Browsing by Author "Martinez, Fabian Duarte"

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    Interaction of stress corrosion cracks in single crystals Ni-Base superalloys
    (Elsevier, 2024-02-02) Elsherkisi, Mustafa; Martinez, Fabian Duarte; Mason-Flucke, Julian; Gray, Simon; Castelluccio, Gustavo M.
    Stress corrosion cracking (SCC) can be detrimental to nickel-based superalloy components exposed to harsh environments in aero-gas turbines. During flight, engines consume contaminants deposited on the surface of a blade, often leading to degradation. Cracking can initiate within minutes and rapidly propagate, depending on the temperature, contaminants, and applied stress. This study investigated the interaction between cracks in single-crystal turbine blades at intermediate temperatures by integrating experimental and computational methods. We performed C-Ring tests to quantify the time required for cracking, along with microscopic characterisation of the damage. In parallel, we developed a finite-element simulation for C-Ring tests using a phase field model calibrated to match the location of the cracks. The results demonstrated that the crack's characteristic spacing and length determine the likelihood of shielding or coalescing mechanisms.
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    Investigation into the effects of salt chemistry and SO2 on the crack initiation of CMSX-4 in static loading conditions
    (Springer, 2020-08-29) Martinez, Fabian Duarte; Morar, Nicolau I.; Kothari, Maadhav; Gibson, G.; Leggett, J.; Mason-Flucke, J. C.; Nicholls, John R.; Castelluccio, Gustavo M.; Gray, Simon
    Although evidence exists of the potential impact of stress, co-incident with corrosive environments at high temperature, for single crystal turbine blades, the mechanism responsible is not fully understood. This work explores the effect of CaSO4, Na2SO4 and sea salt on the scale formation and crack initiation of CMSX-4 at 550°C in 50 ppm of SO2 and synthetic air under a static stress of 800 MPa. The cross-sectional analysis showed that the CaSO4 and the Na2SO4 salted specimens did not undergo a significant degree of corrosion degradation and no cracks were detected after 400 hours of exposure. However, sea salt caused significant degradation to the scale and cracks were detected by X-ray CT scanning after 400 hours of exposure. The findings from this study suggests that the sulfation of chlorine containing species in sea salt led to the formation, vaporisation and re-oxidation of metal chlorides and this mechanism was found to play a key role in the formation of a non-protective scale. An active oxidation mechanism has been proposed to interpret the results. In conclusion, it is hypothesized that due to the synergistic effect of stress and the formation of a non-protective scale, fast diffusion paths for sulfur, oxygen and chlorine ingress were formed. Further work is currently being undertaken to understand the effect of these species on the local embrittlement of CMSX-4 that ultimately led to the initiation of cracks in the specimen.
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    Performance comparison between isothermal hot corrosion and in situ cyclic hot corrosion of nickel-based superalloys
    (Springer, 2021-06-28) Syed, Adnan. U.; Martinez, Fabian Duarte; Roberts, Tracey; Encinas-Oropesa, Adriana; Morar, Nicolau I.; Grohne, Marlene; Frommherz, Martin; Nicholls, John R.; Gray, Simon
    Although a lot of work has been done to understand both major mechanisms of hot corrosion, namely type I (high-temperature hot corrosion) and type II (low temperature hot corrosion), there is very little information available on more representative cyclic performance in these regimes. This work addresses this by assessing the performance of isothermal (type I and type II) hot corrosion tests against combined (short and long) cyclic corrosion tests. Single-crystal alloy PWA 1484 and directionally solidified alloy MAR-M247 were assessed in all test regimes. Pre- and post-exposure dimensional metrology was used to quantify the corrosion damage and characterised using SEM/EDX. This paper highlights that the results of short cycle test conditions are more damaging compared to long cycle and standard isothermal type I and II test conditions. The cast nickel-based alloy MAR-M247 was found to be a better performer compared to PWA 1484 single-crystal alloy.
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    Understanding nanoscale material behaviour for improved precision machining of shape memory alloys; testbed study on elliptical vibration assisted cutting of CuZr SMA.
    (2018-01) Martinez, Fabian Duarte; Goel, Saurav; Georgarakis, Kostas
    The field of ultra-precision machining has gained significant importance in the manufacture of components for the electronic, optical and medical industry. Two crucial factors that play a key role in the machinability of materials are the machining parameters and the material’s physical properties. Certain materials such as hardened steel or nickel-based superalloys are difficult-to-machine but innovations in the field of precision machining have developed a technique known as elliptical vibration assisted machining, which enables to improve the machinability of these materials. CuZr high-temperature shape memory alloy is categorized as a difficult-to-cut material and although EVAM has been applied to a wide range of metals it hasn’t yet been studied in CuZr HTSMA. In this context, the purpose of this thesis is twofold: On the one hand, to characterise the mechanical properties of CuZr SMA using Molecular Dynamics and, on the other hand, to explore the nanoscale mechanism of material removal of CuZr shape memory alloy (SMA) during elliptical vibration assisted machining (EVAM). The conclusions of this thesis can be summarized as follows. To characterise the mechanical properties of Cu₅₀Zr₅₀, Cu₂Zr and Cu₅Zr, a tensile and shear test were carried out using MD. Tensile test was done with crystal orientation and direction of tensile pulling as <010>. The results showed that Cu₅₀Zr₅₀ and Cu₂Zr exhibited a phase transformation (pseudoelasticity) during loading. However, Cu₅Zr showed dislocation nucleation as the main plastic deformation mechanism followed by fracture. Shear tests were done in the same phases with crystal orientation and direction of shear pulling as <100>. Interestingly, the shear test results showed no phase transformation for Cu₅₀Zr₅₀ and Cu₂Zr but the Cu₅Zr composition did show phase transformation during loading. It is important to highlight that all three phases of CuZr binary alloy that we have tested showed a different plastic response during the tensile test and the shear test. As far as machining is concerned, we observed indications that EVAM shows improved machinability compared with conventional machining. Although cutting forces were lower in EVAM, the stresses on the workpiece were slightly higher and both techniques showed the same mechanism of plasticity during machining. Neither dislocation nucleation or martensitic transformation was exhibited in either of the two machining techniques and instead, amorphisation was observed as the main plastic deformation mechanism in both cases. Interestingly, amorphisation has been previously observed by Saitoh and Kubota (2010) during loading NiTi SMA [1]; however, it didn’t show up in every crystal orientation confirming that NiTi shows significant changes in response to loading in different lattice directions. One of the main outcomes from this thesis is that CuZr SMA exhibits different modes of plastic deformation; namely amorphisation, dislocation nucleation and martensitic transformation during loading. The governing mechanism that arises during loading highly depends in the lattice direction in which the load is being applied. These findings can potentially enable reliable predictions and provide guidelines of the microstructural design of CuZr SMA systems