Editorial

5 pages, 159 KiB

Open AccessEditorial

Novel Research for Development of Shape Memory Alloys

by Takuo Sakon

Metals 2018, 8(2), 125; https://doi.org/10.3390/met8020125 - 11 Feb 2018

Cited by 5 | Viewed by 3328

Shape memory alloys have attracted much attention due to their attractive properties for applications as well as their basic aspects of deformation and transformation in structural and magnetic behavior.[...] Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

Research

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4997 KiB

Open AccessArticle

Effect of Ni-Content on the Transformation Temperatures in NiTi-20 at. % Zr High Temperature Shape Memory Alloys

by Matthew Carl, Jesse D. Smith, Brian Van Doren and Marcus L. Young

Metals 2017, 7(11), 511; https://doi.org/10.3390/met7110511 - 21 Nov 2017

Cited by 27 | Viewed by 6631

Abstract

The effect of Ni-content on phase transformation behavior of NiTi-20 at. % Zr high temperature shape memory alloy (HTSMA) is investigated over a small composition range, i.e., 49.8, 50.0 and 50.2 at. % Ni, by differential scanning calorimetry (DSC), high-energy synchrotron radiation X-ray [...] Read more.

The effect of Ni-content on phase transformation behavior of NiTi-20 at. % Zr high temperature shape memory alloy (HTSMA) is investigated over a small composition range, i.e., 49.8, 50.0 and 50.2 at. % Ni, by differential scanning calorimetry (DSC), high-energy synchrotron radiation X-ray diffraction (SR-XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). All samples show a monoclinic B19^′ martensitic structure at room temperature. It is shown that even with these small variations in Ni-content, the alloy shows vastly different transformation temperatures and responds in a drastically different manner to aging treatments at 550 and 600 °C. Lastly, a discussion on H-phase composition with respect to bulk composition is presented. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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1404 KiB

Open AccessArticle

Effect of Hydrogen on the Elastic and Anelastic Properties of the R Phase in Ti₅₀Ni_46.1Fe_3.9 Alloy

by Konstantin Sapozhnikov, Joan Torrens-Serra, Eduard Cesari, Jan Van Humbeeck and Sergey Kustov

Metals 2017, 7(11), 493; https://doi.org/10.3390/met7110493 - 10 Nov 2017

Cited by 3 | Viewed by 3227

Abstract

The linear and non-linear internal friction, effective Young’s modulus, and amplitude-dependent modulus defect of a Ti₅₀Ni_46.1Fe_3.9 alloy have been studied after different heat treatments, affecting hydrogen content, at temperatures of 13–300 K, and frequencies near 90 kHz. It [...] Read more.

The linear and non-linear internal friction, effective Young’s modulus, and amplitude-dependent modulus defect of a Ti₅₀Ni_46.1Fe_3.9 alloy have been studied after different heat treatments, affecting hydrogen content, at temperatures of 13–300 K, and frequencies near 90 kHz. It has been shown that the contamination of the alloy by hydrogen gives rise to an internal friction maximum in the R martensitic phase and a complicated pinning stage in the temperature dependence of the effective Young’s modulus at temperatures corresponding to the high-temperature side of the maximum. Dehydrogenation of the H-contaminated alloy transforms the internal friction maximum into a plateau and minimizes the pinning stage. The internal friction maximum is associated with a competition of two different temperature-dependent processes affecting the hydrogen concentration in the core regions of twin boundaries. The amplitude-dependent anelasticity of the R phase is also very sensitive to hydrogen content, its temperature dependence reflects the evolution of extended hydrogen atmospheres near twin boundaries. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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1996 KiB

Open AccessFeature PaperArticle

Evidence of Change in the Density of States during the Martensitic Phase Transformation of Ni-Mn-In Metamagnetic Shape Memory Alloys

by Rie Y Umetsu, Xiao Xu, Wataru Ito and Ryosuke Kainuma

Metals 2017, 7(10), 414; https://doi.org/10.3390/met7100414 - 04 Oct 2017

Cited by 11 | Viewed by 3972

Abstract

Specific heat measurements were performed at low temperatures for Ni₅₀Mn₅₀₋_xIn_x alloys to determine their Debye temperatures (θ_D) and electronic specific heat coefficients (γ). For x ≤ 15, where the ground state [...] Read more.

Specific heat measurements were performed at low temperatures for Ni₅₀Mn₅₀₋_xIn_x alloys to determine their Debye temperatures (θ_D) and electronic specific heat coefficients (γ). For x ≤ 15, where the ground state is the martensite (M) phase, θ_D decreases linearly and γ increases slightly with increasing In content. For x ≥ 16.2, where the ground state is the ferromagnetic parent (P) phase, γ increases with decreasing In content. Extrapolations of the composition dependences of θ_D and γ in both the phases suggest that these values change discontinuously during the martensitic phase transformation. The value of θ_D in the M phase is larger than that in the P phase. The behavior is in accordance with the fact that the volume of the M phase is more compressive than that of the P phase. On the other hand, γ is slightly larger in the P phase, in good agreement with the reported density of states around the Fermi energy obtained by the first-principle calculations. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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3315 KiB

Open AccessFeature PaperArticle

Temperature Dependences of the Electrical Resistivity on the Heusler Alloy System Ni₂MnGa_1−xFe_x

by Yoshiya Adachi, Yuki Ogi, Noriaki Kobayashi, Yuki Hayasaka, Takeshi Kanomata, Rie Y Umetsu, Xiao Xu and Ryosuke Kainuma

Metals 2017, 7(10), 413; https://doi.org/10.3390/met7100413 - 03 Oct 2017

Cited by 6 | Viewed by 3439

Abstract

Temperature dependences of the electrical resistivity have been measured on the Heusler alloy system Ni₂MnGa_1−xFe_x. The phase diagram of Ni₂MnGa_1−xFe_x was constructed on the basis of the experimental results. The structural and [...] Read more.

Temperature dependences of the electrical resistivity have been measured on the Heusler alloy system Ni₂MnGa_1−xFe_x. The phase diagram of Ni₂MnGa_1−xFe_x was constructed on the basis of the experimental results. The structural and magnetic transition temperatures are consistent with those previously determined by magnetic measurements. The changes of the electrical resistivity at the martensitic transition temperature, ∆ρ, were studied as a function of Fe concentration x. The ∆ρ abruptly increased in the concentration range between x = 0.15 and 0.20. The magnetostructural transitions were observed at x = 0.275, 0.30, and 0.35. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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2402 KiB

Open AccessArticle

Damping Characteristics of Inherent and Intrinsic Internal Friction of Cu-Zn-Al Shape Memory Alloys

by Shyi-Kaan Wu, Wei-Jyun Chan and Shih-Hang Chang

Metals 2017, 7(10), 397; https://doi.org/10.3390/met7100397 - 28 Sep 2017

Cited by 14 | Viewed by 4529

Abstract

Damping properties of the inherent and intrinsic internal friction peaks (IF_PT + IF_I) of Cu-xZn-11Al (x = 7.0, 7.5, 8.0, 8.5, and 9.0 wt. %) shape memory alloys (SMAs) were investigated by using dynamic mechanical analysis. The Cu-7.5Zn-11Al, Cu-8.0Zn-11Al, and [...] Read more.

Damping properties of the inherent and intrinsic internal friction peaks (IF_PT + IF_I) of Cu-xZn-11Al (x = 7.0, 7.5, 8.0, 8.5, and 9.0 wt. %) shape memory alloys (SMAs) were investigated by using dynamic mechanical analysis. The Cu-7.5Zn-11Al, Cu-8.0Zn-11Al, and Cu-8.5Zn-11Al SMAs with

{({IF}_{PT} + {IF}_{I})}_{β_{3} ({L 2}_{1}) \to γ_{3}^{'} (2 H)}

peaks exhibit higher damping capacity than the Cu-7.0Zn-11Al SMA with a

{({IF}_{PT} + {IF}_{I})}_{β_{3} ({L 2}_{1}) \to γ_{3}^{'} (2 H)}

peak, because the

γ_{3}^{'}

martensite phase possesses a 2H type structure with abundant movable twin boundaries, while the

β_{3}^{'}

phase possesses an 18R structure with stacking faults. The Cu-9.0Zn-11Al SMA also possesses a

{({IF}_{PT} + {IF}_{I})}_{β_{3} ({L 2}_{1}) \to γ_{3}^{'} (2 H)}

peak but exhibits low damping capacity because the formation of γ phase precipitates inhibits martensitic transformation. The Cu-8.0Zn-11Al SMA was found to be a promising candidate for practical high-damping applications because of its high (IF_PT + IF_I) peak with tan δ > 0.05 around room temperature. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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8269 KiB

Open AccessArticle

Analytical Investigation of the Cyclic Behavior of Smart Recentering T-Stub Components with Superelastic SMA Bolts

by Junwon Seo, Jong Wan Hu and Kyoung-Hwan Kim

Metals 2017, 7(10), 386; https://doi.org/10.3390/met7100386 - 21 Sep 2017

Cited by 21 | Viewed by 6107

Abstract

Partially restrained (PR) bolted T-stub connections have been widely used in replacement of established fully restrained (FR) welded connections, which are susceptible to sudden brittle failure. These bolted T-stub connections can permit deformation, easily exceeding the allowable limit without any fracture because they [...] Read more.

Partially restrained (PR) bolted T-stub connections have been widely used in replacement of established fully restrained (FR) welded connections, which are susceptible to sudden brittle failure. These bolted T-stub connections can permit deformation, easily exceeding the allowable limit without any fracture because they are constructed with a design philosophy whereby the plastic deformation concentrates on bolt fasteners made of ductile steel materials. Thus, the PR bolted connections take advantage of excellent energy dissipation capacity in their moment and rotation behavior. However, a considerable amount of residual deformation may occur at the bolted connection subjected to excessive plastic deformation, thereby requiring additional costs to recover the original configuration. In this study, superelastic shape memory alloy (SMA) bolts, which have a recentering capability upon unloading, are fabricated so as to solve these drawbacks, and utilized by replacing conventional steel bolts in the PR bolted T-stub connection. Instead of the full-scale T-stub connection, simplified T-stub components subjected to axial force are designed on the basis of a basic equilibrium theory that transfers the bending moment from the beam to the column and can be converted into equivalent couple forces acting on the beam flange. The feasible failure modes followed by corresponding response mechanisms are taken into consideration for component design with superelastic SMA bolts. The inelastic behaviors of such T-stub components under cyclic loading are simulated by advanced three-dimensional (3D) finite element (FE) analysis. Finally, this study suggests an optimal design for smart recentering T-stub components with respect to recentering and energy dissipation after observing the FE analysis results. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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1977 KiB

Open AccessFeature PaperArticle

High Field X-ray Diffraction Study for Ni_46.4Mn_38.8In_12.8Co_2.0 Metamagnetic Shape Memory Film

by Yoshifuru Mitsui, Keiichi Koyama, Makoto Ohtsuka, Rie Y. Umetsu, Ryosuke Kainuma and Kazuo Watanabe

Metals 2017, 7(9), 364; https://doi.org/10.3390/met7090364 - 12 Sep 2017

Cited by 1 | Viewed by 4483

Abstract

The transformation behaviors on metamagnetic shape memory Ni_46.4Mn_38.8In_12.8Co_2.0 film were investigated by X-ray diffraction experiments in the temperature up to 473 K and magnetic fields µ₀H up to 5 T. The prepared film showed [...] Read more.

The transformation behaviors on metamagnetic shape memory Ni_46.4Mn_38.8In_12.8Co_2.0 film were investigated by X-ray diffraction experiments in the temperature up to 473 K and magnetic fields µ₀H up to 5 T. The prepared film showed the parent phase with L2₁ structure at 473 K, and with preferred orientation along the 111 plane. The magnetic field induced reverse transformation was directly observed at T = 366 K, which was just around the reverse transformation starting temperature. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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6283 KiB

Open AccessArticle

A Combined Experimental-Numerical Approach for Investigating Texture Evolution of NiTi Shape Memory Alloy under Uniaxial Compression

by Li Hu, Shuyong Jiang and Yanqiu Zhang

Metals 2017, 7(9), 356; https://doi.org/10.3390/met7090356 - 09 Sep 2017

Cited by 8 | Viewed by 4683

Abstract

Texture evolution of NiTi shape memory alloy was investigated during uniaxial compression deformation at 673 K (400 °C) by combining crystal plasticity finite element method with electron back-scattered diffraction experiment and transmission electron microscope experiment. Transmission electron microscope observation indicates that dislocation slip [...] Read more.

Texture evolution of NiTi shape memory alloy was investigated during uniaxial compression deformation at 673 K (400 °C) by combining crystal plasticity finite element method with electron back-scattered diffraction experiment and transmission electron microscope experiment. Transmission electron microscope observation indicates that dislocation slip rather than deformation twinning plays a dominant role in plastic deformation of B2 austenite NiTi shape memory alloy at 673 K (400 °C). Electron back-scattered diffraction experiment illustrates heterogeneous microstructure evolution resulting from dislocation slip in NiTi shape memory alloy at 673 K (400 °C). {110}<100>, {010}<100> and {110}<111> slip systems are introduced into a crystal plasticity constitutive model. Based on the constructed representative volume element model and the extracted crystallographic orientations, particle swarm optimization algorithm is used to identify crystal plasticity parameters from experimental results of NiTi shape memory alloy. Using the fitted material parameters, a crystal plasticity finite element method is used to predict texture evolution of NiTi shape memory alloy during uniaxial compression deformation. The simulation results agree well with the experimental ones. With the progression of plastic deformation, a crystallographic plane of NiTi shape memory alloy gradually rotates to be vertical to the loading direction, which lays the foundation for forming the <111> fiber texture. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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10192 KiB

Open AccessArticle

Processing Map of NiTiNb Shape Memory Alloy Subjected to Plastic Deformation at High Temperatures

by Yu Wang, Shuyong Jiang and Yanqiu Zhang

Metals 2017, 7(9), 328; https://doi.org/10.3390/met7090328 - 25 Aug 2017

Cited by 18 | Viewed by 5014

Abstract

The processing map of Ni₄₇Ti₄₄Nb₉ (at %) shape memory alloy (SMA), which possesses B2 austenite phases and β-Nb phases at room temperature, is established in order to optimize the hot working parameters. Based on true stress-strain curves of [...] Read more.

The processing map of Ni₄₇Ti₄₄Nb₉ (at %) shape memory alloy (SMA), which possesses B2 austenite phases and β-Nb phases at room temperature, is established in order to optimize the hot working parameters. Based on true stress-strain curves of NiTiNb SMA during uniaxial compression deformation at the temperatures ranging from 700 to 1000 °C and at the strain rates ranging from 0.0005 to 0.5 s⁻¹, according to dynamic material model (DMM) principle, the processing map of NiTiNb SMA is obtained on the basis of power dissipation map and instability map. The instability region of NiTiNb SMA increases with increasing the true strain and it mainly focuses on the region with high strain rate. The workability of NiTiNb SMA becomes worse and worse with increasing plastic strain, as well as decreasing deformation temperature. There exist two stability zones which are suitable for hot working of NiTiNb SMA. In one stability region, the deformation temperature ranges from 750 to 840 °C and the strain rate ranges from 0.0003 to 0.001 s⁻¹. In the other stability region, the deformation temperature ranges from 930 to 1000 °C and the strain rate ranges from 0.016 to 0.1 s⁻¹. The severe microstructure defects, such as coarsening grains, band microstructure, and intercrystalline overfiring appear in the microstructures of NiTiNb SMA which is subjected to plastic deformation in the instability zone. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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1805 KiB

Open AccessArticle

Microstructure, Mechanical Property, and Phase Transformation of Quaternary NiTiFeNb and NiTiFeTa Shape Memory Alloys

by Yulong Liang, Shuyong Jiang, Yanqiu Zhang and Junbo Yu

Metals 2017, 7(8), 309; https://doi.org/10.3390/met7080309 - 12 Aug 2017

Cited by 10 | Viewed by 5210

Abstract

Based on ternary Ni₄₅Ti_51.8Fe_3.2 (at %) shape memory alloy (SMA), Nb and Ta elements are added to an NiTiFe SMA by replacing Ni element, and consequently quaternary Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄ [...] Read more.

Based on ternary Ni₄₅Ti_51.8Fe_3.2 (at %) shape memory alloy (SMA), Nb and Ta elements are added to an NiTiFe SMA by replacing Ni element, and consequently quaternary Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄Ti_51.8Fe_3.2Ta₁ (at %) SMAs are fabricated. The microstructure, mechanical property, and phase transformation of NiTiFeNb and NiTiFeTa SMAs are further investigated. Ti₂Ni and β-Nb phases can be observed in NiTiFeNb SMA, whereas Ti₂Ni and Ni₃Ti phases can be captured in NiTiFeTa SMA. As compared to NiTiFe SMA, quaternary NiTiFeNb and NiTiFeTa SMAs possess the higher strength, since solution strengthening plays a considerable role. NiTiFeNb and NiTiFeTa SMAs exhibit a one-step transformation from B2 austenite to B19’ martensite during cooling, but they experience a two-step transformation of B19’-R-B2 during heating. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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5426 KiB

Open AccessArticle

Deformation Behavior and Microstructure Evolution of NiTiCu Shape Memory Alloy Subjected to Plastic Deformation at High Temperatures

by Shuyong Jiang, Dong Sun, Yanqiu Zhang and Li Hu

Metals 2017, 7(8), 294; https://doi.org/10.3390/met7080294 - 03 Aug 2017

Cited by 8 | Viewed by 5222

Abstract

Deformation behavior and microstructure evolution of NiTiCu shape memory alloy (SMA), which possesses martensite phase at room temperature, were investigated based on a uniaxial compression test at the temperatures of 700~1000 °C and at the strain rates of 0.0005~0.5 s⁻¹. The [...] Read more.

Deformation behavior and microstructure evolution of NiTiCu shape memory alloy (SMA), which possesses martensite phase at room temperature, were investigated based on a uniaxial compression test at the temperatures of 700~1000 °C and at the strain rates of 0.0005~0.5 s⁻¹. The constitutive equation of NiTiCu SMA was established in order to describe the flow characteristic of NiTiCu SMA, which is dominated by dynamic recovery and dynamic recrystallization. Dislocations become the dominant substructures of martensite phase in NiTiCu SMA compressed at 700 °C. Martensite twins are dominant in NiTiCu SMA compressed at 800 and 900 °C. Martensite twins are not observed in NiTiCu SMA compressed at 1000 °C. The microstructures resulting from dynamic recovery or dynamic recrystallization significantly influences the substructures in the martensite phase of NiTiCu SMA at room temperature. Dislocation substructures formed during dynamic recovery, such as dislocation cells and subgrain boundaries, can suppress the formation of twins in the martensite laths of NiTiCu SMA. The size of dynamic recrystallized grains affects the formation of martensite twins. Martensite twins are not easily formed in the larger recrystallized grain, since the constraint of the grain boundaries plays a weak role. However, in the smaller recrystallized grain, martensite twins are induced to accommodate the transformation from austenite to martensite. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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3184 KiB

Open AccessArticle

by Ren-Kae Shiue, Shyi-Kaan Wu, Sheng-Hao Yang and Chun-Kai Liu

Metals 2017, 7(7), 276; https://doi.org/10.3390/met7070276 - 18 Jul 2017

Cited by 4 | Viewed by 4217

Abstract

Infrared dissimilar joining Ti₅₀Ni₅₀ and 316L stainless steel using Cu foil in between Cusil-ABA and BAg-8 filler metals has been studied. The Cu foil serves as a barrier layer with thicknesses of 70 μm and 50 μm, and it successfully [...] Read more.

Infrared dissimilar joining Ti₅₀Ni₅₀ and 316L stainless steel using Cu foil in between Cusil-ABA and BAg-8 filler metals has been studied. The Cu foil serves as a barrier layer with thicknesses of 70 μm and 50 μm, and it successfully isolates the interfacial reaction between Ti and Fe at the 316L SS (stainless steel) substrate side. In contrast, the Cu foil with 25 μm in thickness is completely dissolved into the braze melt during brazing and fails to be a barrier layer. A layer of (Cu_xNi_1−x)₂Ti intermetallic is formed at the Ti₅₀Ni₅₀ substrate side, and the Cu interlayer is dissolved into the Cusil-ABA melt to from a few proeutectic Cu particles for all specimens. For the 316L SS substrate side, no interfacial layer is observed and (Ag, Cu) eutectic dominates the brazed joint for 70 μm/50 μm Cu foil. The average shear strength of the bond with Cu barrier layer is greatly increased compared with that without Cu. The brazed joints with a 50 μm Cu layer demonstrate the highest average shear strengths of 354 MPa and 349 MPa for samples joined at 820 °C and 850 °C, respectively. Cracks are initiated/propagated in (Ag, Cu) eutectic next to the 316L substrate side featured with ductile dimple fracture. It shows great potential for industrial application. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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7467 KiB

Open AccessArticle

Investigation on Deformation Mechanisms of NiTi Shape Memory Alloy Tube under Radial Loading

by Shuyong Jiang, Junbo Yu, Li Hu and Yanqiu Zhang

Metals 2017, 7(7), 268; https://doi.org/10.3390/met7070268 - 13 Jul 2017

Cited by 7 | Viewed by 7650

Abstract

NiTi shape memory alloy (SMA) tube was coupled with mild steel cylinder in order to investigate deformation mechanisms of NiTi SMA tubes undergoing radial loading. NiTi SMA tubes of interest deal with two kinds of nominal compositions; namely, Ni-50 at % Ti and [...] Read more.

NiTi shape memory alloy (SMA) tube was coupled with mild steel cylinder in order to investigate deformation mechanisms of NiTi SMA tubes undergoing radial loading. NiTi SMA tubes of interest deal with two kinds of nominal compositions; namely, Ni-50 at % Ti and Ni-49.1 at % Ti, where at room temperature, B19′ martensite is dominant in the former, and B2 austensite is complete in the latter. The mechanics of the NiTi SMA tube during radial loading were analyzed based on elastic mechanics and plastic yield theory, where effective stress and effective strain are determined as two important variables that investigate deformation mechanisms of the NiTi SMA tube during radial loading. As for the NiTi SMA tube with austenite structure, stress-induced martensite (SIM) transformation as well as plastic deformation of SIM occur with the continuous increase of effective stress. As for NiTi SMA tube which possesses martensite structure, reorientation and detwinning of twinned martensite as well as plastic deformation of reoriented and detwinned martensite occur with the continuous increase in the effective stress. Plastic deformation for dislocation slip has a negative impact on superelasticity and shape memory effect of NiTi SMA tube. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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9145 KiB

Open AccessArticle

Investigation of Dynamic Recrystallization of NiTi Shape Memory Alloy Subjected to Local Canning Compression

by Yanqiu Zhang, Shuyong Jiang and Li Hu

Metals 2017, 7(6), 208; https://doi.org/10.3390/met7060208 - 06 Jun 2017

Cited by 8 | Viewed by 5369

Abstract

Physical mechanism for dynamic recrystallization of NiTi shape memory alloy subjected to local canning compression at various temperatures, 600, 700 and 800 °C, was investigated via electron backscattered diffraction experiments and transmission electron microscopy observations. With increasing deformation temperature, fractions of recrystallized grains [...] Read more.

Physical mechanism for dynamic recrystallization of NiTi shape memory alloy subjected to local canning compression at various temperatures, 600, 700 and 800 °C, was investigated via electron backscattered diffraction experiments and transmission electron microscopy observations. With increasing deformation temperature, fractions of recrystallized grains and substructures increase, whereas fraction of deformed grains decreases. In the case of 600 and 700 °C, continuous dynamic recrystallization and discontinuous dynamic recrystallization coexist in NiTi shape memory alloy. In the case of discontinuous dynamic recrystallization, the recrystallized grains are found to be nucleated at grain boundaries and even in grain interior. The pile-up of statistically stored dislocation lays the foundation for the nucleation of the recrystallized grains during discontinuous dynamic recrystallization of NiTi shape memory alloy. Geometrically necessary dislocation plays as an important role in the formation of new recrystallized grains during continuous dynamic recrystallization of NiTi shape memory alloy. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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679 KiB

Open AccessFeature PaperArticle

Lattice Softening in Fe₃Pt Exhibiting Three Types of Martensitic Transformations

by Takashi Fukuda and Tomoyuki Kakeshita

Metals 2017, 7(5), 156; https://doi.org/10.3390/met7050156 - 27 Apr 2017

Cited by 9 | Viewed by 4116

Abstract

We have investigated the relation between the softening of elastic constants and martensitic transformation in Fe₃Pt, which exhibits various kinds of martensitic transformation depending on its long-range order parameter S. The martensite phases of the examined alloys are BCT ( [...] Read more.

We have investigated the relation between the softening of elastic constants and martensitic transformation in Fe₃Pt, which exhibits various kinds of martensitic transformation depending on its long-range order parameter S. The martensite phases of the examined alloys are BCT (S = 0.57), FCT1 (S = 0.75, c/a < 1) and FCT2 (S = 0.88, c/a > 1). The elastic constants C′ and C₄₄ of these alloys decrease almost linearly with decreasing temperature. Although the temperature coefficient of C′ decreases as S increases, C′ at the transformation temperature is the smallest in the alloy with S = 0.75, which transforms to FCT1. This result implies that softening is most strongly related to the formation of the FCT1 martensite with tetragonality c/a < 1 among the three martensites. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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8272 KiB

Open AccessArticle

Role of Severe Plastic Deformation in Suppressing Formation of R Phase and Ni₄Ti₃ Precipitate of NiTi Shape Memory Alloy

by Li Hu, Shuyong Jiang and Yanqiu Zhang

Metals 2017, 7(4), 145; https://doi.org/10.3390/met7040145 - 19 Apr 2017

Cited by 22 | Viewed by 5319

Abstract

Microstructural evolution of NiTi shape memory alloy (SMA) with a nominal composition of Ni_50.9Ti_49.1 (at %) is investigated on the basis of heat treatment and severe plastic deformation (SPD). As for as-rolled NiTi SMA samples subjected to aging, plenty of [...] Read more.

Microstructural evolution of NiTi shape memory alloy (SMA) with a nominal composition of Ni_50.9Ti_49.1 (at %) is investigated on the basis of heat treatment and severe plastic deformation (SPD). As for as-rolled NiTi SMA samples subjected to aging, plenty of R phases appear in the austenite matrix. In terms of as-rolled NiTi SMA samples undergoing solution treatment and aging, Ni₄Ti₃ precipitates arise in the austenite matrix. In the case of as-rolled NiTi SMA samples subjected to SPD and aging, martensitic twins are observed in the matrix of NiTi SMA. With respect to as-rolled NiTi SMA samples subjected to solution treatment, SPD, and aging, neither R phases nor Ni₄Ti₃ precipitates are observed in the matrix of NiTi SMA. The dislocation networks play an important role in the formation of the R phase. SPD leads to amorphization of NiTi SMA, and in the case of annealing, amorphous NiTi SMA samples are subjected to crystallization. This contributes to suppressing the occurrence of R phase and Ni₄Ti₃ precipitate in NiTi SMA. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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3247 KiB

Open AccessFeature PaperArticle

Dynamic Recovery and Superelasticity of Columnar-Grained Cu–Al–Mn Shape Memory Alloy

by Sheng Xu, Haiyou Huang, Jianxin Xie, Yuta Kimura, Xiao Xu, Toshihiro Omori and Ryosuke Kainuma

Metals 2017, 7(4), 141; https://doi.org/10.3390/met7040141 - 15 Apr 2017

Cited by 14 | Viewed by 5584

Abstract

The columnar-grained Cu_71.5Al_17.5Mn₁₁ shape memory alloy was treated by single-pass hot rolling at 900 °C with a thickness reduction of 67.3% followed by immediate water quenching. Dynamic recovery other than discontinuous dynamic recrystallization occurred during the treatment process, [...] Read more.

The columnar-grained Cu_71.5Al_17.5Mn₁₁ shape memory alloy was treated by single-pass hot rolling at 900 °C with a thickness reduction of 67.3% followed by immediate water quenching. Dynamic recovery other than discontinuous dynamic recrystallization occurred during the treatment process, bringing about retained columnar grains with <001> textures, as well as dislocations introduced into the parent matrix. As a result, a large maximum recoverable strain of more than 11% was maintained due to the retained beneficial grain characteristics. The critical stress for inducing martensitic transformation and stress hysteresis were enhanced mainly due to the existence of dislocations. Full article

(This article belongs to the Special Issue Shape Memory Alloys 2017)

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