Quantitative analysis of grain size in bimodal powders by x-ray diffraction and transmission electron microscopy

In most studies related to milled powders, the grain size¹ is analyzed via X-ray diffraction (XRD) experiments, and a transmission electron microscopy (TEM) image with high magnification, if provided, is used primarily to confirm the results obtained by XRD experiments. This widely used approach is reasonable in light of the difficulties associated with TEM sample preparation. The present study, however, addresses the hypothesis that such an approach may not be valid when there is an inhomogeneous distribution of grains present. TEM examination, carried out in carefully prepared Al-7.5 wt% Mg samples, in which a global region is observable by TEM, provided the opportunity for quantitative analysis of grain size in cryomilled powders having an inhomogeneous distribution of grain sizes. The cryomilled Al-7.5 wt% Mg had a bimodal grain microstructure of 77% (area fraction) fine grains in the range of 10 to 60 nm and 23% coarse grains of approximately 1 m. The results show that the XRD analysis yields a grain size that is close to that present in the fine-grained regions (i.e., 10–60 nm). The present study also systematically investigated the influence of the nine possible combinations of the Cauchy (C) and the Gaussian (G) approximations on the calculated grain size value, and the results show that the CC-CC approximation resulted in the largest calculated grain size, the GG-GG generated the smallest one, and the CG-CG, the approximation recommended by Klug and Alexander [1], led to a calculated grain size that is approximately equal to the average one from the CC-CC and GG-GG approximations. The maximum possible fluctuation of grain size values stemming from the various approximations is 38%.     

A substructure characterizing parameter in creep

A substructure characterizing parameter which is the ratio of applied stress, , in creep test to yield stress, _YS, of the material at the test temperature is introduced. A correlation is found to exist between this parameter and the creep rate for the data obtained in the temperature range 820–975 K when the initial yield strength is modified by (i) introducing different amounts of prior cold work by two modes of deformation at room temperature in a type 316 LN stainless steel and (ii) grain size, chemistry and grain size variation in a type 316 stainless steel. The correlation was found to exist also for a Cr-Mo-V steel at 823 K, in which different yield strengths were due to different heat treatments. Minimum creep rate when plotted against the substructure characterizing parameter yields an exponent similar to Norton's creep exponent and it is postulated that the value of the exponent reflects on the type of substructure developed in creep. Another parameter /F _ys where F _ys is a function of the ratio of the yield strength of a given microstructure to that of a reference microstructure (zero cold work for cold worked material, largest grain size when the microstructure variation is through grain size and solution annealed microstructure among heat treatments) also gives a unique correlation with the minimum creep rate at a test temperature with the exponent identical to Norton's creep exponent.     

Fractal nature and quantitative evaluation of microstructures in metallic materials

The fractal nature of microstructures was investigated using metallic materials containing second-phase particles, grain-boundary reaction (GBR) nodules or creep voids. The area fraction of the precipitates or the creep voids in the specimens was correlated with the scale of the analysis. The microstructures of these specimens exhibited a fractal nature between the lower and the upper critical scales, and could be regarded as the aggregate of the unit pattern with the size of the upper critical scale. The fractal dimension of a given microstructure was generally larger in specimens containing a larger area fraction of the second phase. The lower critical scale was close to the average size of second-phase particles or GBR nodules or the size of a large creep void. The upper critical scale, above which the area fraction of the precipitates or the creep voids did not show a scale dependence, was generally much larger than the average size and the average spacing of the precipitates, but it was almost the same as or a fair degree smaller than the grain size in specimens containing the second-phase particles or the GBR nodules. In the creep-ruptured specimens, the upper critical scale was much larger than the initial grain size and the grain size at rupture. The true area fraction of the second phase or the creep voids corresponding to the upper critical scale was also obtained.     

基于EBSD技术的P91钢蠕变过程中小角度晶界演化行为表征

在620℃、145 MPa条件下对给定的P91钢进行高温蠕变持久与间断试验,采用电子背散射衍射(EBSD)技术研究其在蠕变过程中小角度晶界的演化行为。通过引入EBSD图像中的取向差分布来表征小角度晶界处(0.5~5°)的边界位错密度,分析了边界位错密度在蠕变过程中与小角度边界的数量、塑性应变以及内部微观组织演化之间的关系。此外,通过改变EBSD像素点与像素点之间的计算步长,探讨了步长选择对边界位错密度计算结果的影响。结果表明,小角度晶界处的位错密度在蠕变过程中先迅速上升,在最小蠕变率处达到极值后缓慢下降,直到最后基本保持不变;同时,EBSD的计算步长越小,得到的位错密度值越准确。     

Effect of austenite grain size on isothermal bainite transformation in low carbon microalloyed steel

Abstract

The effect of austenite grain size on isothermal bainite transformation in a low carbon microalloyed steel was studied by means of optical microscopy, SEM and TEM. Two widely varying austenite grain sizes, a fine average grain size (～20 μm) and a coarse average grain size (～260 μm), were obtained by different maximum heating temperatures. The results showed that the morphology of isothermal microstructure changes from bainite without carbide precipitation to bainitic ferrite with a decrease in holding temperature. Coarse austenite grain can retard the kinetics of bainite transformation and increase the incubation time of bainite transformation by reducing the number of nucleation site, but it does not influence the nose temperature of the C curve of bainite start transformation, which is ～534°C.     

Plastic deformation and creep damage evaluations of type 316 austenitic stainless steels by EBSD

The inspection method of plastic and/or creep deformations has been required as the quantitative damage estimation procedure for structural components especially used in electric power plants. In this study, the method using electron backscatter diffraction (EBSD) was applied to the deformation and damage evaluation of austenitic stainless steels strained by tension or compression at room temperature and also tested in creep at high temperature. It was found that the value of Grain Average Misorientation (GAM) which showed the average misorientation for the whole observed area including over several dozen grains, was a very useful parameter for quantifying the microstructural change as either the plastic or creep strain increased. The unique linear correlation was obtained between GAM and plastic strain in tension and compression. For creep damage evaluation, the difference of grain average misorientation from the value of the unstrained specimen (ΔGAM) showed an excellent correlation with the inelastic strain below strain at which the tertiary creep began.     

Inconel 740H蠕变过程中晶界M23C6的粗化行为

为了解700℃超临界电站锅炉过/再热器用主要候选材料Inconel 740H晶界M_(23)C_6相在蠕变过程中的粗化行为,对通过真空感应熔炼和真空电弧重熔双联工艺制备的合金,在750℃/200 MPa条件下分别开展蠕变中断与蠕变断裂试验,并利用扫描电镜、透射电镜、能谱分析仪等测试方法对晶界M_(23)C_6相进行表征.采用Image-Pro Plus软件对相参量进行定量统计,进而分析其变化趋势.研究表明:在所研究温度、应力及时间范围内,随着蠕变时间的延长,晶界富Cr的M_(23)C_6相逐渐由粒状、短棒状发展成为长条、半连续的链状分布;粒子沿平行于晶界方向的长大速率明显高于沿垂直于晶界方向的长大速率,但二者的粗化均表现出尺寸三次方与时间之间呈线性关系的规律;相数量的增加与蠕变时间亦呈线性关系;与尺寸的粗化相比,相数量的增长较为缓慢,外加应力在一定程度上促进了M_(23)C_6的析出.     

Sintering study of nanocrystalline tungsten carbide powders

WC powder with an average grain size of 6 nm was obtained after high energy ball milling under protective gas atmosphere. The kinetics of densification was studied during sintering the powder in a dilatometer up to 1450 °C. The microstructure was investigated by TEM and high resolution SEM after various stages of sintering. The green density of the specimens was 45%. Three stages of sintering were defined: (a) rearrangement of particles at low temperature (850 °C) without grain or particle growth, (b) neckformation between powder particles at 1000–1250 °C and initial grain growth at 1200 °C, (c) pore elimination accompanied by massive grain growth at 1300–1450 °C.     

Tensile creep behaviour of NiAl-Cr(Zr) multiphase intermetallic alloy

Abstract

The microstructure of a multiphase NiAl-33.5Cr-0.5Zr intermetallic alloy was examined by SEM with energy dispersive spectroscopy and TEM. The tensile creep behaviour of the hot isostatically pressed NiAl-33.5Cr-0.5Zr alloy was studied. The results of the creep test indicated that all of the creep curves under the present test have similar characteristics: a short primary creep stage, a dominant tertiary creep stage, and nearly identical creep strains (~45%). The apparent stress exponent and the apparent activation energy were analysed and discussed. The mechanism of the creep deformation was also analysed by the observation of TEM.     

Evolution of grain shape and size during high-temperature creep of a yttria-doped fine-grained alumina

A semi-automatic method of image analysis was used to characterize the evolution of the microstructure of a polycrystalline alumina in terms of grain size and grain shape during different thermomechanical treatments. This study showed that for this material normal grain growth occurs during compressive creep and annealing; in addition, during creep under certain conditions grains flatten in the direction of the applied stress. Three-dimensional estimation of the grain shape was performed by analysing differently oriented plane sections of the specimens.     

Microstructures and Creep Behavior of a Directionally Solidified NiAl-Fe(Nb) Multiphase Intermetallic Alloy

The microstrucure and creep behavior of a DS NiAl-Fe(Nb) multiphase intermetallic alloy have been investigated,This alloy exhibits dedritic structure ,in which dendritic arm is β-(Ni,Fe)(Fe,Al) phase surrounded by interdendritic region of γ′/γ phase ,The results of the creep test indicated that all of the creep curves have similar characteristic,which is a short primary creep stage and a dominant steady state crepp stage ,and the creep strain ranges from 18% to 52%,The apparent stress exponent and the apparent activation energy were analyzed and discussed,The mechanism of the creep deformation was also analyzed by the observation of TEM.     

Ti-6Al-3Nb-2Zr-1Mo合金室温压缩蠕变行为及位错类型研究

研究Ti-6Al-3Nb-2Zr-1Mo合金在不同外加应力下的室温压缩蠕变行为,拟合了蠕变曲线,计算出蠕变发生第二阶段的临界值,并对不同应力水平压缩后的合金显微组织进行TEM观察,研究其位错滑移类型.结果表明:室温条件下,Ti-6Al-3Nb-2Zr-1Mo合金压缩蠕变-时间曲线符合时间强化指数模型,该合金发生蠕变第二...     

用脉冲喷射电沉积法制备纳米晶镍镀层

采用脉冲喷射电沉积方法制备了纳米晶镍镀层,用扫描电子显微镜(SEM)、透射电子显微镜(TEM)和X射线衍射(XRD)等方法研究了镀层的生长形貌和微观结构,并考察了脉冲电流密度对镀层微观结构如晶粒尺寸、织构等的影响.结果表明:镀层内表面(基体一侧)具有比外表面(镀液一侧)更为精细的晶粒结构,说明随着厚度的增加,镀层中的晶粒逐渐粗化.随着电流密度从45 A/dm2增加到180 A/dm2,镀层中晶粒生长的择优取向由(111)织构逐渐转变为强(220)织构.当电流密度从45 A/dm2增加到120 A/dm2时,镀层平均晶粒尺寸逐渐减小;而进一步增加电流密度到180 A/dm2,镀层晶粒尺寸又会有轻微的增大.     

Microstructure and high temperature deformation characteristics of sol–gel derived aluminium titanate–mullite composites

Aluminium titanate (AT)–mullite composites with varying compositions were processed by sol–gel technique. The influence of mullite on the microstructure and creep deformation of AT–mullite composites was investigated. In the composites mullite addition was varied from 0 to 100 vol.%. The AT-80 vol.% mullite composite sintered at 1600 °C resulted in fine-grained microstructure with an average grain size of 2.5 μm. From the steady-state creep analysis of the different AT–mullite composites, the activation energies for the creep deformation and stress exponents were determined. The activation energies in the range 655–874 kJ mol⁻¹ were obtained for various the sol–gel derived AT–mullite composites. Similarly stress exponent values were found in the range 1.5–1.9.     

Effect of grain size on creep of Ti-53.4mol%Al intermetallics at 1100 K

Constant stress creep under compression stress, 100 to 316 MPa, at 1100 K was investigated on single-phase TiAl intermetallics. The material was ingot-cast, isothermally forged, and then annealed to produce stable equi-axed grain structures, whose average grain diameters were 25, 42 and 70m. Creep curves were very similar among the three specimens with different grain diameters and the creep rates at a given strain, as well as the minimum creep rates, depended little on grain size. Two regimes were observed on the stress dependence of the minimum creep rate. The stress exponent under high stresses was about 4.5, independent of grain size. Under stresses lower than about 150 MPa it became about 8.     

On Grain Size Dependence of Minimum Creep Rate

The available experimental results have beensummarized concerning the effect of grain size onminimum creep rate.There are two types of creeprate-grain size relations.One is that there is a criti-cal grain size above which creep rate is independentof grain size,below which creep rate increases withthe decrease of grain size.The other is that there isan intermediate grain size at which creep resistanceis optimum.The first relation usually occurs athigher temperatures(>0.5 T_m),and intermediatestress ranges,while the second relation at interme-diate temperature ranges(0.4-0.5 T_m)and higherstresses.For the two types of creep rate-grain sizerelations,the increase of the creep rates with the de-crease of grain size for small grain sizes is all due tograin boundary sliding.For large grain sizes,a dis-location climb mechanism is dominant in creepdeformation for the first relation,while aHall-Perch grain boundary strengthening effect isbelieved to play an important role by dislocationglide mechanism for the second relation.     

The role of grain boundary sliding and reinforcement morphology on the creep deformation behaviour of discontinuously reinforced composites

In this study, the role of grain boundary sliding behaviour on the creep deformation characteristics of discontinuously reinforced composites is investigated numerically together with the other influencing parameters: reinforcement aspect ratio, grain size and interfacial behaviour between the reinforcement and the matrix. The results obtained for the composites are compared with results obtained for a polycrystalline matrix material having identical grain size and morphology. The results indicate that, with sliding grain boundaries, the stress enhancement factor for the composites is much higher than the one observed for the matrix material and its value increases with increasing reinforcement aspect ratio, reduction in the matrix grain size and sliding interfacial behaviour between the reinforcement and the matrix. In the composites, the contribution of the grain boundary sliding to overall steady state creep rates occurs in a larger stress range in comparison to the matrix material. Experimentally observed higher creep exponent values or stress dependent creep exponent values for the composites could not be explained solely by the mechanism of grain boundary sliding. However, experimentally observed large scale triple point grain boundary cavitation in the composites could occur due to large grain rotations resulting from grain boundary sliding.     

块体纳米晶Ni-Fe合金的制备及表征

采用直流电沉积方法制备块体纳米晶镍铁合金材料,经过工艺参数和成分的优化,提出可连续施镀,并有很高镀厚能力及晶粒尺寸、晶粒结构和合金成分可控的电沉积工艺配方及工艺方法,利用扫描电子显微镜（SEM）、透射电子显微镜（TEM）、X射线衍射分析（XRD）等检测设备进行分析和表征,结果表明,获得的块体纳米晶镍铁合金材料表面光滑致密、结构均匀,材料平均晶粒尺寸在25nm左右。最佳工艺参数为：电流密度Dk=5～dm2,pH值3．5,温度60℃。     

低Re含量第二代柱晶高温合金DZ59的蠕变断裂性能

采用液态金属冷却(LMC)工艺和成分优化设计,制备出一种高温性能优异的低Re含量第二代柱晶高温合金DZ59,通过扫描电镜(SEM),透射电镜(TEM)等方法对其组织特征及蠕变断裂进行了表征,研究了合金的蠕变断裂性能.结果表明,DZ59合金的高温蠕变断裂性能超过一代单晶合金,并接近二代单晶合金的水平.发现柱晶高温合金的二次晶界反应(SGRZ)现象,表明SGRZ受温度和应力的控制,由于增加了合金横向晶界的受力界面,在高负荷下可能成为蠕变空洞萌生和扩展的位置.