Cavity Nucleation Research Articles

Using In Situ TEM Helium Implantation and Annealing to Study Cavity Nucleation and Growth

Noble gases are generated within solids in nuclear environments and coalesce to form gas stabilized voids or cavities. Ion implantation has become a prevalent technique for probing how gas accumulation affects microstructural and mechanical properties. Transmission electron microscopy (TEM) allows measurement of cavity density, size, and spatial distributions post-implantation. While post-implantation microstructural information is valuable for determining the physical origins of mechanical property degradation in these materials, dynamic microstructural changes can only be determined by in situ experimentation techniques. We present in situ TEM experiments performed on Pd, a model face-centered cubic metal that reveals real-time cavity evolution dynamics. Observations of cavity nucleation and evolution under extreme environments are discussed.

Insight into Type IV cracking in Grade 91 steel weldments

In this work, an insight into the premature Type IV cracking was undertaken to clarify its mechanisms in Grade 91 steel pipe weldments. High-resolution microscopy observations of the as-welded heat-affected zone (HAZ) reveal that the commonly recognized fine-grained region susceptible of cracking on the edge of HAZ belongs to the inter-critical HAZ (ICHAZ), rather than the fine-grained HAZ (FGHAZ). Instrumented indentation tests uncover that the ICHAZ is the weakest region across the weld, exhibiting the largest displacement and the lowest hardness in three thermal stages. Localized deformation of matrix grains and high stress triaxiality in the ICHAZ promoted nucleation of creep cavities along grain boundaries. This localized deformation was induced by the creep strength mismatch of matrix grains with different Cr concentrations. Cavity-free regions exhibit a relatively homogenous Cr distribution, whereas, an inhomogeneous Cr distribution is observed in the cavity-containing regions. It is believed that this local Cr inhomogeneity in the ICHAZ is caused by the partial dissolution of Cr-rich M23C6 carbides and an insufficient homogenization during rapid welding thermal cycles.

The development of creep damage constitutive equations for high Cr steel

ABSTRACTThis paper reports (1) an application of the modified hyperbolic sine law to P92 for the minimum creep strain rate over a wider range of stress level, and 2) the calibration of the creep cavity fracture model and its applications to such as P92, E911 and MARBN alloys. It was motivated by the original development of creep cavitation based creep rupture model and application to other alloys, and the need of creep strain and stress relation over a wider range of stress. The results include: (1) the calibration of creep cavity fracture model for E911, and the pre-required cavity nucleation and growth models, for E911, (2) the creep lifetime and stress relation coefficient U’ for P92, and (3) the relationship of the cavity nucleation rate coefficient A2 and stress level for MARBN cross-welds. This paper contributes to the specific knowledge and the methodology. It anticipates that this methodology will find its value and application in modelling other fractures such as ductile and fatigue.

Failure criterion for highly stretchable elastomers under triaxial loading

A failure criterion is introduced for highly stretchable hyper-elastic materials under tensile-dominated triaxial loading. It is based on energy balance using Griffith approach but is presented by the critical state of load as in traditional size independent strength theories. The basic argument is made of the recent assessment about the cavity nucleation in soft materials: the volumetric energy dissipation in the growth of defect and size-insensibility of the critical load in soft materials when the defect is smaller than the material characteristic length. Thus, using the energy approach as in fracture mechanics, we are able to estimate the critical state of load using only one material parameter, which is subsequently replaced by the critical stress under one specific loading, e.g., the strength of uniaxial loading. This leads us to the failure surface in the normalized principal stress space. The predictions of the present theory agree well with the experimental results collected from the literature, including an order of magnitude difference between the strengths of the uniaxial loading and hydrostatic loading. This addresses a variety of difficulties in the previous failure analyses. For the easiness of practical application, we further analyze the data and obtain a criterion using mean stress and the first invariant of Cauchy–Green deformation tensor, I1. We are able to obtain the widely mentioned maximum I1 criterion, and also find its limitation: only applicable when one of the principal stresses is zero. For general triaxial loadings, the material can fail at any allowable I1 and the critical mean stress monotonically increases with I1; and under a constant mean stress that is above the hydrostatic strength, the material is only safe when I1 is above a mean stress-dependent lower bound but below an unknown up-bound.

Determination of creep cavity nucleation rates

ABSTRACTThe product of the temporal strain rate times a coefficient dependent on the Monkman–Grant constant alone is proposed for describing the creep cavity nucleation rate. Comparisons with measurements are presented for various materials.

Experimental and numerical study on convective boiling in a staggered array of micro pin-fin microgap

The flow patterns, heat transfer and pressure drop of convective boiling of dielectric fluid, FC-72, in a micro-gap are investigated experimentally. The surface of the microgap is enhanced with a staggered array of micro pin-fins. The enhanced surface of the microgap with the size of 10 × 10 mm2 is subject to an electrical heat source. The micro-pin-fins are etched as cubic columns of 100 μm size and arranged in a staggered arrangement with 400 μm pitch in both transverse and longitudinal directions. The inside of the micro pin-fins is nucleated with a cavity of cylindrical shape with diameter of 60 μm and an opening with a size of either 15 μm or 45 μm width. The opening of the cavities of the micro-pin-fins is aligned toward the down-stream. For the case of single-phase flow, a numerical analysis is performed, and the pressure drop and velocity fields are investigated in the micro-gap. The experiments were performed for various mass fluxes ranging from 94 to 275 kg/m2s and heat flux ranging from 0 to 10 W/cm2, and at two saturation temperatures of 35 and 50 °C. For the case of single-phase heat transfer, the experimental results are compared with the pin-fins having 45 μm cavity opening and found that the effect of cavity opening is significant when the mass flux is high. The surface-superheat at the onset of boiling is reduced by reducing the cavity opening-width from 45 to 15 μm. The microgap having nucleation cavity with cavity opening-width of 15 μm results in a 3.44 °C smaller surface-superheat than that of 45 μm opening-width. The convective heat transfer coefficient increases with the increase of mass flux, regardless of the flow type. Moreover, the variation of the pin-fin opening-widths does not influence the convection heat transfer rate. For single-phase flow, the heat flux shows a negligible effect on pressure drop, and the pressure drop increases with increasing mass flux. For two-phase flow, the pressure drop increases drastically with increasing heat flux. A smaller cavity opening-width results in a smaller pressure drop when the mass flux is high. The flow observation shows two distinct flow patterns in the microgap at the beginning of the bubble nucleation.

Characterization of a novel 14H-LPSO structure and related elevated-temperature mechanical behaviors in an extruded Mg–Y–Zn–Cu alloy

A Mg-2.1Y-0.5Zn-0.6Cu-0.1Zr (WZC200, in at. %) alloy was developed and conducted to extrusion process to refine the grains as well as LPSO structure, aiming to investigate the mechanical behaviors at ambient and elevated temperatures and their relations with microstructures. A 14H-LPSO structure with stacking sequence of ABCBCACACACBCBA, consisting of twin-related building blocks with a novel CBCA-type, was revealed in the solution-treated WZC200 alloy. Fully recrystallized microstructure with grain size of ⁓14 μm, refined LPSO structure and weaken texture were obtained through extrusion process. The tension results tested at ambient and elevated temperatures revealed ultimate tensile strength (UTS) of 317 MPa along with excellent elongation-to-failure of 26.5% at RT, moderate UTS above 275 MPa with occurrence of serrated flow in the tensile curves in the range 150–200 °C, and obviously declined UTS (⁓150 MPa) along with superplastisity of 155% at 300 °C. The observed serrated flow in the range 150–200 °C was mainly attributed to the dynamic strain ageing. The dramatic drop in UTS at 300 °C was ascribed to fragmenting of LPSO phases and probably significant activation of non-basal slips, these two factors help to coordinate deformation between LPSO and the matrix and restrain the cavity nucleation, resultantly contributing to the observed superplasticity at this temperature.

Nucleate pool boiling heat transfer on etched and laser structured silicon surfaces

Pool boiling experiments were performed using saturated double-distilled and degassed water on 10 × 10 mm silicon samples, which were (i) untreated; (ii) laser structured; or (iii) modified with etched artificial nucleation cavities. The etched silicon surfaces were fabricated with differently sized micro nucleation cavities (5–30 µm) and pitches (0.125–2 mm) to allow a comparative analysis of the fabricated surfaces. The heat transfer coefficient (HTC) comparative analysis conducted at the heat flux of 200 kW/m2 exhibits the highest enhancement of 244% during nucleate boiling on the silicon sample with etched nucleation cavities with a 30 µm diameter and a 0.125 mm pitch. The experimental results consistently show that HTC increases with decreasing the pitch and increasing the size of the nucleation cavities in the range of the experimental conditions. The superheat required for the onset of nucleate boiling and the critical heat flux were not substantially affected with the structured surfaces. However, the boiling phenomena propagates more promptly to the entire available silicon surface, when the sample is laser treated or etched compared to the reference bare silicon sample.

Nucleation and Growth of Cavities in Hydrated Nafion Membranes under Tensile Strain: A Molecular Dynamics Study

Molecular dynamics simulations are performed to investigate the nucleation and growth of cavities in a hydrated Nafion membrane under mechanical deformation. The simulation model used in this study...

Type IV failure in weldment of creep resistant ferritic alloys: II. Creep fracture and lifetime prediction

The creep rupture behavior (Type IV failure) for weldments of creep strength enhanced ferritic steel is numerically analyzed, using an integrated microstructure- and micromechanics-based finite element model. To account for the large microstructure gradients across weldments, a two-dimensional digital microstructure is constructed based on the actual observed microstructure of ferritic steel weldment by using the Voronoi-tessellation method. According to the fracture mechanism studies and literature experimental observations, the Type IV failure is identified as an intergranular creep fracture in the fine-grained or intercritical heated affected zone (FGHAZ or ICHAZ). In the present study, the following micromechanics model is employed to determine the micromechanical and microstructural origins for the failure process above, accounting for the underlying physical fracture mechanisms at different length scales, including nucleation of grain boundary cavities, their growth by competition of grain boundary diffusion and grain interior creep, viscous grain boundary sliding, and the emergence of microcracks by coalescence and their evolution to the ultimate failure. The methodology demonstrates the capabilities in modeling the Type IV failure and providing quantitative creep rupture lifetime prediction which shows an excellent agreement with long-term creep experimental data for creep strength enhanced ferritic steels and their weldments. In particular, the drop-off in time to rupture at high temperatures and low stress levels in the creep rupture curves is quantitatively predicted, and the transition of failure mechanisms from creep-controlled to diffusion-controlled creep fracture mechanism is illustrated.

Deformation behavior at room temperature ranges of fine-grained Mg–Mn system alloys

Tensile properties and deformation behavior were investigated on Mg dilute binary alloys (Mg-0.1Bi and Mg-0.1Zr alloys) and Mg–Mn based ternary alloys (Mg-0.3Mn-0.1Bi and Mg-0.3Mn-0.1Zr alloys). These alloys were processed by hot extrusion to obtain fine-grained structures with an average grain size of ~1 μm. Microstructural observations showed that Mn element was segregated at grain boundaries in the ternary alloys. Precipitates consisting of the α-Mn and/or Mg3Bi2 phase were dispersed in the matrix in the alloys containing Mn and/or Bi elements. All the extruded alloys showed good tensile property of ductility. The elongation-to-failure in tension of Mg-0.3Mn-0.1Zr alloy was more than 100% even at a quasi-static strain rate of 1 × 10−3/s. The Mg-0.1Bi alloy exhibited the largest value of 250% among all in the present examined conditions. These values are superior to those of conventional wrought processed Mg alloys. These superior properties are found to be due to the contribution of grain boundary sliding (GBS) to deformation. Bi and Zr are alternative elements to Mn for use in binary alloys. On the other hand, in low strain rate regimes, a combination of Mn and such micro-alloying elements led to an increase in flow stress, but reduced the elongation-to-failure. This resulted from a high density of precipitates in the matrix, which became cavity nucleation sites. Moreover, non-equilibrium grain boundary structures, which induced atomistic strains, were also unlikely to promote GBS. The normalized stress vs. strain relations including the previous results in the temperature ranges of 298–423 K is found to be fitted by the well-known constitutive equation.

Combined crystal plasticity and grain boundary modeling of creep in ferritic-martensitic steels: I. Theory and implementation

This paper presents a physically-based microstructural model for creep rupture at 600 °C for Grade 91 steel. The model includes constitutive equations that reflect various observed phenomena in Grade 91, and it is incorporated into a mesoscale finite element model with explicit geometry for the prior austenite grains and grain boundaries. Creep within the grains is represented using crystal plasticity for dislocation motion and recovery along with linear viscous diffusional creep for point defect diffusion. The grain boundary models include physics-based models for cavity growth and nucleation that accurately capture tertiary creep and creep rupture. Simulations of creep at 100 MPa are performed, and the contribution of each mechanism is analyzed. The overarching goal is to gain a mechanistic understanding of the material to improve the prediction of creep rupture for long service lives in elevated temperature operating conditions. The creep response of the material at different stress levels, stress states, and temperatures is studied in Part 2 of this paper in order to determine the implications of the simulations on high temperature design practice. Furthermore, the second part explores the effect of triaxial stress states on the creep response and finds a transition from notch-strengthening behavior at high stress to notch-weakening behavior at lower stresses.

Effect of precipitate evolution on creep damage of reduced activation ferritic/martensitic steel friction stir welded joint

This paper reports on the precipitate evolution and its influence on the high temperature creep resistance of 9% Cr martensite ferritic steel FSW joint. The creep rupture time of the FSW joints welded by 400 rpm and 200 rpm are 859.29 h and 1173.8 h respectively, while that of BM is 3398 h. The creep fracture occurs in ICHAZ where a pronounced increase of coarse Laves phase is detected. Most of the Laves phase particles are large in size and mainly nucleate and grow rapidly at the micrograin boundaries adjoining the M23C6 particles where W, and other elements segregate. The large Laves phase particles will deteriorate the pinning effect of M23C6, weaken the solid solution strengthening effect and attribute to the increased cavity nucleation. In addition the size of Laves phase in ICHAZ is bigger than that in stir zone (SZ). These reasons explain why creep fracture occurs in the ICHAZ and the time to rupture will be shorter than base metal.

Kinematic synergies are recognizable in the walker-assisted gait of spinal cord injury patients.

Tensile properties and deformation behavior were investigated on Mg dilute binary alloys (Mg-0.1Bi and Mg-0.1Zr alloys) and Mg–Mn based ternary alloys (Mg-0.3Mn-0.1Bi and Mg-0.3Mn-0.1Zr alloys). These alloys were processed by hot extrusion to obtain fine-grained structures with an average grain size of ~1 μm. Microstructural observations showed that Mn element was segregated at grain boundaries in the ternary alloys. Precipitates consisting of the α-Mn and/or Mg3Bi2 phase were dispersed in the matrix in the alloys containing Mn and/or Bi elements. All the extruded alloys showed good tensile property of ductility. The elongation-to-failure in tension of Mg-0.3Mn-0.1Zr alloy was more than 100% even at a quasi-static strain rate of 1 × 10−3/s. The Mg-0.1Bi alloy exhibited the largest value of 250% among all in the present examined conditions. These values are superior to those of conventional wrought processed Mg alloys. These superior properties are found to be due to the contribution of grain boundary sliding (GBS) to deformation. Bi and Zr are alternative elements to Mn for use in binary alloys. On the other hand, in low strain rate regimes, a combination of Mn and such micro-alloying elements led to an increase in flow stress, but reduced the elongation-to-failure. This resulted from a high density of precipitates in the matrix, which became cavity nucleation sites. Moreover, non-equilibrium grain boundary structures, which induced atomistic strains, were also unlikely to promote GBS. The normalized stress vs. strain relations including the previous results in the temperature ranges of 298–423 K is found to be fitted by the well-known constitutive equation.

Is Iodine Oxidation with Hydrogen Peroxide Coupled with Nucleation Processes?

An intriguing step of the Bray–Liebhafsky oscillatory reaction, that is, iodine oxidation with hydrogen peroxide, was examined. This process is characterized by enormous stochasticity of reaction induction times in the presence of mild mixing. In the present investigations, it was found that even in the presence of mixing, stochasticity is seriously reduced with simple glass powder addition. Using transparent glass, standard deviations changed from 184.9 to 10.6 min for 19 °C, and from 128.2 to 1.9 min for 27 °C. Repeating experiments with amber glass further confirmed those results as standard deviations changed from the mentioned ones to 8.7 min for 19 °C and to 2.5 min for 27 °C. Inert glass particles enhanced heterogeneous nucleation of oxygen formed in chemical reactions. Together with the previous analysis of the involved kinetic barrier of the whole reaction, it is an additional strong evidence of a possible energetic coupling between nucleation of glass cavities and chemical reactions. Such proces...

Crack propagation pattern and trapping mechanism of rolling a rigid cylinder on a periodically structured surface

Recent experiments on the rolling of a cylinder with a poly-dimethylsiloxane (PDMS) film-terminated ridge-channel surface structure against a flat rigid plate have shown significant enhancement in rolling resistance. For rolling perpendicular to the ridges, the rolling resistance initially increases with the ridge spacing. Treating the trailing edge of the rolling interface as an opening crack, this increase has been explained quantitatively by a crack-trapping model. However, beyond a critical value of spacing, the rolling resistance reaches a maximum value and observations suggest that its subsequent decrease is due to two factors. First is the nucleation of cavities ahead of the opening interfacial crack. Second is the growth of these defects parallel to the ridge. These two phenomena limit and eventually attenuate the effect of crack trapping, the primary mechanism of rolling resistance enhancement. Cavitation of the interface is a critical phenomenon limiting rolling resistance, and its mechanism is modeled in this work. We have developed a finite element method (FEM) to simulate the rolling process. Specifically, we developed a special cohesive element and numerical scheme to study how cavities nucleate and grow during rolling. Our simulation captures qualitatively the key experimental observation that cavitation is controlled by ridge spacing. However, our numerical model under-predicts the rolling resistance enhancement due to finite cohesive zone size effects.

Effect of strain on cavity development during Al–Zn–Mg–Cu alloy superplastic flow

ABSTRACTCavitation behaviour has been investigated in an Al–Zn–Mg–Cu alloy with an average grain size of 10 µm during superplastic deformation. The superplastic tensile tests were interrupted at different true strains at 530°C and 3 × 10−4 s−1. The results showed that cavity nucleation occurred above a critical strain in the optimum loading condition. It was easy for cavities to form at the triple junction due to the stress concentration caused by cooperative grain boundary sliding. Since the tensile stress was higher in the middle of the sample, the cavities were arranged in a straight line parallel to the tensile axis in the centre of the sample. A more appropriate cavity growth equation considering the critical strain was proposed to describe the cavitation behaviour.

Combined effect of injected interstitials and He implantation, and cavities inside dislocation loops in high purity Fe-Cr alloys

High purity Fe-Cr alloys with Cr content ranging from 3 to 14 wt% were irradiated by self-ions at 500 °C in dual-beam mode up to 157 displacements per atom (dpa), 17 appm He/dpa. Transmission electron microscopy (TEM) on focused ion beam foils revealed the depth distribution of irradiation induced cavities and dislocation loops. A detailed quantitative analysis of cavity microstructure was performed on Fe14%Cr, while Fe(3, 5, 10, 12)%Cr were analysed qualitatively. A homogeneous distribution of small cavities were observed up to the damage peak. From surface to the damage peak, cavity number density increased almost linearly. The cavity microstructure changed drastically at and after the damage peak, where the size increased and number density decreased. Most notably, the microstructure consisted of a striking mixture of heterogeneously nucleated cavities inside dislocation loops and freely nucleated cavities in the matrix. Further, the depth-dependent void swelling increased continuously along the damage depth. This contrasts with most ion irradiation results where suppression of void swelling occurs adjacent to the damage peak due the injected interstitial effect. We hypothesize a plausible mechanism of the observed swelling variation based on a combined effect of the injected interstitials and helium implantation near the damage peak by comparing the results with those in pure iron irradiated under same conditions. This synergistic effect may develop cavity microstructures which do not necessarily reflect microstructures expected due to injected interstitial effect and can be easily wrongly interpreted. The depth dependent dislocation loop microstructure was studied qualitatively in Fe14%Cr sample. Up to the damage peak, a complex dense network of dislocations formed due to the interaction/impingement of the dislocation loops. Individually resolvable loops were only observed after the damage peak, where their association with cavities was noted. Helium also induced heterogeneous cavity nucleation on dislocations constituting a grain boundary and on pre-existing dislocation lines.

Effect of impurity antimony on the creep behavior of 2.25Cr-1Mo heat-resistant steel

Abstract The creep properties of 2.25Cr-1Mo steel samples, undoped and doped with Sb (0.05 wt%), are examined under different applied engineering stresses (150–400 MPa) and temperatures (773–873 K). The creep behavior follows a unified Monkman-Grant relation e m × t r 1.04 = 0.046 for both the undoped and Sb-doped steels, where e m is the minimum creep rate in h−1 and tr is the rupture time in h. Under the same experimental conditions, the rupture time of the undoped steel is much longer than that of the Sb-doped steel, and meanwhile the minimum creep rate of the former is apparently smaller. Moreover, the threshold stress for creep is much lower for the Sb-doped steel than for the undoped one. This indicates that the impurity element Sb severely lowers the high temperature creep resistance of 2.25Cr-1Mo steel. As shown by boundary microanalysis, there is apparent grain or subgrain boundary segregation of Sb, which could promote the boundary sliding and the nucleation of cavities or microcracks, thereby leading to the creep property deterioration of the steel.

Achieving excellent superplasticity of Mg-7Zn-5Gd-0.6Zr alloy at low temperature regime

Mg-7Zn-5Gd-0.6Zr (wt%) alloy strengthened with quasicrystal phase (I-Mg3Zn6Gd phase) is prepared through hot extrusion and subsequent heat treatments. The low temperature (range from 25 °C to 250 °C) superplastic deformation behavior of the as-extruded, aging treated (T5) and solution and aging treated (T6) alloys are investigated. The results reveal that a superior superplastic elongation of 863% is obtained at 250 °C and strain rate of 1.67 × 10−3 s−1 and the elongation of this alloy increases with the increasing tensile temperature. Detailed microstructural analyses show that I-Mg3Zn6Gd phase and W-Mg3Gd2Zn3 phase are crushed into small particles during extrusion. A high density of nanoscale I-phase precipitates after T5 treatment. Dynamic recrystallization occurs in as-extruded Mg-7Zn-5Gd-0.6Zr alloy. The T5-treated Mg-7Zn-5Gd-0.6Zr alloy shows a relatively weak basal texture intensity, a large number fraction of high angle boundaries and a very finer grain structure (3.01 μm). During superplastic deformation, the nanoscale I-phase is slightly elongated and the microstructure is still equiaxed grains. The superplastic mechanism of the alloy is grain boundary sliding (GBS) accommodated by dislocation movement and static recrystallization. The cavity nucleation at the nanoscale I-phase/α-Mg matrix boundaries or grain boundaries and the cavity stringer formation leads to final fracture.