Papers by Keyword: Phase Stability

Abstract: This work examines the phase stability during hot corrosion and the compressive strength of the AlCrFeNiCu-Nb high entropy alloy (HEA) produced using laser additive manufacturing, emphasizing its prospective uses in energy materials. The alloy's distinctive composition was chosen for its capacity to endure severe environments, including elevated temperatures and corrosive conditions, essential for energy-related applications. Phase stability was evaluated by X-ray diffraction, demonstrating remarkable preservation of critical phases despite high-temperature oxidation exposure. Compressive strength tests revealed the alloy's exceptional mechanical capabilities, underscoring its significant resistance to deformation. The AlCrFeNiCu-Nb HEA demonstrates significant promise for application in rigorous energy sectors, encompassing components for advanced power generation systems, high-temperature reactors, and corrosive conditions inside energy infrastructure.

Abstract: The control of the residually stressed γ’-FCC phase in the grain boundaries that affects super-elasticity in the promising Fe-Mn-Al-Ni shape memory alloy (SMA) and grain size enhancement was an epitome for research in the current study. New composition Fe-33Mn-17Al-8.5Ni (at. %) was designed with the help of thermocalc software TCFE 11 database, produced in an electric arc furnace under an argon atmosphere and systematically investigated in the as-cast and heat-treated conditions. Characterization was performed using optical microscopy, X-ray diffraction measurements (XRD), and compression tests. Controlling the cooling conditions after heat treatment (HT) with high flowrate air cooling helped to reduce on the formation of the detrimental phase, γ’ at the grain boundaries as well as observed some grain growth in the microstructure without necessarily causing cracking as reported previously with quenching in cold water. The yield strength depicting the stress-induced martensitic transformation was 925 MPa for as cast and 909 MPa upon heat treatment. From cyclic compression loading/deloading training, a recovery strain of 2.1% and 2.3% was attained at 800 MPa maximum stress in the as-cast and heat treated-conditions, respectively.

Abstract: In this study, cathode and lithium-ion conducting solid electrolyte composite pellet with 1:1 wt. % composition of LiFePO₄ and Li_7-3XGa_xLa₃Zr₂O₁₂ (x = 0.1) (LiFePO₄|Ga-LLZO) was prepared via solid-state reaction. The aim of the study is to investigate the phase stability between LiFePO₄ cathode and Ga-LLZO solid electrolyte material when heat treated at 400 to 600 °C. The as-mixed LiFePO₄|Ga-LLZO composite was characterized by TG/DTA and the heat treated sample was then analyzed for its structure using XRD and compared to the just as-mixed composite. XRD patterns of the heat treated composite pellet showed that it retains its as-mixed phases of LiFePO₄ and Ga-LLZO when sintered below 500 °C under Ar gas flow environment. However, upon heat treatment at 600 °C, the sample already reacted and decomposed with the formation of other phases.

Abstract: In the past decade, research into High Entropy alloys (HEAs) have gained significant attention due to their outstanding properties and approach to design alloys for high temperature applications. Strengthening of face centered cubic (FCC) based HEAs, by addition of intermetallic phase or precipitate forming elements is a very captivating direction of alloy designing for high temperature structural applications. However, the knowledge regarding the influence of intermetallic phases on the properties of FCC HEAs is rare. The current study focuses on annealing effects on the microstructure of Cr₂₀Co₂₀Fe₂₅Ni₂₅V₅Mo₅ (at. %) alloy, this alloy was synthesized using induction melting, and was homogenized at 1200 °C for 12h. X-ray diffraction analysis indicated that the principle phase was (FCC) identified. Scanning electron microscopy (SEM) together with Energy Dispersion X-ray Spectroscopy (EDS) showed that there is an additional phases that is Mo-rich. In order to understand the effect of the high temperature annealing on phase stability, the homogenized samples were annealed at 700 °C, 800 °C, 900 °C, 1000 °C each for 6h and quenched. The annealing treatments had considerable effect on the crystal structure and the elemental distribution. The Mo-rich phase is precipitated at the grain boundaries at all temperatures. Additionally, at 1000 °C annealing temperature Mo-rich phase had precipitated inside the grains. The lower annealing temperatures inhibited diffusion of Mo, which restricted the Mo-rich phase formation. Additionally, the hardness is increased to 195 HV at 1000 °C due precipitation hardening. At other annealing temperatures the hardness is reduced to 145 – 158 HV.

Abstract: This study assesses the structural stability at ultra-high temperature of the following selected compositions: 6.5 and 14 mol. % of RE₂O₃ (RE = Dy, Y, Er, Yb, and Lu) doped HfO₂. Under thermal cycling and thermal shock, the structural stability was evaluated at 2400°C with water vapor flux using a specific test bench with a 3 kW CO₂ laser. The cubic phase stability, which is theoretically important in the broad temperature range from 25 to 2800°C, was determined by a quantitative analysis of the X-ray diffractograms. Fully and partially stabilized HfO₂, obtained respectively with 14 mol. % and 6.5 mol. % of dopants, showed different behaviors to thermal damage. Thermal expansion was measured up to 1650°C to anticipate dimensional changes of these stabilized samples and to be able to design an optimized material solution fitting with future combustion chamber requirements. All of these results were then considered in order to exhibit a trend on the thermal stability at 2400°C of the ionic radius of the dopants and their optimal doping rates.

Abstract: Hydrogen is increasingly considered as fuel for future mobility or for stationary applications. However, the safe distribution and storage of pure hydrogen is only possible with suitable materials. Interstitially dissolved hydrogen atoms in the lattice of numerous metals are responsible for hydrogen embrittlement (HE). If hydrogen is introduced by an external source, it is called hydrogen environment embrittlement (HEE). Commonly, steels like AISI 316L with a high resistance to HEE include a large number of alloying elements and in high amount. High alloying levels result in a decrease of cost-efficiency. Therefore, the systematic investigation of lean-alloyed austenitic stainless steels is necessary in order to understand the mechanism of HEE. For that purpose, the steel grades AISI 304L and AISI 316L are selected in this work. Tensile tests in air and 400 bar hydrogen gas atmospheres are performed. After tensile testing in H, AISI 304L revealed secondary cracks at the specimen surface, which are related to the local austenite stability, which in turn is affected by the level of micro-segregation. The microstructural investigations of the crack environment directly contribute to the understanding of the micro-mechanisms of HEE. Property-maps generated from experimentally measured distributions of alloying elements allow to correlate the impact of micro-segregations on the local austenite stability. It is shown, that local segregation-bands affect the initiation and propagation of secondary cracks. In this context, the local austenite stability which is significantly affected by the Ni distribution will be discussed in detail by comparison of the metastable austenitic steel grades AISI 304L and AISI 316L.

Abstract: Laser beam welding and friction stir welding of high entropy alloys (HEA) of the CoCrFeNiMn system were studied. The HEAs were produced by self-propagating high-temperature synthesis (SHS). Along with the principal elements, Al, C, S, and Si impurities were detected in the composition of the alloys. The as-cast alloys consisted of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M₂₃C₆ carbides. Laser beam welding resulted in the formation of a defect-free weld joint. Precipitation of nanoscale B2 phase particles in the weld zone leaded to a pronounced increase in microhardness from ~150 HV of the base material to ~220 HV in the fusion zone. Friction stir welding (FSW) of a recrystallized state of the HEA with the average grain size of 3-5 μm resulted in the formation of a fine microstructure with a grain size of ~1.5 μm in the most strained area. Noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the formation of nanosized M₂₃C₆ carbides.

Abstract: The molecular orbital approach to alloy design is reviewed in this paper. This approachis based on the electronic structure calculations by the DV-Xα cluster method. New alloyingparameters are obtained for the first time by the calculations of titanium alloys and used for theprediction of phase stability and alloy properties. For example, it is shown that any titanium alloycan be classified into either the α, or α+β, or β type from the alloy composition by using the newalloying parameters. The corrosion resistance is also treatable along this approach. This theoreticalapproach is useful for the practical design of biomedical titanium alloys.

Abstract: First-principles calculations have been performed to investigate the phase stability, elastic, and thermodynamic properties of Co₃(Al,Mo,Ta) with the L1₂ structure. Calculated elastic constants showed that Co₃(Al,Mo,Ta) is mechanically stable and possesses intrinsic ductility. Young’s and shear moduli of polycrystalline Co₃(Al,Mo,Ta) were calculated using the Voigt-Reuss-Hill approach. It was found that the shear and Young’s moduli of Co₃(Al,Mo,Ta) were smaller than those of Co₃(Al,W). States density indicated the existence of covalent-like bonding in Co₃(Al,Mo,Ta). Temperature-dependent thermodynamic properties of Co₃(Al,Mo,Ta) could be described satisfactorily using the Debye-Grüneisen approach, including entropy, enthalpy, heat capacity and linear thermal expansion coefficient, showing their significant temperature dependences. Furthermore the obtained data could be employed in the modeling of thermodynamic and mechanical properties of Co-based alloys to enable the design of high temperature alloys.

Abstract: The phase instability of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) is widely reported in atmospheres containing carbon dioxide, which affects the long term electrochemical performance. The aim of this study is to investigate the phase stability of BSCF under the influence of milling and calcination temperature. Commercial BSCF powder was milled at 200 and 500 rpm and subsequently calcined at 750, 800 and 900 °C. The BSCF samples were characterized by using X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (FESEM). Secondary phases that were triggered after milling, however reduced with the increase of calcination temperature up to 800 °C. It was also found that the reduction of crystallite size and particle size at increased calcination temperature might be affected by the removal of these secondary carbonate phases. Moreover, the removal of carbonate was clearly evidenced in FTIR spectra by the reduction of carbonate signal intensities. In brief, a minimum calcination temperature of 900 °C was suggested for successful carbonate removal and recovery of single BSCF phase.