Papers by Keyword: Phase Stability

Abstract: Shape memory alloys (SMA) have been at the forefront of research in recent years. They have been used for a wide variety of applications in various fields. This work presents a brief study at the atomic scale of Cu-Al based Shape Memory Alloys. Using first-principles Density Functional Theory (DFT) method, the stability of different austenitic and martensitic phases of Cu₃Al, the effect of intrinsic vacancies, the doping effect by an element X (X = Be, Zn, Ti, Ni, Ag and Au) have been studied.

Abstract: First principle calculation based on Density Functional Theory and U correction (DFT+U) is used to investigate structural change while losing Li atom, average voltage for couple reaction, phase stability, and electronic structure of Li₂Fe_0.5Cr_0.5SiO₄. In this calculation, generalized gradient approach (GGA) of Perdew-Burke-Ernzerhof (PBE) is used for exchange-correlation functional. The initial structure of Li₂Fe_0.5Cr_0.5SiO₄ is obtained from the pmn₂₁ structure of Li₂FeSiO₄ and then the Fe site is substituted by 50 % of Cr. The results of calculation show that the optimized Li₂Fe_0.5Cr_0.5SiO₄ has a monoclinic structure, which has little different with Li₂FeSiO₄ structure. Although the delithiated system (LiFe_0.5Cr_0.5SiO₄) is taken into consideration, the structural geometry does not change significantly. It is indicated that the presence of Cr does not affect to the property of structural change. From the density of states (DOS) analysis, the presence of Cr causes the width of band gap become decrease. Therefore, the electronic properties change from insulator to semiconductor-like behavior. Average voltage for couple reaction M⁺²/ M⁺³ of Li₂Fe_0.5Cr_0.5SiO₄ is about 3.05 V which is lower than Li₂FeSiO₄. Furthermore, the formation energy for Li₂Fe_0.5Cr_0.5SiO₄ and all delithiation have a relatively positive sign compared with Li₂FeSiO₄ that mean that they have poor phase stability than Li₂FeSiO₄.

Abstract: The ability of zirconia ceramics to develop a bioactive surface is of great importance for biomedical applications. For that, it is also required to control phase stability (i.e. to avoid the tetragonal to monoclinic phase transformation) of zirconia surfaces in order to impart a high mechanical resistance. In this work, the phase stability of dense 3 mol% yttria stabilized-zirconia (3Y-TSZ) ceramics after acid and hydrothermal treatments was examined. Ceramic discs were prepared by slip casting from well dispersed 52 vol% suspensions, and subsequently sintered to 1300-1500 °C. The effect of various synthesis parameters and 0.25 wt% alumina doping on the phase transformation after acid and hydrothermal treatments was semi-quantitatively evaluated by XRD. The 3Y-TSZ sintered to 1500°C exhibited higher t→m transformation than samples sintered at lower temperatures. The phase transformation up to10h of hydrothermal treatment occurred by nucleation and growth mechanisms in which the nucleation process predominated. The presence of 0.25 wt% alumina in 3Y-TSZ sintered at 1500°C slightly enhanced the resistance to acid treatment.

Abstract: A series of new alloys including Au-Pd-M (M=Zr, Mo, Y) were developed in this study. The physical properties, electrical properties parameters and thermodynamics database of the alloys were established. The phase structure and composition were analyzed. The calculation results were verified by experiments. The influence of solidification rate to microstructures, morphology and grain size of Au-Pd-M alloys were analyzed. A first-principles plane-wave pseudo-potential method based on the density functional theory was used to investigate the electronic structures, bonding characters and energetic stability of Au-Pd-M alloys, the calculating results indicated that Au-Pd-Mo formed more easily than Au-Pd-Zr and Au-Pd-Y, and the formed Au-Pd-Mo is more stable than Au-Pd-Zr and Au-Pd-Y.

Abstract: A series of new alloys including Pt-Ir-M (M=Zr, Mo, Y) were invented in this study. The physical properties, electrical properties parameters and thermodynamics database of the alloys were established. The phase structure and composition were analyzed. The calculation results were verified by experiments. The influence of solidification rate to microstructures, morphology and grain size of Pt-Ir-M alloys were analyzed. According to First Principle, the electronic structures, bonding characters and energetic stability of the alloys were calculated, the calculate result indicated that Pt-Ir-Mo is formed more easily than Pt-Ir-Zr and Pt-Ir-Y, and the formed Pt-Ir-Mo is more stable than Pt-Ir-Zr and Pt-Ir-Y.

Abstract: The crystalline structure of the hexagonal Ba (Ti_0.95Fe_0.05)O_3−δ (BTF) ceramic were directly measured using a temperature-dependent XRD technique. Different from the traditional structural transitions of rhombohedral-orthorhombic (−90 °C), orthorhombic-tetragonal (−10 °C), and tetragonal-cubic (i.e., the Curie temperature T_C = 130 °C) for BaTiO₃, the phase transition from hexagonal to any phase above for BTF cannot occur at T_C even up to 300 °C. Increasing temperature cannot give rise to the occupation of oxygen vacancies by environmental oxygen absorbed. The lattice electro-neutrality maintained by Fe³⁺-V_O-Fe³⁺ defect complexes is predominant in BTF. This experiment clarifies the phase stability of the hexagonal perovskite structure above T_C up to 300 °C.

Abstract: The present work investigate the possibility of obtainment by mechanical alloying of Ni superalloys based on the Ni-Cr-Nb-C system strengthened by γ”(Ni3Nb), since γ”(Ni3Nb) as γ’ (Ni3Al) are typical coherent phase strengthening mechanisms in nickel superalloys. In order to evaluate this possibility, a composition with 71,65wt%Ni, 7,90wt%Cr, 20,00wt%Nb and 0,45wt%C was processed in a SPEX mill by 8 hours, consolidated and sintered at different temperatures (1200oC, 1250oC and 1300oC). The powder processed by MA and the sintered products were characterized by x-ray diffraction, SEM and EDS.

Abstract: Rare earth oxides co-doped zirconia has been developed for application in thermal barrier coating (TBC) systems to promote the performance and durability. In this paper, the phase stability and thermal physical properties of 1mol.%La₂O₃, 4.5mol.%Y₂O₃ stabilized ZrO₂ (LaYSZ) were investigated. After 100 h heat treated at 1400°C, the content of monoclinic phase in LaYSZ was 2.32 mol%, which is lower than that in YSZ (46.9 mol%). Result indicates that the LaYSZ has better phase stability and lower thermal conductivity than YSZ. The LaYSZ system can be explored as the candidate material as ceramic layer in TBC.

Abstract: The mechanical properties of g-TiAl at elevated temperatures have been investigated extensively over the last 30 years. Designed alloys have been proposed from the first generation alloy (Ti-48Al-2Cr-2Nb) to the second, the third and the fourth generations. However, a decisive chemical composition of g-TiAl has not been agreed among researchers yet. The main reasons for this situation are difficulties in compositional control of Ti-Al-X-Y. In this paper, the high temperature tensile properties of g-TiAl alloy with lots of different composition have been examined from the room temperature to 1200C and the tensile strength data of those specimens have been summarized. It is clear that Ti/Al atomic ratio plays an important role on the behaviors of the high temperature strength since the Ti/Al atomic ratio is strongly related to the phase stabilities between g and a₂ phases in the binary Ti-Al phase diagram. A very narrow confine of a/a₂ atomic ratio exists in the specimens having the comparatively high tensile strength at the elevated temperatures. Moreover, additions of the third elements such as Cr, Nb, Ta and W to g-TiAl contribute on the increase of the tensile strength and the shift of the phase stability among a₂, b and g phases. In order to utilize g-TiAl alloys in the various machine components at high temperatures, the severe process controls of melting, casting, thermo-mechanical treatments and heat treatments are indispensable.

Abstract: Nitrogen addition is known to effectively suppress the athermal γ (fcc) → ε (hcp) martensitic transformation of biomedical Co–Cr–Mo alloys and ultimately provides a combination of high strength and good ductility. In this work, the nanostructural evolution and its influence on dislocation slip as an elementary process in the martensitic transformation were investigated to reveal the origin of their enhanced γ phase stability due to nitrogen addition. The biomedical Co–29Cr–6Mo (wt.%) alloys containing nitrogen in different concentrations (0–0.24 wt.%) were prepared. A single phase γ matrix was attained by adding nitrogen contents higher than 0.1 wt.%. We discovered nanosized Cr₂N precipitates that form on the {111}_γ planes in the N-containing alloy specimens. It was revealed that the nanoscale inhomogeneities function as obstacles to the glide of partial dislocations and consequently significantly retard the γ → ε martensitic transformation. Since the formation of ε martensite plays a crucial role in plastic deformation and wear behavior, the developed nanostructural modification associated with nitrogen addition must be a promising strategy for highly durable orthopedic implants.