保温后用不同方式冷却的试样中存在平衡与非平衡两类硼偏聚,它们各自的形成机制不同,试验条件对它们的影响不同。平衡偏聚在保温时形成,在低温区淬火时起主要作用。在高温加热后,用通常冷却速度淬火时,晶界偏聚主要来源于冷却过程中产生的非平衡偏聚,实际瘁火试样中观察到的硼偏聚是这两类偏聚的叠加。试验指出,Fe-30%Ni合金中偏聚方式有一个转折温度区,这温度受冷却速度影响,在通常冷却速度下,这个转折温度在650~750℃之间。     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Low-Angle Grain Boundaries in Sublimation Grown 6H-SiC Crystals

High-resolution X-ray diffractometry (HRXRD) was used to assess the quality of 6H-SiC crystals grown by sublimation method. The results show the occurrence of low-angle grain boundaries (LB) is close relative to the inclination of the crystal interface. At the central faceted region with 0° inclination the crystal is of high structural perfection. However, at the region close to the facet with less than 5° inclination LB occurs slightly and at the region close to the peripheral polytype ring with more than 5° inclination LB defect occurs heavily. The density of LB can be drastically reduced by decreasing radial temperature gradient that determines the shape of the crystal growth interface.