Boundary Serration Research Articles

Hot working of AA1050—relating the microstructural and textural developments

An aluminum alloy AA1050 was deformed in plain strain at different hot working conditions. An increase in temperature or a decrease in strain rate reduced the relative drop in cube {001}〈100〉 and the relative increase in rolling texture components of Cu {112}〈111〉 and S {231}〈346〉, especially apparent at the higher strain. Along with such textural changes, significant differences in hot worked microstructures were observed. The two distinct microstructural features, as observed by polarized light optical microscopy, were grain boundary serrations (GBS) and in-grain inclined lines (IIL), typically observed at an approximate angle of 35° with rolling direction (RD). At higher temperatures and lower strain rates, and correspondingly lower Zener–Holloman factors ( Z≈10 9−10 10 s −1), coarse but nearly equiaxed grain interior substructures and GBS were observed. Interestingly, orientation imaging microscopy (OIM) clearly showed insignificant/non-noticeable differences between the substructures of different orientation components. An increase in Z aligned the grain-interior low angle boundaries at an angle of approximately 35° with RD and at higher Z ( Z≈10 12−10 13 s −1) the main microstructural feature was the IILs. Development of in-grain long range misorientation (LRM) was estimated to be the mechanism behind the optical visibility of the IILs. The appearance of IILs had two apparent effects—first the substructures of different orientation components were different, and secondly the stability of cube grains dropped noticeably. Generalizing the IILs or 35° inclined cell walls as plastic instabilities or strain localizations, the observed differences in their relative appearance at different deformation conditions and/or texture components could be explained. When formation of such strain localizations are considered as “necessary” for the reorientation of grain segment(s), the cube stability at low Z deformation could also be understood.

Dynamic recrystallization under warm deformation of a 304 type austenitic stainless steel

Warm (and hot) deformation of a 304 type austenitic stainless steel was studied in connection with microstructural developments in compression at temperatures of 873–1223 K (0.5–0.7 Tm) under strain rates of 10−4–10−1s−1. The two deformation domains can be categorized due to their different mechanical and microstructural behaviors. In the region of flow stresses lower than around 400 MPa, the deformation behaviors are typical for hot working accompanied with dynamic recrystallization (DRX). New grains are evolved mainly by dynamic bulging mechanism, which can be accelerated by the development of serrated grain boundaries and strain induced dislocation subboundaries. The relationship between dynamic grain sizes ranged from 2 to 7 μm and peak flow stress can be expressed by a power law function with a grain size exponent of −0.72. In contrast, in the region of flow stresses higher than 400 MPa, the deformation behaviors hardly depend on strain rate and temperature and so can be in the region of athermal deformation. The stress–strain curves under such warm deformation are similar to those affected only by dynamic recovery. The microstructures evolved at high strains are mainly characterized by the dense dislocation walls evolved in pancaked original grains, while grain boundary serration also takes place even at such warm deformation. Mechanisms of this microstructural evolution are discussed in combination with analysis of deformation mechanisms operating under warm deformation.

Grain Boundary Sliding at Serrated Grain Boundaries

A constitutive model is developed for grain boundary sliding (GBS) at serrated grain boundaries. Based on a previously developed GBS model, using the dynamics of grain boundary dislocation pile-up, the present model takes the average of the sliding rate over the characteristic dimensions of grain boundary serrations. Thus, a geometric factor φ is introduced to account for the effects of serration wave length and amplitude on the GBS rate, as compared to the GBS rate at planar boundaries. By considering the role of grain boundary shear stress in stress balancing, the proposed model removes the singularity at planar boundaries which exists in the diffusion-controlled GBS model at serrated grain boundaries. The modified model describes very well the transient creep of complex Ni-base superalloys with and without grain boundary serrations and should be suitable for other engineering alloys (with the exception of columnar grained and single crystal alloys).

Formation and application of grain boundary serrations

Serrations develop in the grain boundaries (GB) of many metals and alloys as a result of local migrations to absorb dislocation walls. The serrations form only under deformation conditions where the subgrains reach about 2–10 μm in diameter; the conditions of temperature and strain rate depend on the metal and the effects of alloying on dynamic recovery. The small serrations evidently diminish GB sliding, making lattice deformation rate controlling. If the conditions are changed to produce lower strain rates, then initial serrations increase the creep life. In alloys with limited dynamic recovery, the serrations are also related to initiation of dynamic recrystallization, which is very important in preventing progression of GB cracking, and thus increasing the ductility in hot-forming operations. The formation of serrations or of a necklace of fine recrystallized grains along the GB of large grains (with good creep resistance) are noted for improving fatigue resistance.

Formation and Application of Grain Boundary Serrations

Formation and Application of Grain Boundary Serrations

Formation and Application of Grain Boundary Serrations

Serrations develop in the grain boundaries (GB) of many metals and alloys as a result of local migrations to absorb dislocation walls. The serrations form only under deformation conditions where the subgrains reach about 2–10 μm in diameter; the conditions of temperature and strain rate depend on the metal and the effects of alloying on dynamic recovery. The small serrations evidently diminish GB sliding, making lattice deformation rate controlling. If the conditions are changed to produce lower strain rates, then initial serrations increase the creep life. In alloys with limited dynamic recovery, the serrations are also related to initiation of dynamic recrystallization, which is very important in preventing progression of GB cracking, and thus increasing the ductility in hot-forming operations. The formation of serrations or of a necklace of fine recrystallized grains along the GB of large grains (with good creep resistance) are noted for improving fatigue resistance. Résumé Des dentelures se développent dans les joints de grains (GB) de plusieurs métaux et alliages, résultat de migrations locales pour absorber les murs de dislocation. Les dentelures se forment seulement sous les conditions de déformations où les sous-grains atteignent à peu près 2–10 μm de diamètre; les conditions de température et la vitesse de déformation dépendent du métal et les effets d'alliages de l'adoucissage dynamique. Les petites dentelures diminuent de manière évidente le glissement des GB, ce qui fait que la vitesse de déformation est contrôlée par le réseau. Si on change les conditions de façon à produire des vitesses de déformation moins grandes, les dentelures initiales augmentent la durée de vie en fluage. Dans les alliages á faible adoucissage dynamique, les dentelures sont aussi reliées à l'initiation de la recristallisation, ce qui est très important pour prévenir la progression de fissurations de GB et donc accroître la malléabilité dans les opérations de formage à chaud. La formation de dentelures ou de colliers de grains fins recristallisés le long des GB de gros grains (avec une bonne résistance au fluage) sont remarquables pour leur résistance accrue à la fatigue.

Effects of stress amplitude and internal stress on the grain boundary deformation behavior under high temperature creep in an Al-2.9%Mg alloy

In an Al-2.9%Mg alloy, the behavior of the static and cyclic creep deformation and the modes of grain boundary deformation with the various stress amplitude ratio under the stress range of 15 to 50 MPa at 573 K were investigated. For static creep, there was a transition from class A to class M behavior with increasing stress. The observed transition stress was about 42 MPa. The dominant deformation characteristics in the class A regime and the class M regime were the grain boundary migration and the grain boundary serration, respectively. However, no transition was observed for cyclic creep, instead, the creep behavior tended to exhibit the class M behavior with increasing δ. The GBC was observed only under the cyclic stressing in the viscous glide regime of the static creep at the conditions of both σ peak > σ i and σ min ≤ σ i and a long enough unloading period for the dislocation recovery to take place. Consequently, the formation of the GBC is considered to be closely related with both the recovery of dislocations in the grain boundary and the homogeneous distribution of Mg solute atoms in the grain boundaries.

Dynamic grain growth and ductility enhancement at intermediate temperature

Dynamic grain growth has been observed during hot tensile tests of copper at intermediate temperatures from 0.50 to 0.75 T m over the strain rate range of about 3 × 10 −5 3 × 10 −3 s −1. Microstructural examination revealed that at strain rates < 3 × 10 −5 s −1, dynamic recovery dominates and the grain sizes of the specimens after the test are comparable to that of the original material. When the strain rate is increased above 3 × 10 −5 s −1, dynamic grain growth occurs in that the sparsely nucleated grains are able to glow freely during the test, resulting in the formation of exceedingly large grains. As the strain rate is increased further above 3 × 10 −3 s −1, dynamic recrystallization sets in and the final grain size drops to below the original value. When dynamic grain growth occurs, the stress-strain curves become serrated with a few coarse ripples of gradually decreasing peaks. Interrupted tests further revealed that the stress drop during the test is associated with the occurrence of rapid grain growth in the material. Boundary serration is also a common feature under the test conditions when dynamic grain growth is observed. Detailed metallographic and fractographic studies further showed that the occurrence of dynamic grain growth is responsible for the upturn of the hot tensile ductility of copper with increasing temperature, during which the fracture mechanism changes from profuse grain boundary cavitation with low ductility to sparse wedge cracking with considerable ductility.

The Dendritic Growth of γ′ Precipitates and Grain Boundary Serration in a Model Nickel-Base Superalloy

The growth pattern of γ precipitates in the grains and at the grain boundaries has been investigated in a Ni-24Co-4Al-4Ti-5Cr-5Mo (weight percent) alloy of very small lattice misfit between the precipitate and the matrix phases under varying heat-treatment conditions. When aged at temperatures lower than the solvus temperature (T s = 1150 °C) by more than 30 °C after direct cooling from the solution-treatment temperature, the nucleation density is high. In this condition, the supersaturation is quickly removed because of the overlapping diffusion fields and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres in the grains and truncated spheres at nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains, and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150 deg) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shapes. When aged at temperatures just belowT s (above 1140 °C), the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just belowT s.

Tensile behavior of the L1 2 compound Al 67Ti 25Cr 8

Temperature-related variations in tensile yield strength and ductility were studied on cast, homogenized and isothermally forged Al 67Ti 25Cr 8. Yield strength dropped discontinously between 623 K and 773 K and then decreased gradually with increasing temperature. Below 623 K, fracture occurred prior to macroscopic yielding. Ductility decreased from 0.2% at 623 K to zero at 773 K, but increased again at higher temperatures. At 1073 K, an elongation of ∼ 19% was obtainable, and ultimate tensile strength and localized necking were observed. Fracture surfaces and deformed microstructures were examined. The 1073 K tensile specimen that exhibited ∼ 19% elongation showed grain boundary serrations and some evidence of recrystallization (likely dynamic) although fracture occurred predominantly via an intergranular mode.

Large-strain torsional deformation in aluminum at elevated temperatures

Abstract A substantial amount of work has been produced recently in the area of large-strain deformation of aluminum at elevated temperatures using torsion tests. These studies have generated much interest because the equivalent uniaxial strain to failure of the aluminum can exceed 100, depending on the purity, strain rate and temperature. The research in this area has been performed principally by four groups and reported largely in less-circulated journals, proceedings and reports. The following review of the subject was written by three representatives from these groups. Emphasis was on establishing areas of substantial agreement as well as delineating aspects where further work will be helpful. The review is divided into two principal sections: macroscopic phenomenology and microstructural observations. The first includes discussions of ductility, stress-strain relationships and constitutive equations. The second includes discussions of grain elongation and thinning and grain boundary serration, subgrain sizes and morphologies, dislocation density, and subgrain boundary misorientation angles, as well as texture analysis. These investigations provide insight into the large-strain mechanical and microstructural phenomenology of materials in which elevated temperature softening proceeds by dynamic recovery.

Effect of heat treatment on precipitation behaviour and high temperature strength in a wrought Co-base superalloy

The effects of solution and ageing temperatures on the grain boundary reaction as well as on matrix precipitation in the interior of the grains were investigated using wrought Co-base superalloy HS-21. The grain boundary reaction occurred during furnace-cooling after solution-heating. The phase that precipitated in the grain boundary reactions nodule was M23C6 carbide. It also occurred during ageing after solution treatment, but the extent of it was considerably influenced by cooling procedure after solution heating. The activation energy of the grain boundary reaction was 244 kJ mol−1 for the early stage of the grain boundary reaction in HS-21 alloy, and was considered to be the activation energy of grain boundary diffusion of chromium. The extent of the matrix precipitation that occurred during ageing was also influenced by the cooling procedure. Creep rupture tests were carried out at 1088 K in air. An excellent combination of long rupture life and large ductility was attained on a specimen, which involved both the grain boundary reaction nodules (about 7% in area fraction) and the matrix precipitates. The improvement of creep rupture properties results from the retardation of brittle intergranular fracture, which is achieved by grain boundary serration owing to the grain boundary reaction and by the increase of strength in the interior of grain due to the matrix precipitation.

インバー材溶接金属の延性低下割れに及ぼす金属学的因子の検討 Ｆｅ‐３６％Ｎｉ合金溶接金属の延性低下割れに関する研究 ３

Mechanism of ductility-dip cracking in weld metal of Invar (Fe-36%Ni alloy) reheated by following welding, pass was studied from the viewpoints of grain boundary precipitation and grain boundary serration. Main conclusions obtained areas follows: (1) Temperature dependency of amount of precipitates on the fracture surface, has good correlation with the characteristic of hot ductility. (2) The fracture surface of the specimen having low crack susceptibility has comparatively little amount of precipitates. (3): The features of intergranular fracture surface under different testing temperature and crosshead speed have intimate connectionn with that of grain boundary serration, and thus this suggests strongly. that grain boundary serration has an important role on'the ductility-dip crack. (4) Thus it is judged that both grain boundary precipitation and grain boundary serration cause cavity foi nation cooperatively accompanied by grain boundary sliding. (5) The mechanism of ductility-dip cracking was proposed synthetically from the viewpoints of grain boundary sliding, grain boundary serration, grain boundary precipitation, grain boundary migration, deformation stress and recrystallization.

Effect of grain boundary serration on the fatigue, creep and cyclic creep strengths of a nickel-base superalloy in air and in hot corrosive environment.

Fatigue, creep and cyclic creep tests have been carried out at 800°C for the variously heat-treated nickel-base superalloy Inconel 751. The heattreatments given are simple aging (SA), two-stages solution treatment (TS) and direct aging (DA), with and without coating of synthetic ash mixture composed of 90% Na2SO4 and 10% NaCl. Effect of grain boundary topography, particularly of the serrated grain boundary produced by the direct aging, on the strength properties is, then, investigated.It is clarified that hot corrosion can alter the fracture mechanism completely. In hot corrosive environment, a stress-enhanced aggressive intergranular penetration of sulfides, oxides and/or chlorides can cause a grain boundary weakening to induce a premature intergranular fracture in a brittle manner regardless of the grain boundary topography. In particular, the cyclic creep is shown to result in minimization of failure life in hot corrosive environment because the combined creep and fatigue loadings promote the aggressive intergranular attack, in contrast to the case in air in which the failure life is mainly determined by the creep damage.In general, the effect of grain boundary topography on the strength properties is significant in hot corrosive environment rather than in air. In hot corrosive environment, both the chromium-depleted zone and the straight grain boundary appear to enhance the aggressive intergranular attack, because the former can induce the low melting nickel sulfides formation and the latter is easily subjected to the locally increased strain concentration so as to provide a stress-activated path. The development of the chromiumdepleted zone tends to give a detrimental effect on the strength properties under all the loading conditions, while the detrimental effect of the straight boundary is more pronounced under the creep-dominant loading conditions. Consequently, in order to improve the strength properties in hot corrosive environment, it is important to eliminate such undesirable microstructures associated with the grain boundary, depending on the loading conditions.It is also confirmed that even in hot corrosive environment, the serrated grain boundary contributes notably to improve the strength under creepdominant loading conditions by virtue of the suppression of such an aggressive intergranular attack.

Ｆｅ‐３６％Ｎｉ合金（インバ）溶接金属の延性低下割れに関する研究 ＩＩ 再現高温延性試験法による低延性特性の検討

Simulated hot ductility test was done to establish the correlation with reheated weld hot cracking test of Fe-36%Ni alloy and to reveal the mechanism of reheated hot crack. Cross lines were drawn with a sharp razor on specimen surface electropolished to study the effects of grain boundary sliding, grain boundary migration, recrystallization and so on. Main conclusions are as follows: 1) There was ductility trough between about 600°C and 1000°C in reheated weld metal, and minimum ductility was located arround 900°C. These behaviors agreed well with those in the actual reheated weld hot cracking test. 2) There was good correlation between brittleness temperature range or minimum ductility evaluated by hot ductility test and total crack length in Cross-head tensile hot cracking test. 3) Grain boundary sliding together with grain boundary precipitates and/or grain boundary serration seems to cause the ductility trough and thus reheated ductility-dip crack by cavity formation process. The grain boundary precipitates contained Al and S. 4) Recovery of dutcility at about 1100°C was associated with active grain boundary migration and recrystallization.

On the mechanism of serrated grain boundary formation in Ni-based superalloys

Grain boundary serration is studied as a function of cooling rate in a number of Ni-based superalloys, e.g. IN 738, Nimonic 115 and Nimonic 105. The mechanism is related to γ′ precipitation in these alloys and a higher γ′ solvus temperature than the M 23C 6 solvus temperature appears to be a prerequisite for the development of serrations. A model based on the grain boundary primary γ′ particle movement causing the displacement of the local grain boundary segment and initiating serrated grain boundary formation has been proposed.

HVEM observations of grain boundary voids in high purity copper

A high voltage electron microscope study has been made of grain boundary cavities produced in copper by high frequency fatigue and by creep. Fatigue-induced voids form first at grain boundary serration peaks where a dislocation tangle or dipole wall intersects the boundary. The existence of a subgrain network is not necessary for cavitation. Little dislocation activity is seen around isolated voids, which appear equiaxed. As fatigue progresses, voids form in clusters which coalesce into long peanutshaped cracks. A high dislocation density is seen around the clusters. It is concluded that individual fatigue-induced voids grow by a vacancy diffusion mechanism limited by grain boundary diffusion but that plastic deformation becomes important as clusters form and voids interact. The nucleation rate for voids appears slower in creep than in fatigue. The majority of creep cavities could not be associated with any distinctive dislocation structure. Plastic deformation is important to the growth of a sizeable fraction of creep cavities.