This paper deals with an investigation of the grain boundaries in alloy 263 using two different kinds of heat treatment. Nibased cr alloys and grain boundaries characterization. This situation can happen in otherwise corrosionresistant alloys, when the grain boundaries are depleted, known as grain boundary depletion, of the corrosioninhibiting elements such as chromium by some mechanism. On a serrated prioraustenite grain boundary, carbide particles had various sizes and shapes, and some parts of the boundary were free from carbide precipitation. The grain boundaries were considerably serrated without the presence of. Figure 1model shows grain boundary serration formation in stainless steel due to m 23 c 6 precipitation, aa grain boundary prior to formation of m 23 c 6. Feb, 2017 these serrated grain boundaries result from grain boundary migration driven by local variations in dislocation density between two neighbouring grains 49,61. Grain boundaries are 2d defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. A grain boundary is a general planar defect that separates regions of different crystalline orientation such as grains within a polycrystalline solid. Based on the re sults of this work and the results pre viously presented in paper i ref.
Grain boundary serration in nickelbased superalloy inconel. The nucleation of drx in 316ln steel was characterized by bulging of serrated grain boundaries. Based on a previously developed gbs model, using the dynamics of grain boundary dislocation pileup, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Kfeldspar porphyroclasts show undulatory extinction and are elongate parallel to the foliation, suggesting intracrystalline plasticity shigematsu et al. Grain boundaries were serrated by grain boundary reaction precipitates. Figure 1b and c are tem micrographs of grains at the center of the laser shock wave after lsp.
Marble myionites in the bancroft shear zone, ontario, canada. Nuclear structural material austenitic stainless steel 316l is a polycrystalline composed of single crystals with a facecentered cubic fcc structure, and the intergranular stress corrosion cracking igscc is closely related to the crystal orientation. Constant and increasing load creep tests show no effect on creep rate from the serrated grain boundaries. Zirconiumrich precipitates were recognized at serrated grain boundaries though their involvement in the occurrence of serration was doubtful. Investigation of nucleation processes during dynamic.
A mechanism of grain boundary serration formation is proposed. Grain boundary serration in nickelbased superalloy. The crackgrowth rates were considerably decreased in the specimens with the serrated grain boundaries. An approach which has shown promise for improvement of cracking resistance of several nickel base superalloys has been the formation of microstructures containing serrated grain boundaries. The serrated grain boundary micro structures show no improvements to creep. Paper open access related content the interplay between. A grain boundary is a general planar defect that separates regions of different crystalline orientation i. A new method based on fourier transforms is used to analyse the multiple wavelike character of the serrated grain boundaries. Heat treatments are applied to produce microstructures with planar and serrated grain boundaries. The serration of grain boundaries in inconel 600 caused by heat treatment is studied systematically.
Chemicalcompositionofinconel718,ati718plusandwaspaloy7. The stress state at the crack tip of structural components in nuclear power plants used in the scc quantitative prediction models is based on the assumption that the polycrystalline material is isotropic and homogeneous at present. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. Grain boundary sliding at serrated grain boundaries article pdf available in advanced performance materials 44. Materials can be made to have good or bad corrosion properties, mechanical properties creep depending on the type of grain boundaries present. In nickel alloys and austenitic stainless steels, where chromium is added for corrosion resistance, the mechanism involved is precipitation of chromium carbide at the. A study on the formation of serrated grain boundaries and. Effect of serrated grain boundaries on the creep property. Pdf effect of serrated grain boundary on stress corrosion.
Dec 24, 2018 grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material. A new metric the serration index is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between. In order to change a metals properties, it is exposed to enough heat so that the grain boundaries dissolve and reform, a process called annealing, where the slower the cooling rate, the larger the grain size formed. Twinning and detwinning via glide and climb of twinning. In almost all grains, calcite twins are present fig. We have demonstrated that serrated grain boundaries have a common asymmetric feature in the curvature distribution and this morphological feature can be used for the determination of direction of the grain boundary migration. On the role of boron on improving ductility in a new.
A new metric the serration index is devised and utilised to quantify the degree of serration and more generally to distinguish objectively between serrated and. A constitutive model is presented to assess the elastic response of anisotropic behavior of single crystals in 316l in this study. On the role of alloying elements in the formation of serrated. To form serrated grain boundaries in material a ma, slow controlled cooling from the temperature of solution annealing to. Thus in certain aluminium alloys, small amounts of iron have been shown to segregate in the grain boundaries and cause intergranular. Grain boundary carbide structure in tempered martensitic. Serrated grain boundaries were observed in the absence of either or carbides.
Is the phenomenon of grain boundary serration of mere academic interest, or are there signi. Formation of serrated grain boundaries and their effect on. Intergranular corrosion is generally considered to be caused by the segregation of impurities at the grain boundaries or by enrichment or depletion of one of the alloying elements in the grain boundary areas. The atoms in the grain boundary will not be in perfect crystalline arrangement. It has also been suggested 6 that alloycompositionplaysa role e for instance by providing a driving force for grain boundary serrations from discontinuous chromium and carbon segregation. Labs formed around the bulged grain boundaries separate the bulge boundaries from the initial grain, leading to nucleation of new grains.
Serrated grain boundary formation potential of nibased. Intragranular strains and grain boundary morphologies of. To form serrated grain boundaries in material a ma, slow controlled cooling from the temperature of solution annealing to 800 c was carried out. Scc cracks preferentially initiated and grew at grain boundaries normal to the tensile loading axis. The serrated grain boundary microstructure contains grain boundary reaction gbr nodules, and two heat treatments producing 10% and16% gbr area fraction are used. Experimental procedure in order to investigate morphology of grain boundary serration in three kinds of nibase. Grain boundaries were serrated by grainboundary reaction precipitates. A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Effects of crystal orientation and grain boundary inclination. However, the crystals in polycrystalline materials are anisotropic with different orientations, which would induce a nonuniform stress to cause the initiation and.
Most grain boundaries are preferred sites for the onset of corrosion and for the precipitation of new phases from the solid. Although carbide particles with various orientations were observed on one serrated prior grain boundary, almost identically oriented carbide particles existed at a locally flat region. A study on the formation of serrated grain boundaries and its. Mechanisms of formation of serrated grain boundaries in. Pdf grain boundary serration in nickel alloy inconel 600.
Grains and grain boundaries in singlelayer graphene atomic patchwork quilts. Characterization of strain accommodation at grain boundaries. Twining took place near the serrated grain boundary in large amount, accelerating the separation of bulging from deformed parent grains to form drx nucleus. Effects of grain orientation on stress state near grain. Samples were heat treated to create either smooth standard or serrated grain boundaries. When a metal part is stressed, the defects and holes in the atomic layers of the metal, called dislocations, move from within the grain toward its grain boundary. Estimates of stress and strain rate in mylonites based on. In the vicinity of serrated grain boundaries, the grains often display strongly misoriented domains up to 5, figure 4 where sgbs form complex structures.
On the role of alloying elements in the formation of. Serrated grain boundaries and straight grain boundaries were obtained by controlled heat treatment. Although carbide particles with various orientations were observed on one serrated priorgrain boundary, almost identically oriented carbide particles existed at a locally flat region. The formation of serrated grain boundaries and its subsequent effect on creep resistance have been investigated in a wrought nickel based superalloy nimonic 263. Grain boundary precipitation in inconel 718 and ati 718plusq1. Microstructureproperty relationship in advanced nibased. Metallurgical investigation into ductility dip cracking in ni. For crystalline materialsrather say polycrystalline, when liquid begin to solidify then the atoms stop moving around much and starts to gather in smaller volume. Strain energy distribution in the xy plane at the center of the film. The effect of a serrated grain boundary on stress corrosion cracking scc of alloy 600 was investigated in terms of improvement of scc resistance. In addition to the serration of grain boundaries, the strain induced labs is also important. Aug 28, 2019 the nucleation of drx in 316ln steel was characterized by bulging of serrated grain boundaries. Grain boundary serration is an effective way to increase the high temperature resistance of superalloys and steels.
Moreover, no dislocations pileup and serrated grain boundaries are observed in fig. Improvement of fatigue crack growth resistance by serrated. An objective of these studies is to better understand the formation of dendritic and serrated grain boundaries in nickel base superalloys and to increase. Metallurgical investigation into ductility dip cracking in. Frontiers nucleation and grain boundary evolution in. It is also noticeable that reducing the cooling rate from 2. Pdf grain boundary sliding at serrated grain boundaries.
The popular alloy 625 nibased superalloy was until now believed not to form serrated grain boundaries based on previous considerations of serrability criteria. In addition, geometrically necessary grain boundary dislocations or disconnections gbds will be present in the serrated ctb figure 1c. Strengthening by grain size reduction continue a finegrained material is harder and stronger than coarsegrained one other grain boundary factors. Estimates of stress and strain rate in mylonites based on the. Some more recent examples include dislocation pairing in low angle grain boundaries 7 and the observation of grain boundary structural phase transitions 8.
The formation of serrated grain boundaries as a result of different precipitation behaviors of. A mechanism of serrated grain boundary formation by which grain boundary particles move and displace the local grain boundary segment is discussed and the prerequisite conditions for its occurrence are highlighted. Most calcite grains are equant and show slightly serrated grain boundaries that meet in triple points. Grain boundaries, misorientation distributions, rodrigues. These serrated grain boundaries result from grain boundary migration driven by local variations in dislocation density between two neighbouring grains 49,61. The present investigation was therefore undertaken to study the different parameters which might control the formation of serrated grain boundaries with the objective of obtaining the largest possible serrations.
Deformation experiments concluded that s train tended to localize at triple points and special boundaries in the standard. If the bicrystal is in an ideal twin misorientation, the net burgers vector of the gbds required to accommodate the tilt is b. One of the important factors to create the pfz in the superalloys can be the difference of the interfacial free energy, i. Grain boundary sliding at serrated grain boundaries. Some grain boundaries exhibit good atomic fit and are therefore resistant to sliding, show low diffusion rates, low energy, etc. A grain boundary gb is the interface between two grains, or crystallites, in a polycrystalline material.
Creep testing is carried out to determine the effect of serrated grain boundary morphology on the creep of a high temperature engineering alloy. The fatigue cracks were largely deflected by the serrated grain boundaries, and brittle intergranular fracture was retarded. Grain boundaries in ultrafine grained materials processed. Abstract the serration of grain boundaries in inconel 600 caused by heat treatment is studied systematically. Microstructure and hardness of laser shocked ultra. Marble myionites in the bancroft shear zone, ontario.
896 1197 1027 1351 1121 1037 597 1437 58 908 318 1023 1052 946 1322 287 561 1224 827 479 674 1206 1074 42 1331 1386 921 1476 335 488 912 67 121