Abstract ：

Recent studies have shown an intriguing phenomenon that by introducing nano-precipitates, its sharp first order martensitic transformation (MT) can be changed into smooth strain glass transition which is characterized by sluggish evolution of nano-sized martensitic domains. However, it remains uncle...

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Geometrical confinement Martensitic transformation Phase field simulation Precipitate Strain glass transition

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Abstract ：

Superelasticity (SE) in shape memory alloys (SMAs) can be attributed to the stress-induced reversible martensitic transformation (MT), which influences the operation temperature of SE. Recent studies have demonstrated that some processes could influence the transition temperatures and related properties of MT such as alloying design, introducing dislocation, thermomechanical treatments, nano-composition, to name a few. In this paper, we propose a method to extend the operating temperature range of SE by designing composition modulation by Ni4Ti3 precipitate dissolution in Ni-rich NiTi SMAs. Phase field calculations in Ni-rich NiTi alloys (Ni50.8Ti49.2 (at.%) and Ni51.5Ti48.5 (at.%)) are used to study the phase transition behavior and related mechanical properties. Calculations have shown that the Ni4Ti3 precipitate could dissolve under solution treatment for a short time, accompanied by residual Ni composition modulation. The residual composition modulation may lead to the continuous martensitic transition behavior under cooling. Further calculations under loading have shown the SE over a wide temperature range due to the continuous MT behavior. In addition, corresponding experiments confirm the precipitate dissolution process and related continuous MT. Our work may provide a simple way to alter the mechanical properties of SMAs by designing composition modulation through precipitate dissolution. © 2023, ASM International.

Keyword ：

Composition modulation; Phase field; Precipitate dissolution; Shape memory alloys; Superelasticity

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Abstract ：

Recent studies have shown an intriguing phenomenon that by introducing nano-precipitates, its sharp first order martensitic transformation (MT) can be changed into smooth strain glass transition which is characterized by sluggish evolution of nano-sized martensitic domains. However, it remains unclear how the precipitates can fundamentally change the nature of the normal MT. In the present study, we reveal the origin of this phenomenon and explore the novel properties associated with the precipitate-induced strain glass using phase field simulations. Our model and phase field simulations show that the shape and size of the Ni4Ti3 precipitates influence the geometrical confinement. The geometrical confinement caused by precipitates with disk shape and high density could produce strong resistance to the growth of large-scale martensitic domains, and can lead to the freezing of nano-sized martensitic domains. When the average size of the B2 pocket between adjacent Ni4Ti3 precipitates is below a critical value (∼16 nm), the avalanche-like behavior of the normal MT is altered to a smooth nanoscale MT with nearly zero transition hysteresis, which exhibits interesting hysteresis-free superelasticity over a wide temperature range. Our results further show that the nanoscale MT exhibits characteristic signatures of strain glass, such as broken ergodicity and Vogel-Fulcher frequency dispersion of internal friction peaks and suggest the non-ergodic nature of the geometrically confined MT. Through precipitate engineering, it is possible to regulate and modify the normal MT into the nanoscale MT and consequently achieve novel properties. © 2022 Acta Materialia Inc.

Keyword ：

Binary alloys Geometry Glass Glass transition Hysteresis Martensitic transformations Nanotechnology Titanium alloys

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Abstract ：

Martensitic transformation (MT), strain glass transition (SGT) and the corresponding stress-strain responses in cold-rolled Ti49.2Ni50.8 shape memory alloys (SMAs) at various amounts of cold-rolling deformation are investigated. A phase diagram describing all the strain states and transitions among ...

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Defect engineering Martensitic transformation Shape memory alloy Strain glass Superelasticity TiNi

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Abstract ：

The formation of ultra-fine α precipitates in β-titanium alloys can significantly improve strength but may lead to rupture and low ductility. Here, we propose a simple two-step aging heat treatment to design a bimodal microstructure with distinctively different populations of fine-scale and coarse α...

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Mechanical properties Pseudo-spinodal decomposition Titanium alloys α precipitation

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A strategy to quantify the second phase strengthening of α precipitates in titanium alloys is proposed to predict the mechanical performance through crystal plasticity finite element (CPFE) models. Considering the different dominant features of equiaxed and lamellar microstructures, statistical and microstructural factors are introduced to describe the slip resistance variations in the β matrix. The significant contribution of the interfaces is incorporated by introducing the interface-affected zone (IAZ) into crystal plasticity constitutive laws of equiaxed microstructures. In particular, the slip resistance variation is derived accounting for the interfacial energy distribution inside the IAZ. Meanwhile, the arrangement of the α lamellae is more compact and complex than that of spherical α grains in equiaxed microstructures; thus, instead of the IAZ, the interface length density is applied to describe the strengthening effects of the interfaces in lamellar microstructures. The elastic interaction energy induced by the semi-coherent interfaces of lamellae is obtained according to the micro-elastic theory. Using the advantages of the aforementioned improved models, CPFE simulations of the tensile behavior of equiaxed and lamellar microstructures at room temperature are performed, with the results matching well with the experimental data. Moreover, differences regarding the lattice structures and grain orientations lead to non-uniform strain partitioning in equiaxed and lamellar microstructures. Equiaxed α grains that favor prismatic slip tend to bear more plastic deformation than those favoring basal slip. The growth direction of α lamellae affects the deformation ability as well. Consequently, the proposed approach can precisely predict the mechanical properties of dual-phase titanium alloys through statistical values of microstructural features, and can be utilized in the investigations of other metals with similar structural characteristics. © 2022 Elsevier Ltd.

Keyword ：

Crystal plasticity; Interface-affected zone; Phase boundary; Titanium alloy

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Abstract ：

Strain glass is a new strain state discovered recently in ferroelastic systems that is characterized by nanoscale martensitic domains formed through a freezing transition. These nanodomains typically have mottled or tweed morphology depending on the elastic anisotropy of the system. Strain glass transition is a broadly smeared and high order-like transition, taking place within a wide temperature or stress range. It is accompanied by many unique properties, including linear superelasticity with high strength, low modulus, Invar and Elinvar anomalies, and large magnetostriction. In this review, we first discuss experimental characterization and testing that have led to the discovery of the strain glass transition and its unique properties. We then introduce theoretical models and computer simulations that have shed light on the origin and mechanisms underlying the unique characteristics and properties of strain glass transitions. Unresolved issues and challenges in strain glass study are also addressed. Strain glass transition can offer giant elastic strain and ultralow elastic modulus by well-controlled reversible structural phase transformations through defect engineering. © 2022 Annual Reviews Inc.. All rights reserved.

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Elasticity Elastic moduli Glass Glass transition Linear transformations Martensitic transformations Shape memory effect

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Abstract ：

The formation of ultra-fine α precipitates in β-titanium alloys can significantly improve strength but may lead to rupture and low ductility. Here, we propose a simple two-step aging heat treatment to design a bimodal microstructure with distinctively different populations of fine-scale and coarse α precipitates in Ti-5Al-5Mo-5V-3Cr-1Zr (Ti-55531, wt. %) by involving two transformation mechanisms, i.e., classical nucleation and growth, and pseudo-spinodal decomposition. Such a multi-scale α microstructure exhibits a synergistic combination of yield strength (∼1.1 GPa) and ductility (∼19.5% elongation). TEM characterization shows that the appearance of deformation twins in coarse α precipitates contribute to increased ductility, and the higher strength may be attributed to dislocation tangles in fine-scale α precipitates. Our work provides a new strategy to overcome the strength-ductility trade-off by designing a heterogeneous microstructure. © 2021 Elsevier B.V.

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Aluminum alloys Chromium alloys Ductility Economic and social effects Microstructure Molybdenum alloys Population statistics Precipitation (chemical) Titanium alloys Vanadium alloys

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A new (α + β) Ti-alloy, Ti-6Al-2V-1Cr-1Fe (wt%), with fine grain sizes, fine precipitates, together with the high strength and excellent ductility in its as-cast state is developed. As compared to the as-cast commercial Ti-6Al-4V alloy, the grain size and α lath thickness of the new alloy are substantially refined by 50∼75%, and the yield strength and ductility are increased by 19.7% and 51.8%, respectively. The grain size refinement is achieved by tuning the supercooling capacity of the alloy through alloying with the aid of CALPHAD calculations. In addition, the alloying of Cr and Fe substantially reduces the α lath thickness. This low-cost Ti alloy with the enhanced properties is anticipated to be highly suitable for various structural applications in its as-cast or as-printed state. © 2021

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Alloying Aluminum alloys Chromium alloys Ductility Grain size and shape High strength alloys Iron alloys Ternary alloys Titanium alloys Vanadium alloys

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The mechanical properties of two-phase γ-TiAl alloys depend strongly on the scale of their lamellar microstructures. In this study, we investigate the effect of alloy composition on lamellar thickness in Ti100-xAlx by simulating microstructural evolution during the α2′ → α2 + γ phase transformation using the phase field method. From both existing experimental data and our simulation results we find that the average thickness of γ lamellae first decreases and then increases with increasing Al content. Further analyses reveal that this unique composition-dependence of the lamellar thickness could be associated with non-conventional pseudospinodal decomposition and congruent structural transformation mechanisms that generate ultra-high densities of nuclei at the initial stage, which impacts the later growth processes. The interplay between the number density of nuclei and the number of coalescence events during growth results in the non-monotonic dependence of the lamellar thickness on the alloy composition. These findings may offer new insights on the design of ultrafine lamellar microstructures for γ-TiAl alloys. © 2020 Acta Materialia Inc.

Keyword ：

Alloying Lamellar structures Phase transitions Precipitation (chemical) Titanium alloys

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