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The calculated T 0 lines for the transformation B2 → B19 Ј , B2 → B19, and B19 → B19 Ј , respectively, in comparison with the
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The pseudobinary Ti0.5Ni0.5-Ti0.5Cu0.5 system was analyzed using thermodynamic models for the Gibbs energy of individual phases in the system. The TiNi intermetallic
phase crystallizes with the B2 CsCl-type structure and is an ordered form of the A2 BCC structure. It was treated with a two-sublattice
model (Cu, Ni, Ti, Va)0.5(Cu, Ni, Ti, Va)0.5, wh...
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Context 1
... optimization, it was found that it was quite easy to form two distinct three-phase regions of in Ref 7. If we consider only phases related to the martensitic transformation,T 0 lines among the B2, B19, and B19 are (liquid B2 B11); such topology seems less likely in reality although not impossible thermodynamically. There- calculated as shown in Fig. 6. The experimental A f tempera- tures used for optimization are also indicated in Fig. 6. The fore, measures had to be taken in order to avoid this topology by introducing certain conditions in the optimization, and appearance of B19 and B19 in Fig. 6 is consistent with experimental observations [18][19][20][21] for martensitic ...
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... of in Ref 7. If we consider only phases related to the martensitic transformation,T 0 lines among the B2, B19, and B19 are (liquid B2 B11); such topology seems less likely in reality although not impossible thermodynamically. There- calculated as shown in Fig. 6. The experimental A f tempera- tures used for optimization are also indicated in Fig. 6. The fore, measures had to be taken in order to avoid this topology by introducing certain conditions in the optimization, and appearance of B19 and B19 in Fig. 6 is consistent with experimental observations [18][19][20][21] for martensitic transforma- the calculated diagram from the final results exhibits a contin- uous area of the ...
Context 3
... less likely in reality although not impossible thermodynamically. There- calculated as shown in Fig. 6. The experimental A f tempera- tures used for optimization are also indicated in Fig. 6. The fore, measures had to be taken in order to avoid this topology by introducing certain conditions in the optimization, and appearance of B19 and B19 in Fig. 6 is consistent with experimental observations [18][19][20][21] for martensitic transforma- the calculated diagram from the final results exhibits a contin- uous area of the three-phase region, as shown in Fig. 4. tion; i.e., B2 transforms to B19 when Cu content in initial alloys is 7.7 at.%; when Cu content is more than 7.7 at.%, From ...
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Citations
... The CALPHAD method has been widely applied to a large number of material systems, although its application to shape memory alloys (SMAs) has been rather modest. In fact, most CALPHAD works on SMAs have focused on NiTi-based systems [44][45][46][47][48]. These trends seem to be changing, though, as there has been an increase in the number of investigations on the phase stability and thermodynamic properties of SMAs. ...
... The model also accounted for possible B2 ? bcc order/disorder transformations, although in Ni-Ti, the B2 phase melts before it experiences disorder [44,48]. In other systems [53], however, it is necessary to consider B2 ? ...
Over the last decade, considerable interest in the development of High-Temperature Shape Memory Alloys (HTSMAs) for solid-state actuation has increased dramatically as key applications in the aerospace and automotive industry demand actuation temperatures well above those of conventional SMAs. Most of the research to date has focused on establishing the (forward) connections between chemistry, processing, (micro)structure, properties, and performance. Much less work has been dedicated to the development of frameworks capable of addressing the inverse problem of establishing necessary chemistry and processing schedules to achieve specific performance goals. Integrated Computational Materials Engineering (ICME) has emerged as a powerful framework to address this problem, although it has yet to be applied to the development of HTSMAs. In this paper, the contributions of computational thermodynamics and kinetics to ICME of HTSMAs are described. Some representative examples of the use of computational thermodynamics and kinetics to understand the phase stability and microstructural evolution in HTSMAs are discussed. Some very recent efforts at combining both to assist in the design of HTSMAs and limitations to the full implementation of ICME frameworks for HTSMA development are presented.
The alloy with the chemical composition of Ni 25 Ti 50 Cu 25 and a weight of 20 g was produced via high-energy ball milling that lasted 100 h, 120 h, and 140 h. After grinding, the powders consisted of an amorphous-nanocrystalline mixture formed from solid solutions based on alloying elements and a phase being a precursor of the intermetallic ß phase. This phase undergoes the reversible martensitic transformation in NiTiCu alloys. Crystallization occurred in several stages, ranging from 361 to 556 °C. Using short-term annealing followed by the X-ray quantitative analysis, the crystallized phases sequence and their weight percentages were determined after each stage. Annealing of the crystallized alloy lasted from 5 min to 20 h in the temperature ranges from 700 to 1000 °C, which revealed the quantitative evolution of the equilibrium phases and the martensitic transformation evolution. Moreover, following thermal treatment steps, the transformation changed from the B2 ↔ B19′ to the B2 ↔ B19, as expected for the Ni 25 Ti 50 Cu 25 alloy.
Recently, concept for an alloy having both glass forming ability and shape memory characteristics has been newly introduced based on the excellent functional properties of existing NiTi alloys. The physical properties of shape memory alloys (SMAs) fabricated through crystallization of glass precursor can be maximized by suppressing the formation of brittle intermetallic compounds and enabling the formation of nanocrystalline structure. The glass precursor is not only applied to small-sized devices easily, but it is possible to produce products with intricate geometry through thermoplastic forming. In addition, the shape memory effect or superelasticity can be manifested after crystallization. In this paper, typical quaternary NiTi-based SMAs and multicomponent NiTi-based SMAs fabricated using glass precursor are reviewed. Firstly, quaternary doping effects on the physical properties in NiTiCu and NiTiHf-based alloys, such as shape recovery characteristics, martensitic transformation, and mechanical behaviors, are introduced. Secondly, multicomponent SMA systems using glass precursor fabricated via rapid quenching are introduced. Glass forming ability, crystallization behavior and consequent transformation properties are discussed. SMA applications using glass precursors, including the micro/nanoscale manipulating tools which utilizes the unique shape memory properties, and the surface imprinting based on the superplasticity are introduced.
A critical thermodynamic re-assessment for the ternary Cu–Ni–Ti system was performed by means of the CALPHAD (CALculation of PHAse Diagram) approach, and a new set of self-consistent thermodynamic parameters for the Cu–Ni–Ti system was obtained. According to the reported crystal structures and homogeneity ranges, appropriate sublattice models were proposed for the τ4 and τ6 phases, which could achieve a better description than the previous stoichiometric compound model for phase compositions and primary phase regions. Three solid-state invariant reactions, CuTi + τ1 ↔ Cu4Ti3 + NiTi at 1074.86 K, Ni3Ti + NiTi + τ1 ↔ τ2 at 1074.39 K and NiTi + τ4 ↔ Ni3Ti + τ1 at 1175.11 K were predicted in the present work. Presently calculated liquidus projection eliminated the liquid miscibility gap of the Cu–Ni–Ti system on Cu–Ni side. Scheil reaction scheme of the entire composition range was also presented accordingly. Comparisons between the calculated results and the measured phase equilibria indicated that all the reliable experimental data were satisfactorily accounted for by the present modeling.
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Ti-50Ni to Ti-55Ni (at.%) can be termed as the pioneer of shape memory alloys (SMA). Intermetallic precipitates play an important role for strengthening. Their influence on the start temperature of the martensitic transformation is a crucial property for the shape memory effect. Efforts for increasing the martensite start temperature include replacement of a part of Ni atoms by Cu. The influence of Cu-addition to Ti-Ni SMA on T0- temperatures and the character of the austenite-martensite transformation is evaluated using a new thermodynamic database for the Ti-Ni-system extended by Cu. Trends of precipitation of intermetallic phases are simulated by combining the assessed thermodynamics of the Ti-Ni-Cu system with assessed diffusion mobility data and kinetic models, as implemented in the solid-state transformation software MatCalc and are presented in the form of time-temperature-precipitation diagrams. Thermodynamic equilibrium considerations, complemented by predictive thermo-kinetic precipitation simulation, facilitates SMA alloy design and definition of optimized aging conditions.
The Cu–Ni–Ti ternary system has been systematically investigated combining experimental measurements with thermodynamic modeling. With selected equilibrated alloys, the equilibrium phase relations in the Cu–Ni–Ti system at 850 °C were obtained by means of SEM/EDS (Scanning Electron Microscopy/Energy Dispersive Spectrum), EPMA (Electron Probe Micro-Analysis) and XRD (X-ray Diffractometry). Phase transformation temperatures were measured by DSC (Differential Scanning Calorimetry) analysis in order to construct various vertical sections in the Cu–Ni–Ti system. The liquidus projection of the ternary system was determined by the identifying primary crystallization phases in the as-cast alloys and from the liquidus temperatures obtained from the DSC analyses. Based on the available data of the binary systems Cu–Ni, Cu–Ti, Ni–Ti and the ternary system Cu–Ni–Ti from the literature and the present work, thermodynamic modeling of the Cu–Ni–Ti ternary system was performed using the CALculation of PHAse Diagram (CALPHAD) approach. A new set of self-consistent thermodynamic parameters for the Cu–Ni–Ti ternary system was obtained with an overall good agreement between experimental and calculated results.
Phase relations of Cu–Ni–Ti ternary system have been experimentally investigated and thermodynamically assessed. Firstly, part of phase relations in the Cu–Ni–Ti ternary system at 800 °C was established by analyzing Ti/Cu–Ni diffusion couple and alloy samples with EPMA (Electronic Probe Micro-Analysis). The Cu4Ti3 + CuTi + NiTi equilibrium reported in literature was presently modified to a three-phase equilibrium CuTi + NiTi + τ1 in the 730–830 °C temperature range. Based on the measured phase equilibria and those experimental investigations about Cu–Ni–Ti system and in combination to the three binary boundary systems optimized in literatures, thermodynamic parameters of various phases in this system were reasonably modeled through CALPHAD (CALculation of PHAse Diagram) approach.
