Short crack initiation and growth at 600 °C in notched specimens of Inconel718

The natural initiation and growth of short cracks in Inconel®718 U-notch specimens has been studied at 600 °C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross-sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short-crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred micrometers in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life

Validating 3D two-parameter fracture mechanics models for structural integrity assessments

In-situ fracture tests were carried out on the I12 beamline at the Diamond Light Source. Four Al-Ti metal-matrix composites (MMCs), with two crack lengths, were studied to assess for the impact of in-plane constraint. Synchrotron X-ray computed tomography and synchrotron X-ray diffraction were used to measure total strain and elastic strain respectively. In this work, the measured elastic strains in the samples are detailed as a function of applied load and compared against those predicted from a 3D elastic-plastic finite element model. The modelled strains increased asymptotically towards the tip of the electro discharge machined notch. The experimental results do not highlight the same response, which is due to a combination of blunting and low experimental spatial resolution. Far field experimental and measured strain fields converged, notably in the test piece containing a long notch (a/W = 0.5) and higher levels of constraint

Measurement of strain evolution in overloaded roller bearings using time-of-flight neutron diffraction

Neutron diffraction is an established method for non-destructively characterising residual stress or observing in situ strain during external stimuli. Neutron based stroboscopic techniques have previously been introduced for measuring strains undergoing cyclic processes but have not been used for tribological applications. This work presents a novel approach for measuring the evolution of radial strain in a rotating bearing through part of the component's lifetime. A cylindrical roller bearing was pre-overloaded to increase the probability of damage within a reasonable experimental time and to help develop further understanding of the influence such events have on bearing life, notably for the application of wind turbine gearbox bearing failure. The stroboscopic neutron diffraction technique was successful in measuring time-resolved contact strain, with a significant increase in compressive radial strain being observed after a suspected failure had been detected using condition monitoring techniques, implemented for validating damage propagation. Cyclic contact strains associated with rolling contact fatigue were also evaluated using neutron diffraction

Development of EM-CCD-based X-ray detector for synchrotron applications

A high speed, low noise camera system for crystallography and X-ray imaging applications is developed and successfully demonstrated. By coupling an electron-multiplying (EM)-CCD to a 3:1 fibre-optic taper and a CsI(Tl) scintillator, it was possible to detect hard X-rays. This novel approach to hard X-ray imaging takes advantage of sub-electron equivalent readout noise performance at high pixel readout frequencies of EM-CCD detectors with the increase in the imaging area that is offered through the use of a fibre-optic taper. Compared with the industry state of the art, based on CCD camera systems, a high frame rate for a full-frame readout (50 ms) and a lower readout noise (<1 electron root mean square) across a range of X-ray energies (6–18 keV) were achieved

Understanding the highly dynamic phenomena in ultrasonic melt processing by ultrafast synchrotron x-ray imaging

In this paper, we present some highlighted findings from our recent research on real-time and in situ studies of the fundamentals of ultrasonic melt processing, including (1) ultrasonic bubble implosion, oscillation in liquid and semi-liquid (semi-solid) metals and their interactions with the growing solidifying phases; (2) enhanced acoustic metal flow and their impact on the liquid-solid metal interface. The real time experimental phenomena were interpreted with the aid of calculating the propagation of acoustic pressure in liquid metals using the Helmholtz equation and bubble wall pressure and velocity profile during bubble oscillation using the classical Gilmore model. The research provides unambiguous real-time evidence and robust theoretical interpretation in elucidating the dominant mechanisms of microstructure fragmentation and refinement in solidification under ultrasound

Data and videos for ultrafast synchrotron X-ray imaging studies of metal solidification under ultrasound.

The data presented in this article are related to the paper entitled 'Ultrafast synchrotron X-ray imaging studies of microstructure fragmentation in solidification under ultrasound' [Wang et al., Acta Mater. 144 (2018) 505-515]. This data article provides further supporting information and analytical methods, including the data from both experimental and numerical simulation, as well as the Matlab code for processing the X-ray images. Six videos constructed from the processed synchrotron X-ray images are also provided

In-situ observation of ultrasonic cavitation-induced fragmentation of the primary crystals formed in Al alloys

The cavitation-induced fragmentation of primary crystals formed in Al alloys were investigated for the first time by high-speed imaging using a novel experimental approach. Three representative primary crystal types, Al3Ti, Si and Al3V with different morphologies and mechanical properties were first extracted by deep etching of the corresponding Al alloys and then subjected to ultrasonic cavitation processing in distilled water. The dynamic interaction between the cavitation bubbles and primary crystals was imaged in-situ and in real time. Based on the recorded image sequences, the fragmentation mechanisms of primary crystals were studied. It was found that there are three major mechanisms by which the primary crystals were fragmented by cavitation bubbles. The first one was a slow process via fatigue-type failure. A cyclic pressure exerted by stationary pulsating bubbles caused the *Manuscript Click here to view linked References 2 propagation of a crack pre-existing in the primary crystal to a critical length which led to fragmentation. The second mechanism was a sudden process due to the collapse of bubbles in a passing cavitation cloud. The pressure produced upon the collapse of the cloud promoted rapid monotonic crack growth and fast fracture in the primary crystals. The third observed mechanism was normal bending fracture as a result of the high pressure arising from the collapse of a bubble cloud and the crack formation at the branch connection points of dendritic primary crystals. The fragmentation of dendrite branches due to the interaction between two freely moving dendritic primary crystals was also observed. A simplified fracture analysis of the observed phenomena was performed. The specific fragmentation mechanism for the primary crystals depended on their morphology and mechanical properties.The authors acknowledge the financial support from UK Engineering and Physical Science Research Council (EPSRC) for the Ultra-Cast project (grant EP/L019884/1, EP/L019825/1, EP/L019965/1). The authors are also grateful to the Diamond Light Source Ltd for the loan of the high speed camera system.UK Engineering and Physical Science Research Council (EPSRC) Ultra-Cast project (grant EP/L019884/1, EP/L019825/1, EP/L019965/1)