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1. Role of Aluminum rejection from isothermal ω precipitates on the formation of α precipitates in the metastable β-titanium alloy Ti-10V-2Fe-3Al

   https://doi.org/10.1016/j.scriptamat.2023.115565  

   Mantri, SA; Dasari, S; Sharma, A; Zheng, Y; Fraser, HL; Banerjee, R (September 2023, Scripta Materialia)

   The formation of isothermal ω phase precipitates and its influence on subsequent fine-scale α precipitation is investigated in a metastable β-titanium alloy, Ti-10V-2Fe-3Al. Atom-probe tomography and high-resolution transmission electron microscopy reveal that the rejection of Al, a potent α stabilizer, from the growing isothermal ω precipitates at 330°C, aids in the formation of α precipitates. Additionally, the presence of α/ω and α/β interfaces conclusively establish that these α precipitates form at the β/ω interface. Interestingly, the local Al pile-up at this interface results in a substantially higher than equilibrium Al content within the α precipitates at the early stages of formation. This can be rationalized based on a novel three-phase β+ω+α metastable equilibrium at a lower annealing temperature (330°C, below the ω solvus). Subsequent annealing at a higher temperature (600°C, above the ω solvus), dissolves the ω precipitates and re-establishes the two-phase β+α equilibrium in concurrence with solution thermodynamic predictions. 

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2. Effect of α″-Ti Martensitic Phase Formation on Plasticity in Ti–Fe–Sn Ultrafine Eutectic Composites

   https://doi.org/10.3390/mi15010148  

   Neelakandan, Deva Prasaad; Kim, Wonhyeong; Prorok, Barton C; Mirkoohi, Elham; Kim, Dong-Joo; Liaw, Peter K; Song, Gian; Lee, Chanho (January 2024, Micromachines)

   Extensive research has been conducted on Ti–Fe–Sn ultrafine eutectic composites due to their high yield strength, compared to conventional microcrystalline alloys. The unique microstructure of ultrafine eutectic composites, which consists of the ultrafine-grained lamella matrix with the formation of primary dendrites, leads to high strength and desirable plasticity. A lamellar structure is known for its high strength with limited plasticity, owing to its interface-strengthening effect. Thus, extensive efforts have been conducted to induce the lamellar structure and control the volume fraction of primary dendrites to enhance plasticity by tailoring the compositions. In this study, however, it was found that not only the volume fraction of primary dendrites but also the morphology of dendrites constitute key factors in inducing excellent ductility. We selected three compositions of Ti–Fe–Sn ultrafine eutectic composites, considering the distinct volume fractions and morphologies of β-Ti dendrites based on the Ti–Fe–Sn ternary phase diagram. As these compositions approach quasi-peritectic reaction points, the α″-Ti martensitic phase forms within the primary β-Ti dendrites due to under-cooling effects. This pre-formation of the α″-Ti martensitic phase effectively governs the growth direction of β-Ti dendrites, resulting in the development of round-shaped primary dendrites during the quenching process. These microstructural evolutions of β-Ti dendrites, in turn, lead to an improvement in ductility without a significant compromise in strength. Hence, we propose that fine-tuning the composition to control the primary dendrite morphology can be a highly effective alloy design strategy, enabling the attainment of greater macroscopic plasticity without the typical ductility and strength trade-off. 

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3. Microstructural impact on flank wear during turning of various Ti-6Al-4V alloys

   Dinh Nguyena, Di Kang (May 2017, Wear)

   Titanium alloys typically do not contain hard inclusion phases typically observed in other metallic alloys. However, the characteristic scoring marks and more distinctive micro- and/or macro-chippings are ubiquitously observed on the flank faces of cutting tools in machining titanium alloys, which is the direct evidence of abrasive wear (hard phase(s) in the microstructure abrading and damaging the flank surface). Thus, an important question lies with the nature of the hard phases present in the titanium microstructure. In this work, we present a comprehensive study that examines the microstructural impact on flank wear attained by turning various Ti-6Al- 4V bars having distinct microstructures with uncoated carbide inserts. In particular, four samples with elongated, mill-annealed, solution treated & annealed and fully-lamellar microstructures were selected for our turning experiments. After turning each sample, the flank surface of each insert was observed with confocal laser scanning microscopy (CLSM) and analyzed to determine the flank wear behavior in relation to each sample' distinct microstructures. To characterize the microstructure, scanning electron microscopy (SEM) together with Orientation imaging microstructure (OIM) was used to identify and distinguish the phases present in each sample and the content and topography of each phase was correlated to the behavior of flank wear. The flank wear is also affected by the interface conditions such as temperature and pressure, which were estimated using finite element analysis (FEA) models. The temperature dependent abrasion models enable us to estimate the flank wear rate for each microstructure, and are compared with the experimentally measured wear data. 

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4. Microstructural Impact on Flank Wear of Various Ti-6Al-4V Alloys

   Nguyen, D. (July 2018, WeAr)

   Titanium alloys typically do not contain hard inclusion phases typically observed in other metallic alloys. However, the characteristic scoring marks and more distinctive micro- and/or macro-chippings are ubiquitously observed on the flank faces of cutting tools in machining titanium alloys, which is the direct evidence of abrasive wear (hard phase(s) in the microstructure abrading and damaging the flank surface). Thus, an important question lies with the nature of the hard phases present in the titanium microstructure. In this work, we present a comprehensive study that examines the microstructural impact on flank wear attained by turning various Ti-6Al-4V bars having distinct microstructures with uncoated carbide inserts. In particular, four samples with elongated, mill-annealed, solution treated & annealed and fully-lamellar microstructures were selected for our turning experiments. After turning each sample, the flank surface of each insert was observed with confocal laser scanning microscopy (CLSM) and analyzed to determine the flank wear behavior in relation to each sample' distinct microstructures. To characterize the microstructure, scanning electron microscopy (SEM) together with Orientation imaging microstructure (OIM) was used to identify and distinguish the phases present in each sample and the content and topography of each phase was correlated to the behavior of flank wear. The flank wear is also affected by the interface conditions such as temperature and pressure, which were estimated using finite element analysis (FEA) models. The temperature dependent abrasion models enable us to estimate the flank wear rate for each microstructure, and are compared with the experimentally measured wear data. 

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5. Thermal stability and phase transformation of α-, κ(ε)-, and γ-Ga2O3 films under different ambient conditions

   https://doi.org/10.1063/5.0214500  

   Tang, Jingyu; Jiang, Kunyao; Tseng, Po-Sen; Kurchin, Rachel C; Porter, Lisa M; Davis, Robert F (August 2024, Applied Physics Letters)

   Phase transitions in metastable α-, κ(ε)-, and γ-Ga2O3 films to thermodynamically stable β-Ga2O3 during annealing in air, N2, and vacuum have been systematically investigated via in situ high-temperature x-ray diffraction (HT-XRD) and scanning electron microscopy (SEM). These respective polymorphs exhibited thermal stability to ∼471–525 °C, ∼773–825 °C, and ∼490–575 °C before transforming into β-Ga2O3, across all tested ambient conditions. Particular crystallographic orientation relationships were observed before and after the phase transitions, i.e., (0001) α-Ga2O3 → (2¯01) β-Ga2O3, (001) κ(ε)-Ga2O3 → (310) and (2¯01) β-Ga2O3, and (100) γ-Ga2O3 → (100) β-Ga2O3. The phase transition of α-Ga2O3 to β-Ga2O3 resulted in catastrophic damage to the film and upheaval of the surface. The respective primary and possibly secondary causes of this damage are the +8.6% volume expansion and the dual displacive and reconstructive transformations that occur during this transition. The κ(ε)- and γ-Ga2O3 films converted to β-Ga2O3 via singular reconstructive transformations with small changes in volume and unchanged surface microstructures. 

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