Boundary Triple Points Research Articles

Thermal faceting on the ductility-dip cracking fracture surfaces of nickel-based alloys – Occurrence, characterization, and implications for the cracking mechanism

Ductility-dip cracking (DDC) fracture surfaces containing thermal faceting (TF) are documented in this study. Research began with the discovery of TF on the DDC fracture surface in the weld metal of high-chromium nickel (Ni)-based filler metal 52M, a valuable alloy to the nuclear industry. Fracture surfaces were generated using strain-to-fracture (STF) and simulated strain ratcheting (SSR) testing on a Gleeble® thermomechanical simulator. Initial fractography performed by scanning electron microscopy (SEM) revealed faceting with preferred grain orientations and faceting interacting with void nucleation near grain boundary (GB) triple points. Internal voids were exposed via metallography, where TF was also observed on the free surface. A thin orientation- and site-specific foil was extracted from the faceted fracture surface using focused ion beam (FIB) nanofabrication. Subsequent scanning transmission electron microscopy (STEM) characterization down to atomic-resolution revealed the facets have perfect crystallographic orientations parallel to close-packed directions. Sharp edges and transitions between individual facets on the scale of individual atomic columns and atomic planes were observed. An analysis of the deformation microstructure and the chemical composition underneath the faceted surface showed a low dislocation density and no evidence of localized segregation or diffusion, respectively. These findings have led to the development of a hypothesis related to the interdependence of TF and DDC and support initial claims regarding TF formation in DDC voids at elevated temperatures.

Effect of Mn doping on the densification and properties of transparent alumina by high-pressure spark plasma sintering

It is common practice to control the densification and microstructure of alumina using additives or dopants. In the present study, the effect of Mn-doping on the sintering behavior, optical properties, microstructure, and mechanical properties of alumina was examined. Addition of 0.5 at.% Mn was shown to significantly reduce densification rate and grain growth during high-pressure spark plasma sintering (HPSPS). The major effects resulted from the formation of MnAl2O4 nano inclusions located at grain boundary triple points. The Mn-doped alumina was highly transparent and had a light-brown tint due to absorption between ∼410 and 580 nm and a cutoff at 300 nm. The MnAl2O4 nano-inclusions caused the Young's modulus to decrease from 400 ± 5 GPa for pure alumina to 370 ± 5 GPa for Mn-doped alumina, while the fracture toughness increased from 2.97 to 3.66 MPa m0.5, and the hardness was the same. Thus, the addition of Mn enables control of the transparent alumina densification process and microstructure development, and it can be used to tailor the optical and mechanical properties for specific applications.

The role of β pockets resulting from Fe impurities in hydride formation in titanium

The corrosion potential of commercially pure titanium in NaCl solutions is dramatically affected by trace Fe additions, which cause the appearance of submicron pockets of β phase at grain boundary triple points. Furthermore, the low solubility of hydrogen in hexagonal close-packed α-Ti makes titanium alloys prone to subsequent hydride-associated failures due to stress corrosion cracking. We analyzed α-α and α-β sections of the abutting grain boundary of a β pocket in a Grade 2 CP-Ti, and the α-β phase boundary. Fe and H partition to β and segregate at the grain boundary, but no segregation is seen at the α-β phase boundary. In contrast, a significant Ni (>1 at%) accumulation is observed at the α-β phase boundary. We propose that the β-pockets act as hydrogen traps and facilitate the nucleation and growth of hydrides along grain boundaries in CP-Ti.

Strain ratcheting limit stresses as a function of microstructure of WC-Co hardmetals under uniaxial cyclic loads under a stress ratio of R = −∞ at elevated temperatures

The aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = σmin/σmax on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = −∞ at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 μm and 2.0 μm, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value εres at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain.

Characterization of the Hot Deformation Behavior and Microstructure Evolution of a New γ-γ′ Strengthened Cobalt-Based Superalloy

Recently, cobalt-based γ-γ′ microstructured superalloys have attracted attention. However, studies on their processing behavior [i.e., processing maps (the variation of strain rate sensitivity (m) with temperature)] are limited. Thus, the high-temperature flow behavior of a γ-γ′ Co-30Ni-10Al-5Mo-2Ta-2Ti-5Cr (at. pct) superalloy was investigated using isothermal compression tests between 1348 and 1498 K at strain rates from 0.001 to 10 s−1. The m contour map was generated using the experimental flow stress values, which were used to locate the optimum hot workability and desired microstructural processing range. A strong dependence of m on the deformation parameters (temperature, strain rate, and strain) was observed. A maximum m value of around 0.3 at 1460 K to 1498 K and strain rates of 0.01 to 0.5 s−1 was found. The deformed samples show a fully recrystallized microstructure at high m. Unstable domains showed the formation of cavities at the grain boundary triple points and cracks along the grain boundaries at high strain rates (1 to 10 s−1), corresponding to m < 0.10. A constitutive model was developed using an Arrhenius hyperbolic sine function, yielding an apparent activation energy of 540 ± 30 kJ mol−1 for hot deformation. This study indicates reasonable formability under certain conditions below the solvus, thus opening possibilities for further thermomechanical treatment.

Effects of the phase interface on initial spallation damage nucleation and evolution in dual phase titanium alloy

The Ti-6Al-4V dual phase alloy samples were dynamic loaded by one-stage light gas gun experiment and samples with initial spallation were softly recovered. During the loading experiment, the velocity of free surface particles was measured by photonic doppler velocimetry (PDV). The effect of α/β phase interface on nucleation, growth, and coalescence of dynamic damage in Ti-6Al-4V were investigated by 2-D or 3-D testing techniques, such as optical microscopy(OM), x-ray computer tomography(XRCT), and electron backscattered diffraction(EBSD). The results showed that the majority of voids were nucleated within α phases, rather than on the α/β phase interface as predicted by quasi-static damage theory. Due to the effects of reflection and transmission of shock wave at the phase interface, a tensile pulse would be formed within α phase when the shock wave transmit from α phase with high impedance to β phase with low impedance. When this tensile pulse was large enough, voids would be formed within α phase. The analyses of OM and XRCT indicated that the voids at the beginning of nucleation were nearly spherical, then grew up along the direction of 45° with the shock loading direction, and finally the rod-shaped voids were formed. Besides, the voids were not randomly nucleated within α phase, and the EBSD analysis showed that the voids were mainly nucleated at grain boundary triple points which composed of grains with large difference of Taylor Factor(TF) value within α phase. This is because the difference of plastic deformation capacity of this grains is larger, and it is easier to produce stress concentration. Thus these sites became the prior nucleation position of voids.

Effect of loading conditions on nucleation of nano void and failure of nanocrystalline aluminum: An atomistic investigation

Deformation mechanism, nano void nucleation and failure of nano crystalline aluminum are investigated for uniaxial, plane strain and equi biaxial loading conditions using atomistic simulation technique. Nano deformation twin formation is noticed for all three loading conditions. Grain boundary triple point is the site of nano void nucleation and the fracture mode is intergranular type. Early nano void nucleation and failure are observed in plane strain loading condition in comparison with uniaxial and equi biaxial loading conditions.

An approach to lifetime prediction for a wrought Ni-base alloy under thermo-mechanical fatigue with various phase angles between temperature and mechanical strain

The damage and lifetime behaviour of Ni-base alloy NiCr22Co12Mo9 (comparable to Inconel Alloy 617) under thermo-mechanical fatigue (TMF) loading with varying phase angles between strain and temperature and optional dwell times at the maximum temperature was studied. Based on the results, a new lifetime prediction approach is proposed.Strain controlled TMF tests with a temperature range of 100–850°C were conducted in air. Phase angles between temperature and mechanical strain were 0° (in-phase, IP), 180° (out-of-phase, OP), +90° (clockwise diamond, CD) and −90° (counter clockwise diamond, CCD). In some tests, optional dwell times of 2, 5 or 30min were introduced at 850°C. The TMF-lifetime depends significantly on the phase angle and increases in the sequence IP<CCD<OP<CD. While for IP and CCD loading intergranular damage dominates, the damage in OP and CD tests is mainly transgranular. For all phase angles, wedge-type cracks at grain boundary triple points could be found. The orientation of these wedge type cracks is perpendicular to the loading axis for IP and CCD loading and parallel to the loading axis for OP and CD loading. This behaviour could be explained by phase angle dependent grain boundary sliding. Introducing dwell times leads to lower stress amplitudes and higher plastic strain amplitudes compared to tests without dwells. For OP and CD loading the two effects apparently compensate each other and the overall effect of dwell times on the lifetime is small. For IP and CCD loading, the intergranular damage is reduced due to lower tensile stress at high temperatures. That way, introducing dwells increases the IP and CCD lifetime. Based on the findings, a life prediction approach incorporating two separate damage parameters for intergranular and transgranular damage was developed. For loading cycles with arbitrary phase angles and dwells both parameters yield a lifetime. The parameter giving the shorter lifetime is used for lifetime prediction and also indicates the dominant damage mechanism. The approach allows for a satisfying life prediction within a wide range of testing conditions.

Deformation and strain localization in polycrystals with plastically heterogeneous grains

A model of a polycrystalline material is studied, where each grain consists of several zones with different plastic properties. The size and configuration of the zones as well as their properties are mimicking the situation inside the crystals of artificially aged Al alloys with precipitate free zones (PFZs). The properties of the different zones are conjectured based on micromechanical models and experimental observations of the AA6000 series of Al alloys. A periodic patch of such a composite material, subjected to plane-strain tension, is modelled using the finite element method. The material behaviour is described by a single crystal plasticity model. The results of the simulations show a number of characteristic features emerging in composite material systems, including sliding and distortion of grain boundaries and shear band formation at grain boundary triple points. The assumptions made about the properties of the different zones in the crystals are evaluated. The distributions of plastic strain and deviatoric and hydrostatic stresses in the composite crystalline systems are discussed.

An investigation on ductility-dip cracking in the base metal heat-affected zone of wrought nickel base alloys—part I: metallurgical effects and cracking mechanism

Ductility-dip cracking (DDC) in the heat-affected zone (HAZ) of NiCr15Fe-type alloys was studied using the programmable-deformation-crack (PVR) and the strain-to-fracture (STF) test. This paper concentrates on the cracking morphology and the effect of metallurgical factors for DDC susceptibility. The obtained results are discussed in the context of the underlying cracking mechanism. Evidence was found that DDC in the heat-affected base metal occurs due to a relative grain boundary movement, which induces high strain buildup at grain boundary triple points, subsequent cavity formation, and intergranular crack propagation. Base metal grain size and intergranular carbide precipitation are shown to have a significant effect on DDC susceptibility and are discussed in the context of the observed grain boundary sliding mechanism.

Simulation and quantification of creep damage

This paper highlights assessment of creep damage of an 11-year service-exposed reformer tube made up of HP40 grade of steel of a petrochemical industry from replicated creep data. The existence of scatter in the void characterization, creep deformation and rupture data leads to substantial amount of uncertainty in the assessment of creep damage. A discontinuous Markov process, which takes into account the associated scatter observed in damage evolution, has been used for creep damage assessment. The damage parameter A* is compared with the damage prediction by Markov process, where A* is derived as the cumulative contribution from microstructurally determined cavities at grain boundaries (A1) as well as number of creep cavities or voids at the grain boundary triple points (A2). This damage parameter (creep cavitation /voids) is in close agreement with the damage prediction curves at low stress whereas at high stresses it could correlate well till the midregion of the predicted curves, above which cavity measurement was practically not feasible. Also the value of damage tolerance parameter λ (ratio of rupture strain to Monkman–Grant constant) infers that growth of cavities has been attributed to a purely diffusion controlled mechanism, grain boundary sliding mechanism or a combination of two.

Influence of Co3O4 addition on the ionic conductivity and microstructural properties of yttria-stabilized zirconia (8YSZ)

Abstract In this study, 0 – 15 wt.% Co3O4 powders were added to 8 mol.% yttria-stabilized cubic zirconia (8YSZ) powders in order to determine the effect of Co3O4 addition and amount on the microstructure, sintering, and electrical properties of yttria-stabilized cubic zirconia. Colloidal processing was used to ensure uniform mixing of the powders and to obtain a homogeneous microstructure. X-ray diffraction results showed that the average lattice parameter of the yttria-stabilized cubic zirconia decreased from 5.146 Å to 5.135 Å upon Co3O4 addition, and that the lattice parameter did not change above a Co3O4 content of 1 wt.%. The initial decrease in the lattice parameter is attributable to the smaller ionic radius of Co3+ dissolved in the yttria-stabilized cubic zirconia matrix. The electrical conductivity of the specimens was measured using a frequency response analyzer in the range of 100 mHz – 13 MHz and 300 – 800 °C. Grain boundary conductivity of yttria-stabilized cubic zirconia was found to be enhanced with increasing Co3O4 addition, with the presence of electronically conductive cobalt oxide at grain boundary triple points increasing the grain boundary conductivity, due to an increase in the Co3+ and Co2+ electronic charge carrier ions.

Magnetic properties of cyclically deformed austenite

In meta-stable austenitic stainless steels, low cycle fatigue deformation is accompanied by a partial stress/strain-induced solid state phase transformation of paramagnetic γ(fcc) austenite phase to ferromagnetic α/(bcc) martensite. The measured characteristic of magnetic properties, which are the saturation magnetization, susceptibility, coercivity, retentivity, and the area under the magnetic hysteresis loop are sensitive to the total strain amplitude imposed and the corresponding material behaviour. The morphologies and nucleation characteristics of deformation induced martensites (i.e., ϵ(hcp), α/(bcc)) have been investigated through analytical transmission electron microscope. It has been observed that deformation induced martensites can nucleate at a number of sites (i.e., shear band intersections, isolated shear bands, shear band–grain boundary intersection, grain boundary triple points, etc.) through multiple transformation sequences: γ(fcc)→ϵ(hcp), γ(fcc)→ϵ(hcp)→α/(bcc), γ(fcc)→ deformation twin →α/(bcc) and γ(fcc)→α/(bcc).

Evaluation of Plastic Workability Limit of Magnesium Alloy by Cavitations in High Temperature Uni and Biaxial Test

Abstract. This study examined the critical plastic formability limit of a fine-structure AZ31 magnesium alloy plate under warm and high temperature based on the strength of a magnesium alloy that has cavities at room temperature. The cyclic hot free-forging process as pre-form working following rolling at a light reduction ratio fabricated a fine-structure AZ31 magnesium alloy plate. The appearance of the cavities was examined in detail together with changes in the structure and preparation methods before further damage at high temperatures with increasing uni-and biaxial plastic deformation. The allowable deformation limit in the super plasticity process can be estimated from the strength of the deformed material and forming limit diagram (FLD) at room temperature. During high-temperature deformation, cavities are produced by stress concentrations at grain boundary triple points and striation bands due to grain boundary sliding. The cavitations growth behavior is dependent upon deformation conditions, and a high percentage of large cavities occupy the sample surface as a large amount of grain boundary sliding is present, i.e., as uniform elongation grows larger, the cavity size also increases. In a case where 200% uniaxial strain was applied to a fine-grained structure material at a temperature of 623K under a strain rate of 10-4s-1, the tensile strength at room temperature decreased about 13%, and elongation was 10% less, compared with that of a material to which no load was applied due to the influence of cavities. In a case of biaxial deformation, the values were 28% lower. It is possible to draw a FLD based on the cavity incidence fraction .

Element-Specific Magnetic Domain Imaging of (Nd, Dy)-Fe-B Sintered Magnets Using Scanning Transmission X-Ray Microscopy

We demonstrate an element-specific observation of magnetic domains in thermally demagnetized Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets using scanning transmission X-ray microscopy (STXM). Clear chemical and magnetic contrast images with the 30-nm spatial resolution were taken by STXM. Both maze-like magnetic domains and stripe magnetic domains with their widths of 200-300 nm are observed in both Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets. In both sintered magnets, multidomain structures are mostly formed within each grain-that is, magnetic domains are likely to be terminated at the grain boundaries. Stripe domains are originated from the grains with the (001)-axis misoriented to the sample normal. From the comparison between chemical and magnetic images, it is found that no clear magnetic domain is observed in Nd-rich phase at grain boundary triple points. Furthermore, it is also found that the interface between Nd2Fe14B phase and Nd-rich phase is chemically abrupt. Similar magnetic domain patterns are observed in Nd-Fe-B and (Nd, Dy)-Fe-B sintered magnets.

Cavitation During Superplastic Forming.

Cavitation is the opening of pores during superplastic forming, typically at grain boundary triple points or on second phase grain boundary particles during slip of grain boundaries. Theories for the initiation of cavitation are reviewed. It seems that cavitation is unlikely to occur by processes intrinsic to metals such as dislocation mechanisms or point defect condensation. It is proposed that cavitation can only occur at non-bonded interfaces such as those introduced extrinsically (i.e., from the outside) during the original casting of the metal. These defects, known as oxide bifilms, are naturally introduced during pouring of the liquid metal, and are frozen into the solid, often pushed by dendritic growth into grain boundaries where they are difficult to detect because of their extreme thinness, often measured in nanometres. Their unbonded central interface acts as a crack and can initiate cavitation. Second phase precipitates probably do not nucleate and grow on grain boundaries but grow on bifilms in the boundaries, explaining the apparent association between boundaries, second phase particles and failure initiation. Improved melting and casting techniques can provide metal with reduced or zero bifilm population for which cavitation would not be possible, promising significant improvements in superplastic behaviour.

Microstructural characterization and electrical properties of spray pyrolyzed conventionally sintered or hot-pressed BaZrO 3 and BaZr 0.9Y 0.1O 3 − δ

A spray pyrolysis route to BaZrO 3 (BZ) and BaZr 0.9Y 0.1O 2.95 (BZY) powders was developed starting from nitrate solutions. Homogeneous powders with a grain size of ~ 100 nm were achieved. A calcination of the powder was necessary to remove carbonates formed during the spray pyrolysis. Hot pressing was in comparison with conventional sintering more effective to enhance densification and suppress grain growth, and dense (> 96%) materials with homogeneous microstructure were obtained. The Y-substitution decreased the densification rate. Minor amounts of a secondary phase was observed at the grain boundary triple points of BZY, but the grain boundaries were otherwise found to be coherent and without significant secondary phase accumulation. Impedance spectroscopy vs T, pO 2 and pH 2O of conventionally sintered BZ and hot-pressed BZY demonstrated that the conductivity of BZ was orders of magnitude lower than compared to BZY. The conductivity of BZ displayed mixed proton and p-type electronic conduction characteristics in the grain interior which was depressed at the grain boundaries. The grain boundaries showed an additional n-type electronic conduction under reducing conditions. The conductivity characteristics were according to core-space charge layer theory. BZ seems to exhibit a larger ratio of p-type electronic to protonic conduction as compared to BZY, contrary to the prediction of simple defect chemistry.

Multiscale strain measurements of plastically deforming polycrystalline titanium: Role of deformation heterogeneities

The purpose of this work was to characterize the spatial distribution of residual deformation at the mesoscale (a few grains) and at the macroscale (hundreds of grains) in titanium subjected to cyclic tensile loading. Using ex situ digital image correlation, we compared the axial residual strain fields obtained at optical magnifications ranging from 3.2× to 50×. To compare the results obtained at different optical magnifications, numerous images at higher magnification had to be assembled to encompass the same field-of-view observed at lower magnifications. The strain fields at the highest optical magnification revealed deformation patterns that were not detectable at lower magnifications. These deformation patterns appeared as inclined slip bands near grain boundaries and grain boundary triple points, with the bands sometimes crossing into neighboring grain interiors. Measurements made at optical magnifications greater than 10× captured an underlying deformation pattern, however, considerably more detail within grains was obtained at 50× magnification. The strain fields obtained at 10× and 50× magnifications were subsequently used to estimate the length scale of a representative volume element (RVE) based on the standard deviation of the average residual strain. The estimated RVE length scale was nearly three times the average grain diameter if extracted from the 50× results. The estimate of the RVE length scale was smaller at lower magnification which was due to a homogenizing effect caused by the low measurement resolution. Thus, care must be taken when experimentally obtaining RVE length scale estimates.

On the Nature and Stability of Phases Present in a Rapidly Solidified Aluminium Alloy 7010 Containing Scandium

The influence of Sc on the nature and stability of phases present in rapidly solidified (RS) Al alloy 7010 (hereafter termed base alloy) produced by melt spinning were examined and compared with the phases present in the same alloys when produced by the conventional ingot metallurgical (IM) route. Transmission electron microscopy (TEM) revealed that the second phases present in the IM alloys have an HCP structure but with varying lattice parameters. On the other hand, the second phase particles present in the base alloy when produced by rapid solidification (RS) route always have a quasi-crystalline, icosahedral structure. In the rapidly solidified 7010 + Sc alloy, however, the icosahedral phase is located only at the grain boundary triple point junctions, and the grain interfaces are populated by the second phase particles having an HCP structure. An analysis of the chemistry of the second phase particles revealed that the icosahedral phase is stabilized by certain minimum concentrations of Zn + Mg/Zn + Mg + Cu. The reason for the absence of the icosahedral phase at the grain interfaces of the rapidly solidified 7010 + Sc alloy is the partitioning of relatively lower concentrations of the major alloying elements into the second phases formed at those sites. Micro-hardness measurements revealed higher hardness values for the RS 7010 + Sc alloy in both as-melt spun as well as artificially aged conditions. These results are correlated with the microstructures developed in these materials.

Analysis of micro void formation at grain boundary triple points in monotonically strained AA6022-T43 sheet metal

This study reports on the micro voids formed in a 1 mm thick sheet of rolled aluminum alloy AA6022 subjected to monotonic tensile loading. In this research micro voids were found to occur at certain grain boundary triple point (GBTP) junctions. The orientations of the grains at various GBTP junctions were determined using an electron backscatter diffraction detector mounted in a field-emission scanning electron microscope. Because damage evolution and eventual final failure are driven by local tensile hydrostatic stresses arising from microhetereogenities, the hydrostatic stress state surrounding the GBTP junction was evaluated using a crystal plasticity material model implemented in a finite element analysis code. Grain orientations that result in a local hydrostatic state of tension surrounding the GBTP junction were found to have a tendency to form micro voids.