Final Rupture Research Articles

Premature fracture of high-strength suspension springs caused by corrosion fatigue cracking

In this work, the mechanisms responsible for the premature fatigue fracture of four suspension springs during road test were investigated. Failure analysis combined with finite element method (FEM) and stress corrosion cracking (SCC) experiments were conducted. The results indicate that the corrosion and wear damage occur on the failed spring surface caused by the stone chipping and acid rain. The unexpected failure of suspension springs was caused by the presence of surface cracks formed by the corrosion fatigue cracking (CFC) mechanism. The fatigue strength of spring reduces to as low as 420 MPa due to the presence of these surface cracks, which is much lower than the service stress. As a result, the fatigue crack propagates quickly without the help of corrosion and leading to the final rupture.

Abnormal three-stage plastic deformation in a 17-4 PH stainless steel fabricated by laser powder bed fusion

A 17-4 PH stainless steel has been additively manufactured (AM) using laser powder bed fusion (LPBF). Different from traditional counterpart, the AM 17-4 PH steel exhibits a peculiar dual-phase microstructure containing large amount of retained austenite (66 vol%) and the rest martensite. This metastable structure is highly susceptible to surface polishing, thus widely scattered data of phase constitution are reported in many previous papers. It is found that the thermal martensite transformation is suppressed by the ultra-fine microstructure produced by LPBF, thus giving rise to a large amount of austenite retained. And as a consequence, plastic deformation of the AM 17-4 steel displays an obvious three-stage behavior: (I) yield plateau, (II) severe work hardening, and (III) final necking. Underlying mechanisms are revealed, respectively. At the beginning of plastic deformation, because there is lacking of further strengthening mechanism upon yielding in the fine-grained structure, a plastic instability caused by local softening occurs and leads to the formation of complex Lüders band. When the accumulated mechanical energy plus the intrinsic chemical energy exceeds activation energy of martensite transformation, further plastic deformation results in severe strain-induced martensite transformation (SIMT). The SIMT endows the AM 17-4 steel with extremely high work hardening ability in stage II, consequently, flow stress rises sharply from 490 MPa to the maximum level of 1090 MPa, and most of the retained austenite is exhausted in this stage. Afterward, the work hardening rate drops below the true stress, an increase in strain produces no additional load increase, local necking occurs and leads to final rupture of the AM 17-4 steel.

Lower expression of NINJ1 (Ninjurin 1), a mediator of plasma membrane rupture, is associated with advanced disease and worse prognosis in serous ovarian cancer.

Gasdermin proteins (GSDMs) form pores in cell membranes upon various stimuli, leading to the release of certain proinflammatory molecules such as IL-1β and IL-18, and this ultimately results in pyroptotic cell death. NINJ1 (Ninjurin 1) has recently been identified as a cell membrane protein responsible for the final complete plasma membrane rupture following lytic cell death mechanisms including pyroptosis, causing the release of relatively larger molecules such as HMGB1 and LDH. In this study, we reported the presence of higher GSDMD and lower GSDME protein levels in ovarian tumors compared to surrounding non-malignant stromain the tumor microenvironment. GSDME protein levels are also lower in the tumorsof theomentum compared to adjacentstromal cells.We found that NINJ1 expression decreases from early to late stage in serous ovarian cancer, and the percentage of NINJ1 copy number loss events is the highest in ovarian cancer among other cancers. Moreover, we showed that low expression of NINJ1 is associated with shorter overall survival of patients with ovarian cancer. In support of the findings showing that low NINJ1 expression contributes to worse prognosis in this most lethal gynecological malignancy, NINJ1 expression was found to be lower in cisplatin-resistant ovarian cancer cells compared to cisplatin-sensitive counterparts in vitro. We suggest that the members of gasdermin family might have distinct functions in serous ovarian cancer, and low levels of NINJ1 might contribute, at least in part, to the progression and poorer prognosis of ovarian cancer. A complete picture of how pyroptosis and subsequent plasma membrane rupture are involved in ovarian cancer will be of high importance in order to identify actionable therapeutic vulnerabilities within this newly identified group of proteins.

Mechanisms ruling the lifetimes of films of liquid mixtures

Foams and bubbles formed in liquid mixtures have lifetimes longer by several orders of magnitude than in pure liquids of similar viscosities. We have shown recently that this effect results from slight differences in molecular concentrations between bulk and surfaces, which give rise to a thickness-dependent surface tension of liquid films. We present a quantitative description of the enhanced stability of liquid films in binary mixtures, based on experimental data and theoretical analysis. Experiments were performed with mixtures of different natures and compositions: foams of stationary heights were obtained by continuous injection of gas, on one hand, and single bubbles were swollen under the surface of a liquid bath, on the other hand. Remarkably, the lifetimes measured in both experiments exhibit the same variations with mixture composition, and follow a power law with the microscopic length that characterises the amplitude of the thickness-dependent surface tension. In addition, the lifetimes vary with the squared film thicknesses at the onset of bursting. We suggest that two stages occur between the birth of liquid films and their rupture. We show how the thickness-dependent surface tension allows an equilibrium shape to be reached at the end of a first stretching stage. We give an analytical description of this shape, which is fully consistent with experimental findings. We suggest a possible mechanism for final rupture of the film, and discuss it in light of existing theoretical predictions. Finally, we compare the properties of liquid mixtures and surfactant solutions, and in particular their surface rheology.

Phase-field modeling of fatigue crack growth during tooth flank fracture in case-hardened spur gears

Tooth flank fracture (TFF) is a fatigue failure mode in gears in which the crack is initiated in a larger depth below the surface within the tooth volume. The crack propagates load-dependently and is not visually observable until final rupture of the tooth. The present publication focuses on numerical fatigue crack simulation of TFF in case-hardened spur gears. Herein, an existing phase-field model for fatigue fracture is revisited and extended to account for the simultaneous subsurface crack initiation and growth in high cycle fatigue during TFF. For this purpose, a gear tooth finite element model is implemented and parameterized using experimental data. Inhomogeneous material properties and residual stresses resulting from the manufacturing process are explicitly considered. The main interest of the study lies in the ability of the phase-field model to reproduce experimentally observable behavior during TFF such as subsurface crack initiation and the emerging subsurface crack path.

The aspect of the corrosion pitting with fretting fatigue on Aluminum Alloy: A nuclear reactor safety or an aerospace structural failure phenomenon

Since wear and other surface decaying processes as Stress Corrosion Cracking (SCC) and Gridto-rod fretting failure (due to fuel rod vibration) can risk the lifespan of nuclear reactors, surface modification and other life degradation reducing attempts are being adopted for Aluminum alloys in such sophisticated applications like nuclear or aerospace components. In this research work, fretting fatigue and artificial pitting effect on heat-treated Al-Mg-Si alloy (Al 6061-T6) have been studied. Experimental observations have been authenticated by developing a FEM (Finite Element Method) model using simulation software ANSYS. Fretting causes a noticeable reduction in fatigue life at high cycle fatigue tests. However, it is difficult to differentiate between fretting and normal fatigue curve lines at low cycle fatigue. The fracture surface schematic shows how fretting affects crack initiation to final rupture. Interestingly, it has been found that pitting reduces the lifetime of Aluminum more abruptly than normal and even fretting fatigue. This study suggests more concentration on pitting by corrosion or others as well as geometry irregularities than the fretting aspect comparatively.

C27 SHOCKWAVE DEFERRED USE IN A CASE OF INFERIOR STEMI

Abstract A 62–years–old female patient, smoker, hypertension, dyslipidemia and TIA in history, was admitted for inferior STEMI; urgent coronary angiography from radial access was done, revealing diffuse calcific coronary artery disease with thrombotic occlusion of the proximal right coronary artery. Primary PCI was done with a direct implantation of a 3,0/24 mm drug–eluting stent. The stent expansion was not optimal in the mid–portion, due to severe calcification, requiring then post–dilatations. Nevertheless, the use of several non–compliant balloons (3,0/12 mm, 3,0/8 mm, 3,0/6 mm, 3,25/12 mm) was not effective to fully expand the stent, even dilating at very high pressure (rupturing one of the balloon). A ventricular fibrillation episode complicated the procedure, promptly resolved. After the stenting, even if not fully expanded, TIMI 3 flow was gained, with ST segment elevation resolution, in a quite asymptomatic patient; no more devices were available, so the procedure was stopped and the patient transferred in the coronary care unite. A second procedure was scheduled with the support of the intravascular lithotripsy device (Shockwave). Through the radial access the second PCI was done, using at first the Shockwave balloon 2,5/12 mm, with its total 8 treatments; then the Shockwave balloon 3,0/12 mm was used, effective at the seventh treatment in the final rupture of the calcific lesion, obtaining the full expansion of the stent. A good final angiographic result was documented, the patient tolerated very well the procedure.

Experimental study on the effect of dwell-fatigue life of titanium alloy for pressure hull

Titanium alloy is widely applied in the pressure hull of the manned submersible for its good mechanical properties. The operations of the manned submersible include diving, cruising and ascent. The load spectrum of the pressure hull is a typical dwell-fatigue process. The dwell effect is known to induce a reduction in the fatigue life of titanium alloys at room-temperature. The dwell fatigue of the new titanium alloy is the key issue to ensure a safe submersible operation. In this paper, the dwell-fatigue behavior of a new titanium alloy for pressure hull is investigated. It is found that the accumulated strain at the final rupture increases with the increase of dwell time, and the total cycle decreases. The total cycle life of the specimen decreases, and the accumulated strain increases with increasing peak stress level. The introduction of the dwell time at the peak stress will significantly reduce the dwell-fatigue life and the mechanism is due to the accumulation of the plastic strain.

Three-dimensional progressive damage and failure analysis of double-lap composite bolted joints under quasi-static tensile loading

Bolted joints are the dominant connection method in assembling composite structures. To investigate the failure mechanism of composite bolted joints under tensile loading, a three-dimensional progressive damage model for composite bolted joints was developed and implemented using the subroutine UMAT in Abaqus/Standard. This model considered several significant damage phenomena, such as the matrix crack orientation, the closure effect of matrix crack, and the longitudinal compressive responses of failed material under transversal constraints in the crush zone. The model utilized Hashin criterion for fiber fracture and Mohr–Coulomb based criterion for matrix cracking prediction separately. For validation of the model, a composite double-lap single-bolt joint configuration was adopted. The simulation results show high accuracy and precision compared with experimental results from the literature concerning inflection load, failure load, load–displacement response, and failure modes. Such a high-fidelity progressive damage model can overcome the inherent limitations of the experimental method. The bearing damage onset and propagation in the distinct plies and interfaces were investigated in detail at different load levels. It was found that matrix cracking initiates the joint damage onset and triggers fiber fracture. The initiation of interface delamination was identified as the mechanism by which the load–displacement response becomes nonlinear. The study revealed that matrix cracking is the dominant failure mode and induces the final rupture of the joints.

Anisotropic Composition and Mechanical Behavior of a Natural Thin-Walled Composite: Eagle Feather Shaft

Flight feather shafts are outstanding bioinspiration templates due to their unique light weight and their stiff and strong characteristics. As a thin wall of a natural composite beam, the keratinous cortex has evolved anisotropic features to support flight. Here, the anisotropic keratin composition, tensile response, dynamic properties of the cortex, and fracture behaviors of the shafts are clarified. The analysis of Fourier transform infrared (FTIR) spectra indicates that the protein composition of calamus cortex is almost homogeneous. In the middle and distal shafts (rachis), the content of the hydrogen bonds (HBs) and side-chain is the highest within the dorsal cortex and is consistently lower within the lateral wall. The tensile responses, including the properties and dominant damage pattern, are correlated with keratin composition and fiber orientation in the cortex. As for dynamic properties, the storage modulus and damping of the cortex are also anisotropic, corresponding to variation in protein composition and fibrous structure. The fracture behaviors of bent shafts include matrix breakage, fiber dissociation and fiber rupture on compressive dorsal cortex. To clarify, ‘real-time’ damage behaviors, and an integrated analysis between AE signals and fracture morphologies, are performed, indicating that calamus failure results from a straight buckling crack and final fiber rupture. Moreover, in the dorsal and lateral walls of rachis, the matrix breakage initially occurs, and then the propagation of the crack is restrained by ‘ligament-like’ fiber bundles and cross fiber, respectively. Subsequently, the further matrix breakage, interface dissociation and induced fiber rupture in the dorsal cortex result in the final failure.

Monitoring damage evolution with acoustic emission in two types of glass epoxy laminates

This study concerns the identification and monitoring by acoustic emission (AE) of the different damage mechanisms leading to the final rupture of glass/epoxy laminates. The series of specimens are subjected to static and fatigue loading in order to follow the progression of the various damage mechanisms over time and to identify the most critical ones leading to material failure. The specimens are made with two stacking sequences ([0/90]s and [90/0]s) using the vacuum infusion process. Several parameters such as the energy, the amplitude, the rise-time, the counts and the duration are used for the AE technique in order to identify the damage mechanisms evolutions to classify the collected events (AE). The damage mechanisms recorded in each stacking sequence are identified and characterized using a multi-parametric analysis. The damage accumulated in the test specimen is first of all transverse cracking, followed by interlaminar decohesion and finally fiber breakage leading to total composite failure.

Fracture surface formation of notched 2017A-T4 aluminium alloy under bending fatigue

The effect of cyclic loading on facture surface topology in notched components made by aluminium alloys is not completely clear. Fractography and fracture mechanics can help to understand this interdependency. This paper aims to study the distribution of the fracture surface roughness of notched 2017A-T4 aluminium alloy after bending fatigue using an optical focus-variation surface measurement technique by applying the fracture zone concept. The effects of stress level at the notch root and the load ratio on fatigue crack growth and fracture surfaces are analysed. Profile and areal surface parameters of four fracture surface regions were investigated at incremental crack lengths of the specimens. Studies have confirmed that the surface areas associated with the main stages of fatigue phenomenon (i.e. crack initiation, crack propagation, and final rupture) have significant differences in roughness which can be explained by the different loading scenarios. Overall, fatigue fracture surfaces have smallest roughness values at the crack initiation stage and a gradual increase during the fatigue crack growth stage.

Tensile mechanical characteristics of ultra-thin carbon sulfur nanothreads in orientational order

Carbon sulfur nanothreads (CSNTs) mainly composed of two chiral long alkane chains have been recently fabricated from thiophene by a pressure-induced phase transition in low-temperature, but their mechanical properties remain unexplored. Here, the critical roles of morphology and temperature on the tensile characteristics of CSNTs are for the first time examined using molecular dynamic simulations with a first-principles-based ReaxFF forcefield. It is revealed that CSNTs exhibit high tensile Young's modulus, high tensile strength and excellent ductility, and their tensile properties are morphology and temperature dependent. Morphologically, atomic arrangement with various configurations makes every CSNTs possess unique mechanical properties. Thermally, as temperature varies from 1 to 1500 K, CSNTs become mechanically weakened. In comparison with conventional diamond nanothreads (DNTs) and carbon nitride nanothreads (CNNTs), CSNTs show distinct axial elongation mechanisms, with relatively insignificant changes in chemical bond orders and bond length in the skeleton prior to the final rupture. Instead, the stretching of bond angle and dihedral angle mainly contribute to the global axial elongation, while the torsional deformation is limited due to their perfect global symmetry in the configuration. This study provides fundamental insights into the mechanics of ultra-thin CSNT structures.

FinDerS(+): Real-Time Earthquake Slip Profiles and Magnitudes Estimated from Backprojected Displacement with Consideration of Fault Source Maturity Gradient

The Finite-Fault Rupture Detector (FinDer) algorithm computes rapid line-source rupture models from high-frequency seismic acceleration amplitudes (PGA). In this paper, we propose two extensions to FinDer, called FinDerS and FinDerS+, which have the advantage of taking into account a geological property of the source fault, its structural maturity, as well as its relation to the earthquake slip distribution. These two new algorithms calculate real-time earthquake slip profiles by backprojecting seismic and/or geodetic displacement amplitudes onto the FinDer line-source. This backprojection is based on a general empirical equation established in previous work that relates dynamic peak ground displacement (PGD) at the stations to on-fault coseismic slip. While FinDerS projects PGD onto the current FinDer line-source, FinDerS+ allows the rupture to grow beyond the current model extent to predict future rupture evolution. For an informed interpolation and smoothing of the estimated slip values, FinDerS and FinDerS+ both employ a generic empirical function that has been shown to relate the along-strike gradient of structural maturity of the ruptured fault, the earthquake slip distribution, and the rupture length. Therefore, while FinDer derives magnitudes from a relatively uncertain and general empirical rupture length-magnitude relations, FinDerS and FinDerS+ provide alternate and better informed magnitude estimates using the mean slip of the profiles derived from the integration of fault source maturity. The two new algorithms can incorporate both seismic strong-motion and geodetic displacement data. In order to recover PGD from strong-motion instruments, we double-integrate and high-pass filter (>0.075 Hz) the seismic acceleration records. Together, the three algorithms exploit the full spectrum of ground-motions, including high frequencies to derive a source fault model (FinDer) and low frequencies to determine the static offsets along this model (FinDerS and FinDerS+). We test the three algorithms for the 2019 MW7.1 Ridgecrest (California), 2016 MW7.0 Kumamoto (Japan), and 2008 MW7.9 Wenchuan (China) earthquakes. Conclusively, low-frequency PGD data and integration of the fault maturity gradient do not speed-up calculations for these events, but provide additional information on slip distribution and final rupture length, as well as alternative estimates of magnitudes that can be useful to check for consistency across the algorithm suite. The FinDer algorithms systematically outperform previously established real-time PGD-based magnitude estimates in terms of speed and accuracy. The resulting slip distributions can be useful for improved ground-motion prediction given the observed relationship between seismic radiation and fault maturity.

The onset of seafloor spreading at the northeastern continent-ocean boundary of the South China Sea

The transition from continental to oceanic crust and the initial formation of typical oceanic crust at passive rifted margin are still poorly understood. In this study, deep crustal structures in the transitional domain of the northeastern South China Sea (SCS) margin are investigated and studied according to newly-acquired high-resolution reflection seismic data and gravity modeling. Three strong reflectors (R1, R2 and R3) are imaged in the narrow transitional domain. About 1 s in the two-way travel time (TWTT) beneath the top basement, R1 shows a similar morphology to the top basement, and quite likely represents a partial serpentinization interface. The presence of R1 suggests a tectonically dominated extensional stage prior to the final lithospheric rupture of the SCS. R2 is from magmatic intrusions or plutons (sills or dikes) from decompression melting. R3 is observed at ~2.5–2.2 s TWTT beneath the top basement but shoals seawards. R3 likely resulted from magmatic relicts in the paleo-melt transfer channels and/or from frozen molten materials in a shear zone of the lithospheric and asthenospheric mantle. The occurrence of R3 indicates the onset of a robust seafloor spreading system. The gradual seaward increase in magmatic activity highlights that, towards the final lithospheric breakup, tectonic extension was overtaken by magmatism. Increased magmatism and partial serpentinization formed a narrow hybrid transitional crust. Nevertheless, the breakup was not an instantaneous event. The last-stage transient magmatism occurred in a very narrow zone and does not rule out that the margin was predominantly amagmatic during the rifting and breakup. Based on reflection seismic and gravity modeling, we propose that the onset of typical seafloor spreading occurred at about 30 Ma around the study area, and the continent-ocean boundary (COB) should be located further seawards in the northeastern SCS margin.

Microstructure and fatigue behaviour of spin-arc welded AISI C1018 low carbon steel

This work investigates the fatigue performance of 10 mm AISI C1018 low carbon steel plates welded with ER70S-6 using a gas metal arc welding-based spin-arc welding process. Welded joint microstructure is characterized by bainite, acicular ferrite, and allotriomorphic ferrite along with pearlite in the ferritic matrix. The tensile strength of the weld metal was comparable with base metal and meets the mechanical property requirements in accordance with the ASTM A311/A311M-04 (2020) standard. The fatigue strengths of base metal and weld metal are 121 and 126 MPa, respectively, after sustaining 106 cycles. During cyclic loading, fracture surfaces were distinctly noticed as the crack initiation, crack propagation, and final rupture regions. The decrease in alternating stress increased the fatigue cycles to final rupture, and the nature of fatigue fracture was ductile with dimples and voids.

Longevity of small-scale (‘baby’) plumes and their role in lithospheric break-up

SUMMARYControversy between advocates of ‘active’ (plume-activated) versus ‘passive’ (driven by external tectonic stresses) modes of continental rifting and break-up has persisted for decades. To a large extent, inconsistencies between observations and models are rooted in the conceptual model of plumes as voluminous upwellings of hot material sourced from the deep mantle. Such large-scale plumes are expected to induce intensive magmatism and topographic uplift, thereby triggering rifting. In this case of an ‘active’ rifting-to-break-up system, emplacement of plume-related magmatism should precede the onset of rifting that is not observed in many rifted continental margins, thus providing a primary argument in favour of an antiplume origin for continental break-up and supercontinent fragmentation. However, mantle plumes are not restricted to whole-mantle (‘primary’) plumes emanating from the mantle-core boundary but also include ‘secondary’ plumes originating from the upper mantle transition zone or shallower. Over the last decades a number of such ‘secondary’ plumes with horizontal diameters of only ∼100–200 km (therefore, sometimes also called ‘baby’ plumes) have been imaged in the upper mantle below Europe and China. The longevity of such small-scale plumes and their impact on geodynamics of continental break-up have so far not been explored. We present results of a systematic parametrical analysis of relatively small thermal anomalies seeded at the base of the lithosphere. In particular, we explore the effects of variations in initial plume temperature (T = 1500–1700 °C) and size (diameter of 80–116 km), characteristics of the overlying lithosphere (e.g. ‘Cratonic’, ‘Variscan’, ‘Mesozoic’ and oceanic) and intraplate tectonic regimes (neutral or far-field extension of 2–10 mm yr–1). In tectonically neutral regimes, the expected decay time of a seismically detectable ‘baby’-plume varies from ∼20 to >200 Myr and is mainly controlled by its initial size and temperature, whereas the effect of variations in the thermotectonic age of the overlying lithosphere is modest. These small but enduring plumes are able to trigger localized rifting and subsequent continental break-up occurring from ∼10 to >300 Myr after the onset of far-field extension. Regardless of the thermomechanical structure of the lithosphere, relatively rapid (tens of Myr) break-up (observed in models with a hot plume and fast extension) favours partial melting of plume material. In contrast, in the case of a long-lasting (a few hundreds of Myr) pre-break-up phase (relatively cold plume, low extension rate), rifting is accompanied by modest decompressional melting of only ‘normal’ sublithospheric mantle. On the basis of the models presented, we distinguish two additional modes of continental rifting and break-up: (1) ‘semi-active’ when syn-break-up magmatism is carrying geochemical signatures of the deep mantle with deformation localized above the plume head not anymore connected by its tail to the original source of hot material and (2) ‘semi-passive’ when the site of final lithospheric rupture is controlled by a thermal anomaly of plume origin but without invoking its syn-break-up melting. These intermediate mechanisms are applicable to several segments of the passive continental margins formed during Pangea fragmentation.

Rupture Passing Probabilities at Fault Bends and Steps, with Application to Rupture Length Probabilities for Earthquake Early Warning

ABSTRACTEarthquake early warning (EEW) systems can quickly identify the beginning of a significant earthquake rupture, but the first seconds of seismic data have not been found to predict the final rupture length. We present two approaches for estimating probabilities of rupture length given the rupture initiation from an EEW system. In the first approach, bends and steps on the fault are interpreted as physical mechanisms for rupture arrest. Arrest probability relations are developed from empirical observations and depend on bend angle and step size. Probability of arrest compounds serially with increasing rupture length as bends or steps are encountered. In the second approach, time-independent rates among ruptures from the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), are interpreted to apply to the time-dependent condition in which rupture grows from a known starting point. Length probabilities from a Gutenberg–Richter magnitude–frequency relation provide a reference of comparison. We illustrate the new approach using the discretized fault model for California developed for UCERF3. For the case of rupture initiating on the southeast end of the San Andreas fault we find the geometric complexity of the Mill Creek section impedes most ruptures, and only ∼5% are predicted to reach to San Bernardino on the eastern edge of the greater Los Angeles region. Conditional probabilities of length can be precompiled in this manner for any initiation point on the fault system and thus are of potential value in seismic hazard and EEW applications.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.

Innovative Densification Process of a Fe-Cr-C Powder Metallurgy Steel

In this study, the efficacy of an innovative ultra-fast sintering technique called electro-sinter-forging (ESF) was evaluated in the densification of Fe-Cr-C steel. Although ESF proved to be effective in densifying several different metallic materials and composites, it has not yet been applied to powder metallurgy Fe-Cr-C steels. Pre-alloyed Astaloy CrM powders have been ad-mixed with either graphite or graphene and then processed by ESF. By properly tuning the process parameters, final densities higher than 99% were obtained. Mechanical properties such as hardness and transverse rupture strength (TRS) were tested on samples produced by employing different process parameters and then submitted to different post-treatments (machining, heat treatment). A final transverse rupture strength up to 1340 ± 147 MPa was achieved after heat treatment, corresponding to a hardness of 852 ± 41 HV. The experimental characterization highlighted that porosity is the main factor affecting the samples’ mechanical resistance, correlating linearly with the transverse rupture strength. Conversely, it is not possible to establish a similar interdependency between hardness and mechanical resistance, since porosity has a higher effect on the final properties.