Sensitivity to Fatigue Cracks Initiation of Symmetrical Tilt Grain Boundaries in Pure Copper Bicrystals in 1M NaNO2 Solution

2007 ◽  
Vol 561-565 ◽  
pp. 2415-2418
Author(s):  
Hiroyuki Miyamoto ◽  
K. Kuroda ◽  
Takura Mimaki
Keyword(s):  
Plastic Strain ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Fatigue Cracks ◽  
Pure Copper ◽  
Large Angle ◽  
Compression Tests ◽  
High Susceptibility ◽  
Axial Tension ◽  
Pile Up

Sensitivity to corrosion fatigue (CF) crack initiation has been investigated in a series of pure copper bicrystals with a symmetrical <110>-tilt grain boundary. Tests were performed by axial tension-compression tests in 1M NaNO2 solutions. The small-angle tilt bicrystals fractured in both intergranular and transgranular manners accompanied by a large amount of plastic strain to fracture while the large-angle bicrystals fractured in almost intergranular manner with a smaller plastic strain. Susceptibility to CF cracks increases with increasing misorientation. It seems that effect of grain boundaries structures, i.e., Σ-values is small in this experiment. Stress concentration generated by the pile-up of trapped dislocations at the grain boundary could account for the high susceptibility of the intergranular cracks in large-angle grain boundaries

Simulation of the interaction of plastic zone dislocations with the grain boundary at brittle-plastic transition temperatures in molybdenum

2021 ◽  
Vol 2021 (3) ◽  
pp. 77-85
Author(s):  
K. M. Borysovska ◽  
◽  
N. M. Marchenko ◽  
Yu. M. Podrezov ◽  
S. O. Firstov ◽  
...  
Keyword(s):  
Fracture Toughness ◽  
Grain Size ◽  
Grain Boundary ◽  
Plastic Zone ◽  
Grain Boundaries ◽  
Crack Tip ◽  
Dynamics Simulation ◽  
Density Of Dislocations ◽  
Grain Sizes ◽  
Pile Up

The (DD) method was used to model the formation of the plastic zone of the top of the cracks in polycrystalline molybdenum. Special attention was paid to take into account the interaction of dislocations in the plastic zone with grain boundaries. Structural sensitivity of fracture toughness was analyzed under brittle-ductile condition. Simulations were performed for a range of grain sizes from 400 to 100 μm, at which a sudden increase in fracture toughness with a decrease of grain size was experimentally shown. We calculated the value of K1c taking into account the shielding action of dislocations. The position of all dislocations in the plastic zone at fracture moment was calculated. Based on these data, we obtained the dependences of dislocation density on the distance from the crack tip thereby confirming significant influence of the grain boundaries on plastic zone formation. At large grain sizes, when the plastic zone does not touch the boundary, the distribution of dislocations remained unchanged. As grains reduce their size to size of the plastic zone, they start formating a dislocation pile – up near the boundaries. Dislocations on plastic zone move slightly toward the crack tip, but the density of dislocations in the middle of the grain remains unchanged, and fracture toughness remains almost unchanged. Further reduction of the grain size leads to the Frank-Reed source activation on the grain boundary Forming dislocation pile-up of the neighbor grains. Its stress concentration acts on dislocations of the first grain and causes redistribution of plastic zone dislocations. If the reduction in grain size is not enough to form a strong pile-up, density of dislocations on plastic zone increases slightly and crack resistance increases a few percent. Further reduction of grains promotes strong pile-up, dislocations move to crack tip, and its density on plastic zone increases. Crack is shielded and fracture toughness increases sharply. The calculation showed that the fracture toughness jump is observed at grain sizes of 100—150 μm, in good agreement with the experiment. Keywords: dislocation dynamics simulation, molybdenum, fracture toughness, grain size, plastic zone, brittle-ductile transition.

A dynamical model of a crystal structure - IV. Grain boundaries

1952 ◽  
Vol 212 (1111) ◽  
pp. 576-584 ◽  
Keyword(s):  
Crystal Structure ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Large Angle ◽  
Gradual Transition ◽  
Small Angle Boundary ◽  
Pure Metals ◽  
And Diffusion ◽  
Large Angle Boundary ◽  
Dislocation Walls

Four photographs of bubble rafts are used as a basis for discussion of the structure of grain boundaries in pure metals. In these photographs one can follow the gradual transition from a small-angle boundary made up of clearly separate dislocations to a large-angle boundary where the dislocation structure is hardly recognizable. As the angle is increased, a continuous shortening of the dislocations, accompanied by the widening of a crack on the tensile side, is seen, and the process culminates in a structure which is perhaps best described in terms of local fit and misfit. The fact is also illustrated that the dislocation content of the boundary depends on the angle of the boundary, as well as on the disorientation of the crystals that it separates. If a boundary turns it must therefore gain or lose dislocations. The bearing of this on the measurement of grain-boundary energies is discussed. Other points considered concern the range of validity of calculations of the energy of dislocation walls, and slip and diffusion along grain boundaries.

Quantitative measurements of stresses at grain boundaries in polycrystalline metals

1988 ◽  
Vol 46 ◽  
pp. 620-621
Author(s):  
W. A. T. Clark
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Direct Transmission ◽  
Static Mode ◽  
Quantitative Measurements ◽  
Polycrystalline Metals ◽  
Dynamic Studies ◽  
Pile Up ◽  
Mode A

It has long been recognized that the deformation of polycrystalline metals proceeds by the movement of individual dislocations both within the grains and across the grain boundaries which separate them. It is known, for example, that the yield stress is directly affected by the density of grain boundaries in a metal; in the familiar Hall-Petch relationship it is inversely proportional to the grain diameter. Various models have been proposed to account for this behaviour, all of which involve the interaction between dislocations and grain boundaries (for a review see e.g. ref. 1). Microscopically, these interactions can be accomplished by several different mechanisms, which include the nucleation of new dislocations, direct transmission of dislocations across the interface, the absorption and desorption of dislocations into and out of the interface.The TEM can be used for both static and in-situ dynamic studies of these interactions. In the static mode, a TEM is used to analyze fully the crystallography of dislocation pile-up/grain boundary interactions; one such pile-up is shown in Fig. 1.

The Micro Structural Characteristic Parameters of High Grade Pipeline Steel and its Mechanical Properties

2006 ◽  
Author(s):  
Xiaoli Zhang ◽  
Chuanjing Zhuang ◽  
Lingkang Ji ◽  
Yaorong Feng ◽  
Wenzhen Zhao ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Pipeline Steel ◽  
Structural Parameters ◽  
Large Angle ◽  
Lower Bainite ◽  
Granular Bainite ◽  
High Grade ◽  
Pipeline Steels

The microstructure of high grade pipeline steels, including X65, X70, X80, X100, were studied by SEM and EBSD, respectively. It was found that the microstructures of high grade pipeline steels were composed of lower bainite, granular bainite and acicular ferrite. The phases of kinds of pipeline steels were composed of Fe3C, retained austenite and ferrite. And their percentage content, grain size and its distribution were studied respectively also. These micro structural parameters were correlated to the mechanical properties of kinds of pipeline steels. Furthermore, all kinds of angles of grain boundaries were studied, and the relationship between the angles of grain boundaries and mechanical properties was obtained. It was shown that as the improving of the steel grade, the grain boundary including small angle and large angle increased. And only when grain boundary was greater than 15 degree, it was effective to the toughness behavior.

Nanoindentation study of slip transfer phenomenon at grain boundaries

2009 ◽  
Vol 24 (3) ◽  
pp. 607-615 ◽  
Author(s):  
T.B. Britton ◽  
D. Randman ◽  
A.J. Wilkinson
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Cold Work ◽  
Pure Copper ◽  
Displacement Curve ◽  
Interstitial Free ◽  
Face Centered Cubic ◽  
Slip Transfer ◽  
Individual Contributions ◽  
Face Centered

Nanoindentation was undertaken near grain boundaries to increase understanding of their individual contributions to the material’s macroscopic mechanical properties. Prior work with nanoindentation in body-centered cubic (bcc) materials has shown that some grain boundaries produce a “pop-in” event, an excursion in the load–displacement curve. In the current work, grain boundary associated pop-in events were observed in a Fe–0.01 wt% C polycrystal (bcc), and this is characteristic of high resistance to intergranular slip transfer. Grain boundaries with greater misalignment of slip systems tended to exhibit greater resistance to slip transfer. Grain boundary associated pop-ins were not observed in pure copper (face-centered cubic) or interstitial free steel ~0.002 wt% C (bcc). Additionally, it was found that cold work of the Fe–0.01 wt% C polycrystal immediately prior to indentation completely suppressed grain boundary associated pop-in events. It is concluded that the grain boundary associated pop-in events are directly linked to interstitials pinning dislocations on or near the boundary. This links well with macroscopic Hall–Petch effect observations.

An in-situ TEM study on the slip activation across grain boundaries

1990 ◽  
Vol 48 (4) ◽  
pp. 542-543
Author(s):  
T. C. Lee ◽  
I. M. Robertson ◽  
H. K. Birnbaum
Keyword(s):  
Stainless Steel ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Polycrystalline Material ◽  
In Situ Tem ◽  
310 Stainless Steel ◽  
Pile Up

The movement of lattice dislocations in a polycrystalline material is usually impeded by the existence of grain boundaries. Depending on the local chemistry and the structure of the boundary, this impediment (or the interaction of dislocations and grain boundaries) can create dislocation pile- ups and eventually lead to one of the several possible reactions: 1) dislocations can be transferred directly through the grain boundary, 2) dislocations can be absorbed and become grain boundary dislocations, 3) dislocations can be ejected back into their original grain, 4) dislocations can be accommodated in the grain boundary, followed by the emission dislocations into the adjoining grain or 5) nucleation of a crack at or near the intersection of the dislocation pile-up and the grain boundary.In order to understand the mechanism of the these interactions and their sensitivity to the chemistry of the grain boundary, the in-situ TEM straining technique has been used. The material studied were 310 stainless steel, hyper- and hypo-stoichiometric Ni3Al with and without boron.

Improvement of Fatigue Crack Growth Resistance by Serrated Grain Boundary at High Temperature

1991 ◽  
Vol 113 (1) ◽  
pp. 9-14 ◽  
Author(s):  
Hiroshi lizuka ◽  
Manabu Tanaka ◽  
Fumio Ashihara
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Crack Growth ◽  
Growth Rates ◽  
Fatigue Cracks ◽  
Crack Growth Resistance ◽  
Fatigue Crack Growth Resistance ◽  
Crack Growth Rates

Effects of serrated grain boundaries on the improvement of fatigue-crack growth resistance were investigated using austenitic 21Cr-4Ni-9Mn heat-resisting steel at 973K in air. Grain boundaries were serrated by grain-boundary reaction precipitates. The crack-growth rates were considerably decreased in the specimens with the serrated grain boundaries. The fatigue cracks were largely deflected by the serrated grain boundaries, and brittle intergranular fracture was retarded. The improvement of the crack-growth resistance was obtained especially under the conditions of low crack-growth rates of less than 30 μm/cycle. The widths and the heights of the deflected portions of the cracks were in the range from about a few μm to 30 μm.

scholarly journals Crossing grain boundaries in metals by slip bands, cleavage and fatigue cracks

2015 ◽  
Vol 373 (2038) ◽  
pp. 20140131 ◽  
Author(s):  
André Pineau
Keyword(s):  
Fracture Toughness ◽  
Grain Boundaries ◽  
Crack Path ◽  
Fatigue Cracks ◽  
Large Angle ◽  
Cleavage Crack ◽  
Pressure Vessel Steel ◽  
Vessel Steel ◽  
Bcc Metals ◽  
Slip Transfer

The size and the character (low and large angle, special boundaries, tilt and twist boundaries, twins) of the grain boundaries (GBs) in polycrystalline materials influence their strength and their fracture toughness. Recent studies devoted to nanocrystalline (NC) materials have shown a deviation from the Hall–Petch law. Special GBs formed by Σ3 twins in face-centred cubic metals are also known to have a strong effect on the mechanical behaviour of these metals, in particular their work-hardening rate. Grain orientation influences also crack path, the fracture toughness of body-centred cubic (BCC) metals and the fatigue crack growth rate of microstructurally short cracks. This paper deals both with slip transfer at GBs and with the interactions between propagating cracks with GBs. In the analysis of slip transfer, the emphasis is placed on twin boundaries (TBs) for which the dislocation reactions during slip transfer are analysed theoretically, experimentally and using the results of atomic molecular simulations published in the literature. It is shown that in a number of situations this transfer leads to a normal motion of the TB owing to the displacement of partial dislocations along the TB. This motion can generate a de-twinning effect observed in particular in NC metals. Crack propagation across GBs is also considered. It is shown that cleavage crack path behaviour in BCC metals is largely dependent on the twist component of the GBs. A mechanism for the propagation of these twisted cracks involving a segmentation of the crack front and the existence of intergranular parts is discussed and verified for a pressure vessel steel. A similar segmentation seems to occur for short fatigue cracks although, quite surprisingly, this crossing mechanism for fatigue cracks does not seem to have been examined in very much detail in the literature. Metallurgical methods used to improve the strength of the materials, via grain boundaries, are briefly discussed.

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