Duplex Grain Research Articles

Grain Growth in Si3 N4-Based Materials

The growth of Si3N4 grains in the usual sintering process occurs in an oxynitride liquid formed by reactions between sintering additives, impurity SiO2, and Si3N4. The shape of an Si3N4 grain in the liquid matrix is a hexagonal rod, although the aspect ratio (c/a) varies considerably, depending on the processing conditions and final crystalline forms of α or β. Figure 1 shows two types of microstructures observed in sintered Si3N4-based materials. The microstructure shown in Figure la is normal with unimodal grain-size distribution and that of Figure 1b is abnormal with a microstructure of exceptionally large elongated grains within fine matrix grains. When normal grain growth occurs, the microstructure varies little with sintering time, and the development may be described by a simple law. But, when abnormal grain growth occurs, a duplex grain structure with a bimodal grainsize distribution results; then no simple kinetic law can describe the microstructure development.The grain growth in the microstructures shown in Figure 1 exemplifies the growth of faceted grains in a liquid matrix. The grain growth in a matrix occurs via growth of larger grains and dissolution of smaller ones by material transport through the liquid phase. The driving force of the material transport for an individual grain is determined by the difference between its size and the critical grain size, which is invariant at the moment of observation. Since the driving force is usually low, the volume change of each grain is relatively slow and analogous to the crystal growth or dissolution in a liquid matrix under low super- or undersaturation. Therefore, knowledge of the growth behavior of faceted crystals under low supersaturation in a liquid may provide fundamental understanding of the grain growth in Si3N4-based materials.

The influence of solidification mode on heat affected zone microfissuring in a nickel-iron base superalloy

The heat affected zone (HAZ) microfissuring of thermomechanically processed Incology 903 with a duplex grain structure, has been examined with a view to understand the mechanism(s) of, and to reduce the incidence of microfissuring. Extensive formation of liquid films on the HAZ grain boundaries primarily due to the complete constitutional liquation of preexisting MNP phosphides, MC carbides and partial constitutional liquation of primary MX carbide insolubles was observed. The liquid films resulted in considerable microfissuring on the long warm worked grain boundaries and extensive grain boundary liquid film migration (LFM) on the fine recrystallized grain boundaries. The liquid films formed on the warm worked boundaries were observed to have undergone normal solidification accompanied with the formation of dendritic gamma and terminal interdendritic constituents. The exclusive occurrence of LFM only on the fine grains and not on warm worked grains is attributed to the increased driving force and velocity for LFM arising due to substantial interface curvature of the fine grains. It is shown that HAZ microfissuring is minimized if the liquid films on the HAZ grain boundaries can partially equilibrate by LFM instead of by normal solidification.

Changes in the duplex grain structure of UO 2 with a bimodal grain size distribution and irregular grains

Grain growth and the transition from irregular to regular grains have been studied on the duplex grain structure of UO 2. The powder compacts were sintered in CO 2 at 1300 and 1500°C for 0.1 to 40 h. The duplex grain structure which consists of fine regular grains and coarse irregular grains develops at 1300°C after 0.1 h. Grain growth occurs preferentially in the cluster of fine regular grains, so that the medium-sized grains of about 7 μm develop. Continued grain growth yields coarse grains, and simultaneously the irregular grain changes into the regular one. The initial bimodal grain size distribution of the duplex grain structure changes eventually into a monomodal distribution via transient bimodal distributions. Factors influencing the changes in the duplex grain structure are discussed.

Effects of sintering processes on the duplex grain structure of UO 2

The grain structure of UO 2 has been investigated under the conditions of three sets of sintering atmospheres and powder properties: (1) CO 2 atmosphere and the powder derived from the AUC process, (2) H 2CO 2 atmosphere and powder from the AUC process, and (3) CO 2 atmosphere and powder from the AU process. Powder compacts have been sintered at 1300°C for 0.1 h or partially sintered. The duplex grain structure which consists of coarse and irregular grains and fine and regular grains develops only under the conditions of both CO 2 atmosphere and powder derived from the AUC process. It is found that the difference in uranium diffusivity between the U 4O 9-z and the UO 2+x phases does not play a dominant role in the development a duplex grain structure. The microstructural features suggest that the coarse and irregular grains develop abnormally. The reason for the development of the duplex grain structure is discussed on the basis of the inhomogeneity of the powder compacts derived from the AUC process.

Image analysis measurements of duplex grain structures

Several types of duplex grain size distributions in five different alloys were evaluated using image analysis. Most of the grain structures contained annealing twins. Those with straight interfaces could be recognized and deleted from the image, leaving only grain boundaries. One specimen exhibited curved twin boundaries, caused by deformation, and they could not be discriminated by the system as currently programmed. Grain areas were measured and grouped according to their relationship to the ASTM grain size scale. An area-weighted histogram was shown to be excellent for revealing the nature of the distribution, while a numerical-frequency histogram was insensitive. The intersection of these two curves separated only one of the four bimodal distributions. A deconvolution approach, using the area-weighted curve only, should be evaluated. An arithmetic grain area classification approach using 25 classes based on the data range, to split the two grain area populations based upon the intersection of the number percent and area percent curves, worked well for two of the four specimens. Image analysis detection of grains results in a small portion of the image (about 6–12%) assigned to the grain boundaries. In manual measurement methods, the area occupied by the grain boundaries is not considered, and it does not influence measurements. Thus, compared to manual methods, image analysis undersizes grains slightly producing a relatively small positive bias in the grain size number, which could be ignored, but can be eliminated or reduced.

Development of a necklace microstructure during isothermal deformation and its properties relative to uniform microstructures

A necklace microstructure containing a controlled amount of fine (i.e., 4 to 6 μm) grains along the boundaries of larger (i.e., 30 to 40 ώm) warm-worked grains is intended to achieve a balance in mechanical properties for applications such as gas turbine disks. The development of this duplex grain microstructure critically depends on the starting microstructure, the strain and strain rate of the deformation process, and the subsequent heat-treatment conditions. These aspects of microstructural development were studied using isothermal compression tests on powder metallurgy (PM) ASTROLOY consolidated by “hipping” or extrusion. Tensile, creep, and stressrupture (S/R) properties of the necklace microstructure were also evaluated and compared with those of uniform grain microstructures.

Three-dimensional distribution of grain structure and image analysis.

The aim of this research is to establish a stereological method to characterize three-dimensional (3-D) distribution of grains in a metal from its sectional images with an application of the current image processing techniques. First of all, 3-D models of metals consisting of homogeneous grains and duplex grains were generated by simulating the processes of nucleation and crystallization with various nucleus distributions and growth rates. The frequency distribution of cut-lengths of the ASTM or JSPS standard chart of austenite (No. 7 or 8) obtained by cutting them with equiinterval parallel lines comes into a normal distribution. It was confirmed that there is no significant difference between the standard chart and the homogeneous grain model by statistical tests, and relations among mean cut-length, grain size number, mean diameter and mean volume were derived on the homogeneous grain. For duplex grains, formulae were derived to calculate the degree of duplication and the fraction of volume occupied by different sizes of grains from the distribution of duplex grains.The results can practically be utilized to obtain 3-D information about homogeneous and duplex grain structures with image processing techniques.

The influences of impurity content, tensile strength, and grain size on in-service temper embrittlement of CrMoV steels

The influences of impurity levels, grain size, and tensile strength on in-service temper embrittlement of CrMoV steels have been investigated. The samples for this study were taken from several steam turbine CrMoV rotors which had operated for 15 to 26 years. The effects of grain size and tensile strength on embrittlement susceptibility were separated by evaluating the embrittlement behavior of two rotor forgings, which were made from the same ingot, after giving an extended step-cooling treatment. The results reveal that among the residual elements in the steels, only P produces a significant embrittlement. The variation of P and tensile strength of the steels in the ranges investigated has no effect on in-service temper embrittlement susceptibility, as measured by the shift in fracture appearance transition temperature (FATT). However, the prior austenite grain size plays a major role on in-service embrittlement. The fine grain steels with a grain size of ASTM No. 9 or higher are virtually immune to in-service embrittlement. In steels having duplex grain sizes, the embrittlement susceptibility is controlled by the size of coarser grains. For a given steel chemistry, the coarse grain steel is more susceptible to in-service embrittlement, and a decrease in ASTM grain size number from 4 to 0/1 increases the shift in FATT by 61°C (110°F). It is demonstrated that long-term service embrittlement can be simulated, except in very coarse grain steels, by using the extended step-cooling, treatment. The results of step-cooling studies also show that the coarse grain rotor steels take longer time during service to reach a fully embrittled state than the fine grain rotor steels. This difference in the kinetics of embrittlement is believed to be related to the variations in Mo content in the matrix and the grain size of the steels.

Constrained cavity growth models of longitudinal creep deformation of oxide dispersion strengthened alloys

Two models of constrained cavity growth are developed to describe the long-term longitudinal creep behavior of nickel based oxide dispersion strengthened (ODS) alloys. For both models the rupture time is taken as the time for a transverse grain boundary to cavitate fully. A diffusive cavity growth law is assumed to govern cavitation. The applicability of the respective models is determined by the particular grain morphology achieved by thermal-mechanical processing. The first model assumes that longitudinal grain boundaries are unable to slide; hence displacements due to cavitation must be matched by displacements due to dislocation creep in adjoining grains. This model predicts a low stress exponent at the transition from single crystal to cavitation creep behavior, and higher stress exponents at stresses below this transition. Good agreement is found between the model predictions and creep data for MA 754 at 1000 and 1093 °C. A second model considers a grain morphology wherein longitudinal grain boundaries are able to slide by means of deformation of pockets of fine grains. Cavitation of transverse grain boundaries is thus controlled by grain boundary sliding. This model predicts a stress exponent of 1 at low stresses, and serves as an upper bound for the creep rate when a duplex grain morphology is present. Model predictions are in good agreement with creep data for a heat of MA 754 with a duplex grain morphology.

The effect of grain morphology on longitudinal creep properties of INCONEL MA 754 at elevated temperatures

The longitudinal creep behavior of two heats of coarse grained INCONEL* MA 754 have been examined at temperatures of 1000 °C and above. Both heats exhibit a pronounced transition in deformation behavior. At high stresses, dislocation creep is observed and high stress exponents (n ∼40) are measured. Fracture in this regime is transgranular with high creep ductilities. At lower stresses, the stress exponents are low and fracture is intergranular. In this regime, the stress exponent depends strongly on the grain morphology. Heat 1, with a uniform fiber grain morphology, exhibits significantly higher stress exponents than Heat 2, which has a duplex grain morphology consisting of coarse grains along with pockets of fine, equiaxed grains. Microstructural examination of specimens deformed at the lower stresses provides evidence that cavitation of the transverse grain boundaries occurs by means of diffusive cavity growth. In the heat with the uniform fiber morphology, cavity growth is constrained by creep of adjacent grains. Cavity growth for the heat with the duplex grain morphology is apparently limited by the sliding of pockets of fine grains. The implications of these results for optimizing creep resistance of MA 754 are discussed.

Effect of initial grain size on the secondary grain growth in molybdenum sheet

In order to understand in more detail the secondary recrystallization behaviour of rolled molybdenum sheet doped with CaO and MgO, the effect of initial grain size on the secondary grain growth behaviour was investigated using optical microscopy and X-ray diffraction. For the case where the initial grain size was smaller than 83 μm, a uniform and finegrained recrystallized structure was obtained after an anneal at 1500 or 1800°C, whilst an abnormal grain growth occurred after an anneal at 2000°C and a single crystal was obtained. In contrast, for the case where the initial grain size was larger than 150 μm, a partially recrystallized structure was obtained after an anneal at 1500°C, whilst normal and abnormal grain growth occurred concurrently and a duplex grain structure was obtained after an anneal at 1800 or 2000°C.

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 preparation and characterization of dense, highly conductive Na 5GdSi 4O 12 nasicon (NGS)

Na 5GdSi 4O 12 has been prepared via conventional ball-milling technique and through spray-freezing/freeze-drying. The ball-milled materials were calcined at 700°C or 925°C and sintered at 1050°C/3.5h to 89% dense multiphase ceramics. Spray-frozen/freeze-dried powders were calcined at 530°C and sintered at 1050°C/25 min to 99 ± 1% theoretical density. The latter material was single-phase NGS nasicon with a 300°C resistivity of 3.8 Ω·cm, an activation energy for Na +-conduction of 4.4 kcal/mol, a flexural strength of 105 MPa, a duplex grain structure with average sizes 0.4 μm and 3 μm and a unique linear thermal expansion coefficient (25–540°C; α = 12.6 × 10 −6/°C±4%).

Exploration of heterogeneous duplex grain structure in type 304 austenitic stainless steel using ultrasonic spectroscopy

Cyclic stress-strain tests at different temperatures for a given constant strain range on a single reference heat (9T2796) of Type 304 austenitic stainless steel showed many unexpected features. The metallographic investigations of the fatigued specimens showed existence of heterogeneous duplex grain structure of ASTM grain size numbers ranging from 1.6 to 3.2. Since it was not feasible economically to cut even a small portion, for the metallographic examination, of the machined and heat treated specimen ready for the test, a non-destructive technique was needed to sort out specimens according to their grain structures prior to testing. Results of using ultrasonic spectroscopy for this purpose are described in this work.

Hot‐Pressing of MgO with NaF

The results of using NaF additives in hot‐pressing MgO are described and compared with the results when LiF is used. To obtain nearly theoretical density, the temperatures required for hot‐pressing with NaF are ∼100°C higher than those required with LiF. Specimens made with NaF were generally translucent after hot‐pressing or subsequent annealing. Sodium fluoride inhibits grain growth and can produce a duplex grain structure, whereas LiF enhances grain growth during annealing. Both NaF and LiF wet MgO and lubricate it well. The present study and other investigations of fluoride densification aids indicate that the formation of a liquid phase, with resultant wetting and lubrication, is important, especially in the early stages of densification. Possible solution of anions and cations is discussed.

蒸着法で製造したAg-Ni,Ag-C合金の組織と機械的性質

By applying the vacuum-deposition method, it may be possible to prepare alloys constituted of elements mutually insoluble in the liquid state. From this point of view, Ag-Ni and Ag-C alloys were successfully produced, and their structure and the mechanical properties were investigated. The following results were obtained: (1) The solid solution of silver containing up to 9 at%Ni could be produced. The solid solubility of carbon in silver could not be detected by means of the X-ray method, but the change in the electric resistivity of 1 and 2 at%C-Ag alloys with temperature indicated the decomposition of the solid solution into silver and carbon. (2) By alloying nickel and carbon in silver, the duplex grain structure in silver films as-deposited were normalized and refined. (3) After the tempering of 4 at%Ni-Ag alloy at 400°C and of 1 and 2 at%C-Ag alloys at 200°C, they were hardened by the precipitation of fine nickel and carbon particles. (4) The linear relationship between proof stress vs. (grain diameter)−1⁄2 for bulk silver polycrystals can be applied to vacuum-deposited films up to about 0.1 μ in grain diameter, but the proof stress of ultra-fine grained silver with grain diameters of less than 0.1 μ was smaller than the value expected from the Petch’s relation.

Effect of Grain Size Variations on the Long-Time Stability of Alloy 718

Uniform, fine grain size is generally preferred in forgings of Alloy 718 for optimum properties. However, due to limitations imposed by forging equipment, forging size and forging configuration, it is often impossible to produce this desired grain size; instead, some grain duplexing or grain coarsening occurs in certain areas throughout the forging. Stress-rupture tests were conducted before exposure, and after exposing specimens at 1250°F, 50 ksi or 1300°F, 50 ksi for 500 hrs. on material having (1) uniform fine grain size (ASTM 7 or finer), (2) duplex grain size, and (3) uniform coarse grain size (ASTM 5 or coarser). These grain sizes were obtained in forgings with a simple pansake configuration using varying forging techniques. The results of the rupture tests conducted before exposure showed an excellent correlation with grain size and Ni3Cb plate size. After exposure, rupture ductility increased and strength decreased due to overaging. In duplexed samples the controlling factor appears to be the amount and size of the coarse grain. The optimum grain size for the best balance of rupture properties appears to be ASTM 6–7.

オーステナイト結晶粒の成長性におよぼす高温加工の影響について

For medium-carbon steels and a medium carbon Cr-Mo steel having different grain size, the effect of hot working on growth characteristics of austenite grains was studied by changing the forging ratio from 2 to 10.From this work, it was concluded that hot working lowered the coarsening temperature of austenite and that annealing after forging much lowered it. These were more clearly observed in a duplex-grained steel than in a fine-grained steel. It was also found that the effect of hot working and annealing on coarsening temperature was much smaller than that of the difference of ingot size. It was veritied, therefore, difference of grain size and grain growth characteristics between a ladle sample and a finished product was mainly due to the difference of teeming conditions.Moreover, taking account of the behavior of AlN in steels, it was deduced that the effect of hot-working and annealing was the function of the state of aggregation of AlN and the occurrence of duplex grain structure, detected in this work, in as-forged state was also related to the behavior of AlN in austenite.