Grain Boundary Fracture Surfaces Research Articles

Ductile-brittle transition temperature shift controlled by grain boundary decohesion and thermally activated energy in Ni-Cr steels

Abstract Temper embrittlement induced by segregation of metalloid solutes to grain boundary (GB) was evaluated by a shift of the ductile-brittle transition temperature (DBTT). DBTT was found to be linearly correlated with the amount of metalloid on the GB (X gb) for both dynamic and static displacement rates (dδ/dt) in high and medium hardness steels. Recent first-principles calculations have determined the GB embrittling potency (Δe p) of segregated Sb, Sn and P. In both high and medium hardness steels, the slope (α) of DBTT vs. X gb was found to be linearly dependent on Δe p regardless of the segregated solutes. In high hardness steels, the slope is independent of dδ/dt, while in medium hardness steels the α is dependent on dδ/dt. An Arrhenius plot of dδ/dt vs. the reciprocal DBTT was used to drive the thermal activation energy (E act), which represents a barrier to plasticity. It was found that E act correlates to a reduction in the GB fracture surface energy. The E act depends strongly on GB decohesion in high hardness steels but only weakly depends on it in medium hardness steels.

Effects of helium on mechanical properties of tungsten for fusion applications

The effects of helium (He) on mechanical properties of hot-rolled pure W were investigated using a high-energy He-implantation technique and post-implantation tensile tests. After the He implantation, the tensile specimens were heat treated at 1100 °C for 100 h in order to recrystallize the pure W. The post-implantation tensile tests were conducted in vacuum at 400 and 700 °C. The post-implantation annealed specimens showed almost the same hardness as-received unimplanted specimens. Tensile properties of He-implanted specimens after the post-implantation annealing indicated almost the same trend as those of as-received specimens under these experimental conditions. Dimples were formed and ductile fracture was observed on the ruptured surface of all specimens. No grain-boundary fracture surface was observed under these experimental conditions. The results showed that He clusters would suppress the recrystallization of hot-rolled pure W because the He clusters might form on dislocations in low angle grain boundaries or in cell walls in the matrix. Because of the suppression of recrystallization by the He clusters, the tensile properties of He-implanted specimens after post-implantation annealing show almost the same trend as those of as-received specimens under the experimental conditions. The results indicate that 20 appm of He implantation is level enough to suppress the recrystallization of pure W.

Auger electron spectroscopy analysis of grain boundary microchemistry in an Al–Cu–Li alloy

In situ impact fracture combined with Auger electron spectroscopy (AES) was employed to determine grain boundary (GB) microchemistry, namely lithium (Li) concentration, for an Al–Cu–Li alloy. The Li/Al ratio of GBs in the naturally aged condition was revealed to be significantly higher than in the overaged condition (or in the bulk alloy). GB fracture surfaces for the naturally aged alloy were depleted in Cu, whereas in the overaged condition the GB fracture surfaces were depleted in Li and enriched in Cu relative to the bulk composition. These findings are discussed in the context of intergranular corrosion and the precipitation sequence in Al–Cu–Li alloys.

Fractal Dimension of Grain-boundary Fracture for Characterization of High-temperature Fracture in Heat-resistant Alloys

The fractal dimension of the grain-boundary fracture, Df, (2<Df<3), which represents the fracture surface pattern with grain-boundary microcracks in three-dimensional space, is proposed for characterization of high-temperature fracture in materials. The value of Df as well as its two dimensional value, Dfp (the fractal dimension of the grain-boundary fracture surface profile, 1<Dfp<2), was estimated in the length scale range more than about one grain-boundary length using the height data of fracture surfaces of heat-resistant alloys obtained by the stereo matching method. The value of Df increased with increasing fractal dimension of the grain-boundary surface profile (DGB, 1<DGB<2) in the specimens of the HS-21 alloy ruptured at 1089 K. Both rupture life and creep ductility increased with increasing value of Df in these specimens. Similar results were obtained by the two-dimensional fractal analysis on other specimens of cobalt-base, nickel-base and iron-base heat-resistant alloys. Creep fracture process including the growth of the main creep crack was examined by the fractal analysis using the fractal dimension map (FDM, a color-coded map) on the surface notched specimens. The result of the fractal analysis was compared with that of the FRASTA (fracture surface topography analysis) in the Inconel X-750 alloy. The fractal analysis used in this study is more convenient and more advantageous than the FRASTA, and is widely applicable to the investigation of high-temperature fracture in materials.

Correlation Between Grain‐Boundary Segregation and Grain‐Boundary Plane Orientation in Nb‐Doped TiO2

Grain‐boundary (GB) segregation has been measured in Nb‐doped TiO2 (rutile) by Auger electron spectroscopy analyses of the two surfaces exposed by inter‐granular fracture. In addition, the orientations of the exposed GB planes on both sides of the fracture surface have been determined. A strong anisotropy of GB composition is found. Measurements on 166 individual GB fracture surfaces show that boundary planes with low and high levels of segregation tend to be clustered along the (110)–(010) and (001)–(011) regions of the stereographic triangle, respectively. The matching halves of GB planes with either high or low segregation also tend to be terminated by planes in the same respective regions of orientation space. High and low levels of segregation occur in regions of GB plane orientation space estimated to correspond to high and low GB energy, respectively. This trend is consistent with qualitative arguments based on Gibbsian adsorption.

519 Cu-SiO_2 分散強化合金の高温疲労中の粒界破壊とメカニズム

Orientation-controlled Cu-SiO_2 bicrystals having different misorientation angles were cyclically deformed at temperatures between 673K and 873K in vacuum. Although the fatigue lives of the most bicrystals tended to increase monotonically with decreasing stress amplitude at a higher stress amplitude region, some of them showed discontinuous and abrupt life shortening at a lower stress amplitude region. This transition of the cyclic ftigue behavior was brought by the occurrence of brittle intergranular fracture. This transition stress amplitude was also dependent on testing temperature and misorientation angle. At the lower stress region, elongated voids, which would be formed by GBS were observed at grain-boundary fracture surfaces. At the higher stress region, however, grain-boundary steps those were densely formed by plastic deformation of the matrices effectively suppressed GBS, resulting in the absence of brittle intergranular fracture.

Automated Image Processing and Analysis of Fracture Surface Patterns Formed during Creep Crack Growth in Austenitic Heat-Resisting Steels with Different Microstructures.

A computer program of automated image processing was developed for fractal analysis of creep fracture surface profiles in this study. Change in the creep fracture surface patterns during crack growth was examined by the computer-aided image processing and analysis. Fractal analysis was then made using software on the processed images. Creep crack growth experiments were carried out on the surface notched specimens of the austenitic 21Cr-4Ni-9Mn steel at 973 K. Creep crack growth rate was lower in the specimens with serrated grain boundaries than in those with straight grain boundaries. The fractal dimension of the grain-boundary fracture surface profiles was larger in the former specimens than in the latter ones. The fractal dimension was larger in the specimens tested under the lower stress, and decreased with increasing distance from notch root. Effects of grain-boundary microstructures and stress on the fracture patterns were correlated to the microstructure and stress dependence of the density of grain-boundary microcracks linked to the fracture surface. Quantitative evaluation of fracture surface patterns may give an important information about the fracture origin or crack growth direction.

ESTIMATION OF THE FRACTAL DIMENSION OF FRACTURE SURFACE PATTERNS BY BOX-COUNTING METHOD

In the box-counting method, positioning of images do not significantly affect the estimation of the fractal dimension of river pattern on the brittle fracture surface, and that of dimple pattern on the ductile fracture surface of materials. A reasonable estimation of the fractal dimension can be made using the box-counting method by a single measurement on the fracture surface pattern. The fractal dimension of dimple pattern in pure Zn polycrystals (about 1.50) is larger than that of river pattern in soda-lime glass (about 1.30). Personal difference in image processing does not have a large influence on the estimation of the fractal dimension of grain-boundary fracture surface profile, compared with the effects of local variation in fracture pattern concerning image size.

Fractal nature of creep-fracture surface patterns in pure Zn polycrystals

Mandelbrot et al. [1] ®rst quantitatively evaluated the fracture surface patterns in impact-loaded and fractured steels using fractal geometry and studied the relationship between the fractal dimension of the fracture surfaces and the absorbed energy. Since then, the concept of fractal geometry has been applied to many studies in materials science. For example, the fractal nature of microstructures such as grain boundaries, has been investigated in deformed or annealed metals and alloys [2±5] and in heat-treated alloys [6]. A correlation has also been found between the fractal dimension of the grain boundaries, the fractal nature of the grain-boundary fracture surfaces and the creep-rupture properties in heat-resistant alloys [6, 7]. Ishikawa [8] recently examined the size distribution of dimples in ductile fracture surfaces of steels and found that the value of the fractal dimension is about 1.5 and is close to that of the Sierpinski gasket (about 1.585). However, fracture surface patterns are generally associated with the fracture mechanisms in materials, and it is not uncommon that both equiaxed and elongated dimples are observed on the ductile fracture surface of a single specimen. Ductile fracture surfaces with dimples can also be regarded as a network of ridges (the walls of dimples) formed by a large amount of plastic deformation between dimples. In this study, the fractal dimension of the dimple patterns, which consisted of a network of ridges on ductile fracture surfaces, was estimated by the boxcounting method on fracture surfaces with elongated or equiaxed dimples in creep-ruptured specimens of pure Zn polycrystals. According to Ishikawa [8], the fractal dimension of the dimple size distribution was also estimated on some creep-ruptured specimens to compare with that of the dimple patterns. Commercial pure Zn polycrystals of 99.99 wt % were used in this study. Specimens 16 mm in diameter and 90 mm in length were cut from forged Zn bars and were solution-heated for 1.8 ks at 473 K and then air-cooled. The average grain diameter of the heat-treated specimens was 12 im. These specimens were machined into creep-rupture specimens 5 mm in diameter and 30 mm in length. Creeprupture experiments were carried out under the initial creep stresses of 14.7, 19.6 and 29.4 MPa at 373 K. Ruptured specimens were sectioned normally to the tensile axis. Fracture surfaces of the ruptured specimens were observed and photographed in a scanning electron microscope at magni®cations of 200 or 10003. The direction of microscopic observation is parallel with the tensile axis of specimens. The dimple size, x, is the diameter of a dimple for equiaxed dimples, but the value of x is the mean diameter estimated by x ˆ (d1d2) for elongated dimples, where d1 is the length of semimajor axis and d2 is that of semi-minor axis of an elongated dimple. The cumulative number of dimples, N, whose size is equal to or larger than x, is related to the value of x by a power law with the fractal dimension of the dimple size distribution, D9, as [8, 9]: N / xyD9 (1)

Effects of high-temperature ageing on the creep-rupture properties of high-tungsten cobalt-base superalloys

The effects of high-temperature ageing on creep-rupture properties were studied using cobalt-base superalloys containing about 14–20 wt% tungsten (W) at 1089 K (816 °C) and 1 311 K (1038 °C) in air. A high-temperature ageing for 1080 ks at 1273 K after solution treatment caused grain-boundary and matrix precipitates of W solid solution and carbide phases in these alloys, and grain boundaries were serrated especially in the alloys with higher W content. The high-temperature ageing largely improved the rupture life in the alloys with higher W content, particularly under lower stresses at 1089 K, whereas it caused the creep ductility to decrease a little in the alloy containing 20% W. The high-temperature ageing also improved the rupture life without decreasing creep ductility in these alloys under higher stresses at 1311 K. Under the same ageing conditions of 1080 ks at 1273 K, the initiation of grain-boundary cracks was retarded in the solution-treated and aged specimens, as well as in the aged specimens with serrated grain boundaries, for the alloys with higher W content at both 1089 and 1311 K. A large amount of grain-boundary serration also occurred in the non-aged specimens of the alloys with higher W content during creep at 1311 K, and contributed to the strengthening of the alloys. The solution-treated and aged specimen had almost the same rupture strength as the aged specimens with serrated grain boundaries in these cobalt-base alloys. The rupture strength of the solution-treated and aged specimens largely increased with increasing W content under the lower stresses at 1089 K and under the higher stresses at 1311 K. A ductile grain-boundary fracture surface, which was composed of dimples and grain-boundary ledges associated with grain-boundary precipitates, was observed in the solution-treated and aged specimens, as well as in the aged specimens with serrated grain boundaries at both 1089 and 1311 K. The fracture surface of the non-aged specimens was a brittle grain-boundary facet at 1089 K, but it became a ductile grain-boundary fracture surface, as serrated grain boundaries were formed owing to grain-boundary precipitates occurring during creep at 1311 K.

Mechanical properties of individual grain boubdaries in Ni 3Al using a miniaturized disk-bend test

The mechanical behavior of individual grain boundaries in boron-free Ni 3Al was investigated using miniaturized disk-bend tests performed on specimens 3 mm in diameter and nominally 200 μm thick. A directionally-solidified ingot containing 24 at.% Al was annealed between 1300 and 1350°C to produce an average grain size well over 1 mm, and fifteen disk specimens were extracted from it such that one long grain boundary extended across the diameter of each disk. Electron channeling patterns were used to determine the relative orientation of the grains on either side of the boundary. Low-angle boundaries are so strong they do not fracture, whereas high-angle boundaries always fracture, as indicated by a load drop in the curve of load vs displacement. Considerable plastic deformation of the individual grains occurs in all tests regardless of grain boundary character. Slip traces were observed on many of the grain-boundary fracture surfaces, providing some evidence for slip transmission across high-angle boundaries. The relative fracture strengths of the high-angle boundaries were estimated by extrapolating the elastic portion of the load-displacement curves to the displacement at fracture, yielding values ranging from about 2 to 4 GPa, with an average of 3.06±0.71 GPa. These are roughly an order of magnitude smaller than the fracture strengths of special boundaries predicted by computer simulations. No correlation was found between the fracture stresses and the relative orientations of the high-angle boundaries, as defined by the coincidence site lattice model.

Characterization of grain-boundary configuration and fracture surface roughness by fractal geometry and creep-rupture properties of metallic materials

Grain-boundary configuration in heat-treated specimens and fracture surface roughness in creep-ruptured specimens of several kinds of metallic material were quantitatively evaluated on the basis of fractal geometry. Correlations between the fractal dimension of grain boundary, that of fracture surface profile, the creep-rupture properties and the fracture mechanisms of the alloys are discussed. In heat-resistant alloys, the fractal dimension of a nominally serrated grain boundary was always larger than that of a straight grain boundary in the same alloy. The relative importance of the ruggedness of grain boundaries was estimated by the fractal dimension difference between these two grain boundaries. There was a quantitative relationship between the increase of the fractal dimension of the grain boundary and the improvement of rupture ductility and rupture strength owing to grain-boundary serration in the alloy. A similar correlation was also found between the increase in the fractal dimension of the fracture surface profile and the improvement of the creep-rupture properties, since in some cases the fractal dimension of the fracture surface profile was correlated with that of the grain boundary. Both grain boundary and fracture surface profile were assumed to exhibit a fractal nature between one grain boundary length (upper bound) and an interatomic spacing (lower bound). In carbon steels with ferrite-pearlite structure, according to the increase in pearlite volume fraction, the rupture ductility decreased and the fracture mechanism changed from transgranular fracture in pure iron and low-carbon steels to intergranular fracture at ferrite-pearlite grain boundaries in medium-carbon steels, and further to intergranular fracture at pearlite grain boundaries in high-carbon steels. The correspondence between the fractal dimension of the grain boundary and that of the fracture surface was confirmed in ruptured specimens of ferrite-pearlite steels when the grain boundary was the fracture path.

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys were investigated at 1089 and 1311 K in air. The specimens with serrated grain boundaries and those with normal straight grain boundaries were aged for 1080ksec at 1273 or 1323 K to cause the matrix precipitates of tungsten-rich b c c phase and M6C carbide. The creep-rupture strength of both specimens were improved by the high-temperature ageing. The rupture strength at 1311 K was the highest in the specimens with serrated grain boundaries aged at 1273 K, while the specimens with straight grain boundaries aged at 1273 K of the highest matrix hardness had the highest rupture strength at 1089 K. The high-temperature ageing did not decrease the rupture ductility of specimens. The ruptured specimens with serrated grain boundaries exhibited a ductile grain-boundary fracture surface which consisted of dimple patterns and steps, regardless of whether high-temperature ageing was carried out. The fracture mode of the specimens with straight grain boundaries was changed from the brittle grainboundary fracture to the ductile one similar to that of the specimens with serrated grain boundaries by high-temperature ageing, since large grain-boundary precipitates which gave nucleation sites of dimples were formed during the ageing. The grain-boundary cracks initiated in the early stage of creep (transient creep regime) in both non-aged and aged specimens of L-605 alloys in creep at 1089 and 1311 K, although the time to crack initiation is shorter in the specimens with straight grain boundaries than in those with serrated grain boundaries. Thus, the period of crack growth and linkage occupied most of the rupture life. The strengthening mechanisms of the aged specimens were also discussed.

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as α2 phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (α2) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich α2 phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles andβ-Co matrix, such that (0 1 1)α2 ¶ (1 1 1)β-Co and [1\(\overline 1 \) 1]α2 ¶ [1\(\overline 1 \) 0]β-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.

Observations on Work-Hardening and Fracture of Pure Polycrystalline Zinc in Tension

High-purity zinc wires of different grain sizes were subjected to increasing tensile loading at different temperatures, and the associated changes in length, internal friction, and shear modulus were simultaneously measured. The work-hardening coefficient was found to be parabolic and to decrease with increase of grain size and temperature. This was interpreted in terms of the associated variations of the dislocation slip distance. An internal friction peak was observed at low stresses before excessive slip took place, and was attributed to the freeing and subsequent locking of the loosely pinned dislocations under the action of the applied stress. In fine-grained samples, a rise in the shear modulus was also observed and was attributed to the dislocations created by activated Frank—Read sources, which pile up against grain boundaries. The grain-boundary fracture surface energy, calculated from the observed dependence of the fracture and yield stresses on grain size, amounted to 1280 ergs/cm2 at room temperature.