Void nucleation in astroloy: theory and experiments

Dislocation arrangements in deformed polycrystalline 3% silicon-iron

Stacking fault energy in short-range ordered γ-phases of Ni-based superalloys

The nucleation of grain boundary voids in a nickel-base superalloy during high temperature creep

Deformations of a planar doubly periodic array of square elasticinclusions in an isotropically hardening elastic-viscoplastic matrixare analysed. The arrays considered have multiple inclusions per unitcell, but in each array all inclusions have the same size. Overallplane strain tension with a superposed tensile biaxial stress isimposed. A finite deformation formulation is used with a cohesivesurface constitutive relation describing the bonding between theinclusion and the matrix. A characteristic length is introduced fromdimensional considerations since the cohesive properties include thework of separation and the cohesive strength. The system analysed isused to study inclusion distribution effects on void nucleation, withthe aim of providing background for incorporating the effect ofclustering on void nucleation into phenomenological constitutiverelations for progressively cavitating plastic solids. For low valuesof the triaxiality of the imposed stress state, void nucleation occursafter extensive overall plastic straining and regular distributionshave a higher value of the void nucleation strain than randomdistributions. For larger values of stress triaxiality, where void nucleation occurs at relatively small overall plastic strains, theeffect of inclusion size dominates the effect of inclusiondistribution and smaller inclusions give rise to higher void nucleation strains. The ability of various scalar measures ofclustering to characterize the computed dependence of void nucleationon inclusion distribution is explored. Within the context of aphenomenological description of void nucleation, it is found that theeffective void nucleation stress is approximately a linear function ofthe overall hydrostatic tension with a coefficient 0.40-0.44 forregular distributions and 0.25-0.35 for random distributions.The results also suggest a possible dependence of the effective voidnucleation stress on a simple scalar measure of clustering.

Stress corrosion cracking (SCC) of single-crystal type 321 (UNS S32100) stainless steel (SS) was investigated in 42% boiling magnesium chloride (MgCl2) solution. Tests were performed under Mode II loading to study the SCC mechanism. Maximum normal stress and maximum shear stress were located at different positions on the notch surface in the Mode II notch specimens. Results showed SCC initiated at the maximum normal stress position. No SCC occurred at the maximum shear stress position. SCC initiated on the (111) plane and propagated on the normal stress plane. Slip bands appeared around the maximum normal stress position but not in the area around the main crack. Many microcracks ~ 2 μm long were observed on the slip bands. Their direction was inclined ~ 35° to the slip bands instead of along them. Nucleation resulted from interaction of dislocation pileup stress and local anodic dissolution. Multiple microcracknucleations increased stress around crack tips of the main and microcracks. Highly concentrated local stress from dislocation pileups increased the anodic dissolution rate locally.

Direct observations of martensite nuclei in stainless steel

The ductile fracture of a spheroidized 1518 steel has been investigated using three types of tensile specimens — smooth tensile, notched tensile, and plane-strain tensile. It was found that void nucleation has two different modes (Type I and Type II) depending on local conditions, the most important of which are the size, shape, and distribution of the particles. By identifying the low-strain-range nucleation behavior (Type I), it was possible to determine the value of plastic strain, eN, after which void nucleation at average-sized carbide particles (Type II) begins; eN is 0.45 for the smooth tensile case, 0.30 for the notched, and 0.25 for the plane strain. The critical stress for Type II void nucleation, σc, is of the order of 1200 MPa. Void growth depends on the macroscopic stress-strain state: longitudinal growth is given by a linear function of applied plastic strain, ep, whereas lateral growth shows a linear dependence on the triaxial stress, σT. When the local value ofV f reaches a critical volume fraction of voids (V f cri = 5 ± 0.5 pct), void coalescence occurs in a catastrophic manner, leading to final separation within a highly localized zone. The stress concentration caused by the notched tensile specimen geometry and the localized mode of plastic flow caused by the constraint of the plane-strain state in a Clausing-type specimen were found to affect the substeps of void nucleation, growth, and coalescence.

Microstructural changes in a single crystal Ni-base superalloy induced by plastic straining

In this paper, the probability of fatigue cracking along different kinds of grain boundary (GB) and persistent slip bands (PSBs) is considered in the light of data obtained by cyclic deformation of copper bicrystals and columnar crystals. It is found that, in copper bicrystals, fatigue cracks always nucleate and propagate along large-angle GBs, irrespective of whether the GB is perpendicular, parallel or inclined to the stress axis. On the contrary, for columnar copper crystals containing small-angle GBs, PSB-matrix interfaces become the preferential sites for initiation of fatigue crack; fatigue cracking along the small-angle GBs was never observed. For a special [1-34]/[182-7] copper bicrystal with a Sigma = 19b GB and a common primary slip plane, GB cracking also results in fatigue failure. Based on the results above, the interactions of dislocations carried by PSBs with GBs, including 'pile-up of dislocations', 'passing through of dislocations' and 'partial passing-through dislocations', are discussed. It is suggested that the probability of fatigue cracking in fatigued copper crystals increases in the order of small-angle GBs, PSBs and large-angle GBs.

Crystal dislocations provide the ultimate source of localized damage enhancement within solid materials. Vortices are the dislocation counterparts within liquids and gases. For energetic crystals, tubular holes might run along the pre-existent dislocation line lengths and act as shock-induced in-situ hot spots. Beyond this consideration, nearly invisible clouds of dislocations are possibly generated at point defects or point defect clusters by the shear stresses at a shock front. Multiple fine scale dislocation movements provide a mechanism for the shock to move to a hydrostatic stress state. Interatomic or intermolecular separations of the order of critical reaction coordinate distances are achievable during the unit dislocation displacements --- without change in material volume. Such nanoscale dislocation predictions connect with microscale experiments in a number of cases where larger scale defect considerations are involved. Dislocation pile-ups in slip band avalanches, often associated with cracking, account for very appreciable and localized heating that is deformation rate dependent. Complex dislocation slip band interactions occur within the plastic zones of macroscopic crack tips to control the fracture toughness properties of energetic and related materials.

Most materials used in nuclear reactors are prone to radiation damage and their mechanical properties degrade with time, limiting their service life. Defects produced by high energy particle radiation can be highly mobile at high temperatures with their mobility being influenced by the local stress fields associated pre‐existing defects and grain boundaries. The relaxation of radiation defects can produce clusters and induce defect diffusion to interfaces and other pre‐existing defects, where they can be absorbed and alter the microstructure that can detrimental to the material's mechanical properties. Radiation mechanisms that promote biased point defect diffusion can induce significant microstructural change and degrade properties. For instance, interstitials, being more mobile than the vacancies, are quickly absorbed by nearby dislocations, inducing creep by dislocation climb and dislocation multiplication that results in work hardening and embrittlement. Likewise, a small excess of remnant vacancies can agglomerate, leading to the formation of voids that cause swelling, increased residual stresses, the formation of microcracks, and the eventual failure of the material. The long‐term stability of a microstructure under irradiation depends on its neutrality towards biased point defect dynamics. Materials with a high density of sites that can act as sinks for the point defects produced by high‐energy particles would thus be an enabling technology for reliable and clean nuclear energy. We have explored radiation damage mechanisms in one such material, nanotwinned copper, which has a microstructure comprising a high density of coherent twin boundaries distributed in a “latter‐like” morphology within columnar grains having high misorientation angle intercolumnar boundaries. By performing in‐situ particle irradiation in a MeV TEM and characterizing the evolution in the microstructure using the Nanomegas ASTAR system, we have studied the irradiation induced damage and correlated the radiation induced grain boundary migration and void preferential void formation to the local grain boundary character and network. In‐situ observations were made at two different temperatures, room temperature and temperature above 573 K that stimulate void nucleation and growth. Figure 1 shows an example montage of the irradiation induced void formation and grain boundary migration with increased time and dose. The radiation induced grain boundary migration (RIGM) of several high‐angle random and low–angle grain boundaries within the irradiated zone was observed at both temperatures. However, low‐energy CSL type boundaries were not observed to migrate even at elevated temperatures as well as grain boundaries connected to junctions that contain one or more Σ3 boundaries. Thus, coherent twins are thought to stabilize the microstructure from radiation induced coarsening. At temperature above 573 K during high‐energy electron irradiations, copious amounts of voids were observed to nucleate and growth. They were predominantly observed to nucleate and grow in the regions with a high density of the coherent twin boundaries (CTBs). Very few voids were observed around HARGBs and LAGBs as shown in Figure 2, and only a limited of number of small voids were observed in the vicinity of incoherent twin boundaries. Non‐coordinated boundaries having excess free volume are efficient sinks for both vacancies and interstitials, and thus do not stimulate biased point defect diffusion that promote void formation, explaining the observed lack of voids in the vicinity of HARGBs. On the other hand, the observed high density of large voids in regions with finely spaced CTBs may arise from the biased diffusion of point defects. Interstitials being more mobile that are able to diffuse through CTBs can rapidly migrate and annihilate at free surfaces or other sinks such HARGBs. This leaves an excess of vacancies in the vicinity of the CTBs which can coalesce and cause the nucleation of voids. Given these observations, a nanocrystalline material having a high fraction of CTBs, though being resistant to radiation induced coarsening, may not possess the ideal topology for radiation resilience and may be prone to increased void formation and swelling.

Loading frequency and microstructure interactions in intergranular fatigue crack growth in a disk Ni-based superalloy

Void growth in copper during high-temperature power-law creep

The effects of void nucleation occurring during the deformation history on forming limit curves are considered for both in-plane and punch stretching employing a constitutive model of a porous plastic solid. Both plastic strain controlled and stress controlled nucleation processes are simulated by a two parameter void nucleation criterion. For in-plane stretching, plastic strain controlled nucleation can have, in certain circumstances, a significantly destabilizing effect on the forming limit curve. However, within the framework of plane stress theory which neglects the enhancement of the hydrostatic stress due to necking, a stress controlled nucleation process is not found to be significantly destabilizing. In punch stretching a ductile rupture criterion, which limits the maximum volume fraction of voids, as well as the appearance of a well defined thickness trough, is adopted as a localized necking criterion. Only plastic strain controlled void nucleation is considered here in out-of-plane stretching. The resulting forming limit curves have the same shape as those obtained previously with void nucleation neglected.

ABSTRACTStructural integrity plays an important role in any industrial activity, due to its capability of assessing complex systems against sudden and unpredicted failures. The work here presented investigates an unexpected new mechanism occurring in structures subjected to monotonic and cyclic loading at high temperature creep condition. An unexpected accumulation of plastic strain is observed to occur, within the high-temperature creep dwell. This phenomenon has been observed during several full inelastic finite element analyses. In order to understand which parameters make possible such behaviour, an extensive numerical study has been undertaken on two different notched bars. The notched bar has been selected due to its capability of representing a multiaxial stress state, which is a practical situation in real components. Two numerical examples consisting of an axisymmetric v-notch bar and a semi-circular notched bar are considered, in order to investigate different notches severity. Two material models have been considered for the plastic response, which is modelled by both Elastic-Perfectly Plastic and Armstrong-Frederick kinematic hardening material models. The high-temperature creep behaviour is introduced using the time hardening law. To study the problem several results are presented, as the effect of the material model on the plastic strain accumulation, the effect of the notch severity and the mesh element type and sensitivity. All the findings further confirm that the phenomenon observed is not an artefact but a real mechanism, which needs to be considered when assessing off-design condition. Moreover, it might be extremely dangerous if the cyclic loading condition occurs at such a high loading level.