Damping characteristics of Ti50Ni49.5Fe0.5 and Ti50Ni40Cu10 ternary shape memory alloys

The damping characteristics of Ti₅₀Ni_49.5Fe_0.5 and Ti₅₀Ni₄₀Cu₁₀ ternary shape memory alloys (SMAs) have been systematically studied by resonant-bar testing and internal friction (IF) measurement. The damping capacities of the B19′ martensite and the B2 parent phase for these ternary alloys are higher than those for the Ti₅₀Ni₅₀ binary alloy. The lower yield stress and shear modulus of these ternary alloys are considered to be responsible for their higher damping capacity. For the same ternary alloy, the B19/B19′ martensite and R phase also have a higher damping capacity than does the B2 parent phase. In the forward transformations of B2 → R, R → 519′, and B2 → 519′ for Ti₅₀Ni₅₀ and Ti₅₀Ni_49.5Fe_0.5 alloys, the damping capacity peaks appearing in the resonant-bar test are attributed to both stress-induced transformation and stress-induced twin accommodation. The lattice-softening phenomenon can promote the stress-induced transformation and enhance the damping capacity peaks. The Ti₅₀Ni₄₀Cu₁₀ alloy had an unusually high plateau of damping capacity in the B19 martensite, which is considered to have arisen from the easy movement of twin boundaries of B19 martensite due to its inherently very low elastic modulus. The peaks appearing in the IF test for the Ti₅₀Ni₄₀Cu₁₀ alloy are mainly attributed to the thermal-induced transformation due to T ⊋ 0 during the test.     

Shape memory properties of Ni-Ti based melt-spun ribbons

Shape-memory properties of equiatomic NiTi, Ni₄₅Ti₅₀Cu₅, and Ni₂₅Ti₅₀Cu₂₅ ribbons made by melt spinning have been studied by temperature inducing the martensitic transformation under constant tensile loads. Recoverable strains above 4 pct can be obtained under ∼100 MPa loads for the NiTi and Ni₄₅Ti₅₀Cu₅ ribbons, transforming to B19’ martensite. The B19 martensite is formed in the Ni₂₅Ti₅₀Cu₂₅ ribbon after crystallization, and according to the lowering in transformation strain as Cu content increases, the recoverable strain is close to 2.5 pct for ∼150 MPa load. The transformation temperatures exhibit a linear dependence on the applied stress, which can be quantitatively described by means of a Clausius-Clapeyron type equation. The NiTi and Ni₄₅Ti₅₀Cu₅ ribbons exhibited some degree of two-way shape-memory effect (TWSME) after thermomechanical cycling. Texture analyses performed on the different ribbons allow us to better understand the transformation strains obtained in each ribbon. The amounts of shape-memory effect (SME) and nonrecoverable strain shown by the studied ribbons are of the same order as those already observed in bulk materials, which makes melt spinning an ideal substitute to complicated manufacturing processes if really thin samples are needed. However, applicable stresses in melt-spun ribbons are limited by a relatively “premature” brittle fracture caused by irregularities in ribbon thickness.     

Transformation behavior in a thermomechanically cycled TiNiCu alloy

The effect of thermomechanical cycling under 150 MPa on the transformation behavior in a TiNi₄₀Cu₁₀ (at pct) alloy annealed at different temperatures was investigated using electrical resistivity measurements and differential scanning calorimetry (DSC). It was found that thermomechanical cycling to failure could increase or decrease the transformation temperature for specimens annealed below or above the recrystallization temperature, respectively, but there was no obvious change of the transformation temperature for specimens annealed at the recrystallization temperature. The DSC and electrical-resistance experiments show that the B2 ⇋ B19 and B19 ⇋ B19′ two-stage transformations occurred in cold-worked and thermomechanically cycled specimens and that the electrical-resistance change due to the B2 → B19 transformation is larger than that of annealed specimens.     

Study of the Ni<Subscript>41.3</Subscript>Ti<Subscript>38.7</Subscript>Nb<Subscript>20</Subscript> wide transformation hysteresis shape-memory alloy

The shape-memory characteristics in the Ni_41.3Ti_38.7Nb₂₀ alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M _s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ _s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M _s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni_41.3Ti_38.7Nb₂₀ alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.     

A study of B2↔B19↔B19′ two-stage martensitic transformation in a Ti50Ni40Cu10 alloy

The B2↔B19↔B19′ two-stage martensitic transformation in a Ti₅₀Ni₄₀Cu₁₀ alloy has been investigated by electrical resistivity, DSC, X-ray diffraction and internal friction measurements. The shear modulus of B19 martensite has an unusually low value over a broad temperature range between the two shear modulus minima. The B2↔B19 transformation is thus proposed to proceed under the condition of deep shear modulus softening. X-ray diffraction results show that the B19↔B19′ is an incomplete transformation and that the monoclinic angle β of B19′ martensite will increasing with decreasing temperature. This indicates that the B19↔B19′ transformation has the characteristic of the continuously monoclinic distortion of B19′ martensite, which is similar to that of the continuously rhombohedral distortion of R-phase. The opposite behavior observed in electrical resistivity and DSC measurements for B2↔B19 and B19↔B19′ transformations is also discussed.     

Phase transformation and stabilization of a high strength austenite

An investigation of the phase transformation and the austenite stabilization in a high strength austenite has been made. An Fe-29Ni-4.3Ti austenite age-hardened byγ′(Ni₃Ti) precipitates showed a further increase of strength after martensitic and reverse martensitic phase transformations. The stability of ausaged austenite as well as ausaged and transformation-strengthened austenite was improved significantly through an isothermal treatment at 500°C. TheM _s temperature of the strengthened austenite was restored to nearly that of annealed austenite while the austenite was hardened toR _c 41 through precipitation and phase transformations. The observed austenite stabilization is attributed to the formation of G.P. zones or short-range order of less than ∼10? size. Formerly with University of California, Berkeley     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Effect of CO2 laser welding on the shape-memory and corrosion characteristics of TiNi alloys

A CO₂ laser has been employed to join binary Ti₅₀Ni₅₀ and Ti_49.5Ni_50.5 shape-memory alloys (SMAs), with an emphasis on the shape-memory and corrosion characteristics. Experimental results showed that a slightly lowered martensite start (M _S) temperature and no deterioration in shape-memory character of both alloys were found after laser welding. The welded Ti₅₀Ni₅₀, with an increased amount of B2 phase in the weld metal (WM), had higher strength and considerably lower elongation than the base metal (BM). Potentiodynamic tests revealed the satisfactory performance of laser-welded Ti₅₀Ni₅₀ in 1.5 M H₂SO₄ and 1.5 M HNO₃ solutions. However, the WM exhibited a significantly higher corrosion rate and a less stable passivity than the BM in artificial saliva. On the other hand, the pseudoelastic behavior of the laser weld was investigated only for the Ti_49.5Ni_50.5 alloy, to facilitate tension cycling at room temperature. The cyclic deformation of Ti_49.5Ni_50.5 indicated that the stress required to form stress-induced martensite (σ _m) and the permanent residual strain (ε _p) were higher after welding at a given number of cycles (N), which were certainly related to the more inhomogeneous nature of the WM.     

Influence of Al and Ni concentration on the Martensitic transformation in Cu-Al-Ni shape-memory alloys

The martensitic transformation temperatures and the types of martensitic phases have been determined in a wide concentration range of technological interest for Cu-Al-Ni shape-memory alloys (SMAs) A stability diagram of martensitic phases as a function of alloy concentration has been determined. It is found that when the aluminum content increases, the transformation changes from β ₃ ⇒ β′₃ to β ₃ ⇒ γ′₃, with an intermediate concentration range where both martensites coexist due to a β ₃ ⇒ γ′₃+β′₃ transformation. On the other hand, an increase of nickel content stabilizes the martensite β′₃, changing from a mixed β ₃ ⇒ γ′₃ + β′₃ to a single β ₃ ⇒ β′₃ transformation. Furthermore, linear relationships between M _s and Al and Ni concentrations have been obtained for all types of martensitic phases.     

<Emphasis Type="Italic">In-Situ</Emphasis> High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition in a Ni<Subscript>2</Subscript>MnGa Ferromagnetic Shape-Memory Crystal

The full information on the changes in many crystallographic aspects, including the structural and microstructural characterizations, during the phase transformation is essential for understanding the phase transition and “memory” behavior in the ferromagnetic shape-memory alloys. In the present article, the defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a uniaxial pressure of 50 MPa applied along the [110] crystallographic direction were studied by the in-situ high-energy X-ray diffuse-scattering experiments. The analysis of the characteristics of diffuse-scattering patterns around different sharp Bragg spots suggests that the influences of some defect clusters on the pressure-induced phase-transition sequences of Ni₂MnGa are significant. Our experiments show that an intermediate phase is produced during the premartensitic transition in the Ni₂MnGa single crystal, which is favorable for the nucleation of a martensitic phase. The compression stress along the [110] direction of the Heusler phase can promote the premartensitic and martensitic transition of the Ni₂MnGa single crystal. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

X-ray study of phase transformations in martensitic Ni-Al alloys

The formation of the Ni₅Al₃ and Ni₂Al phases in Ni-Al alloys with L1o ↔ B2 thermoelastic martensitic transformation has been studied by X-ray analysis. Ni₅Al₃ can form both the L1o and B2 structures, but the kinetics of L1o → Ni₅Al₃ and B2 → Ni₅Al₃ reactions are significantly different. A homogeneous mechanism for the former reaction and a mechanism of precipitation and growth for the latter are proposed. Ni₂Al forms from the B2 structure by the complex rearrangement of atoms. The initial stage of this reaction proceeds very rapidly and involves segregation of Ni atoms into Ni-rich zones leading to a Ni depletion in the surrounding regions. The nucleation of Ni₂Al retards the Ni₅Al₃ formation, so preaging in the B2 region affects the kinetics of the L1o → Ni₅Al₃ reaction on further aging in the L1o region. The microstructural mechanism for this effect is suggested.     

Preparation and Properties of Ti<Subscript>50</Subscript>Cu<Subscript>28</Subscript>Ni<Subscript>15</Subscript>Sn<Subscript>7</Subscript> Bulk Metallic Glass by Vacuum Hot Pressing

Amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders were synthesized by a mechanical alloying (MA) technique. Differential scanning calorimetry (DSC) results showed that, after 7 hours of exposure to the milling process, amorphous Ti₅₀Cu₂₈Ni₁₅Sn₇ alloy powders exhibit a wide supercooled liquid region of 61 K. Consolidation of amorphous powders were performed at a temperature slightly higher than the glass transition temperature under a pressure of ∼1.2 GPa, and bulk metallic glass (BMG) discs can be prepared successfully. However, we noticed partial crystallization during the hot pressing process and were not able to achieve full densification of BMG. The Vickers microhardness of Ti₅₀Cu₂₈Ni₁₅Sn₇ BMG was 634 kg/mm², and the trace of the indentation revealed that pre-existing particle boundaries or interfaces between nanocrystals and amorphous matrix may serve as the crack initiation sites. Thus, typical brittle failure of Ti₅₀Cu₂₈Ni₁₅Sn₇ BMG was observed and resulted in relatively low fracture stress compared to that estimated by the microhardness. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Effects of ternary additions on the thermoelastic martensitic transformation of NiAl

Nickel-rich β-NiAl alloys, which are potential materials for high-temperature shape-memory alloys, show a thermoelastic martensitic transformation, which produces their shape memory effect. However, the transformation to Ni₅Al₃ phase during heating of NiAl martensite can interrupt the reversible martensitic transformation; consequently, the shape memory effect in NiAl martensite might not appear after heating. The phase transformation process in binary Ni-(34 to 37)Al martensite was investigated by differential thermal analysis (DTA) method, and we found that the condition of reversible martensitic transformation was not the β → Ni₅Al₃ transformation, but rather the M → Ni₅Al₃ transformation occurring at 250 °C to 300 °C. Therefore, the transformation temperature of M → Ni₅Al₃ determined the highest operating temperature for the shape memory effect. For verifying the critical temperature, the phase transformation process was investigated for eight ternary Ni-33Al-X alloys (X=Cu, Co, Fe, Mn, Cr, Ti, Si, and Nb). Only Ti, Si, and Nb additions were found to be effective in dropping the M _s temperature, and they facilitated the shape memory effect in Ni-33Al-X alloys. In particular, the addition of Si and Nb raised the transformation temperature of M → Ni₅Al₃, a potentially beneficial effect for shape memory at higher temperatures. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Origin of Microstructural Irreversibility in Ni-Ti Based Shape Memory Alloys during Thermal Cycling

Different microstructures of Ni-Ti- and Ni-Ti-Fe-based shape memory alloys were subjected to thermal cycling: dipping in liquid nitrogen, for approximately 5 minutes, and then bringing it back to room temperature or austenite (cubic: B2) ↔ martensite (monoclinic: B19′) reversible solid-state phase transformation. Direct electron backscattered diffraction (EBSD) observations could bring out aspects of microstructural irreversibilities: namely, changes in grain size, misorientation buildup, and presence of retained martensite. The average changes in grain size (Δd) differed by almost 2 to 4 times between different microstructures. The highest Δd was typically observed in structures having maximum clustering of fine (d < 5 μm) grains. The sample with highest Δd was also subjected to multiple thermal cycling. Although Δd scaled linearly with d after the first thermal cycle, the scatter increased during subsequent thermal cycles. Grain or orientations deviating from the linear behavior were clearly anisotropic crystallographically. With repeated thermal cycling, the patterns of changes in Δd, austenite misorientation, and retained martensite content were similar. A phenomenological model or hypothesis, based on 40 deg á 001 ñ \left\langle {001} \right\rangle orientation relationship between austenite and martensite phases, was proposed to address the observed patterns of microstructural irreversibility.     

A study of electrical resistivity,internal friction and shear modulus on an aged Ti49Ni51 alloy

The 400°C aged Ti₄₉Ni₅₁ alloy can exhibit the transformation sequence of B2 →r premartensite R-phase →r martensite. In the early aging stage, only the premartensitic transformation is observed due to the M_s point being deeply depressed by the coherent stress of Ti₁₁Ni₁₄ precipitates. In the later aging stage, internal friction peaks associated with premartensitic and martensitic transformations are all observed on both heating and cooling. The sharp peaks associated premartensitic transformation on heating is believed to be related to the “bias” effect of the coherent stress induced by the Ti₁₁Ni₁₄ precipitates. The serrations of internal friction appearing significantly in the temperature around −30 to −80°C are found to be caused by the stress induced accomodation of R-phase or martensite variants, and are not associated with the transformation. The Ti₁₁Ni₁₄ precipitates can enhance the amount of martensite formed by unit of temperature or time during the martensitic transformation.     

Preparation and thermal stability of Zr-Ti-Al-Ni-Cu amorphous powders by mechanical alloying technique

This study examined the amorphization feasibility of Zr_70−x−y Ti _x Al _y Ni₁₀Cu₂₀ alloy powders by the mechanical alloying (MA) technique. According to the results, after 5 to 7 hours of milling, the mechanically alloyed powders were amorphous basically in the ranges of 0 to 12.5 at. pct Ti and 2.5 to 17.5 at. pct Al. These ranges are larger than those of bulk amorphous alloys prepared by a squeeze mold casting technique. Most of the amorphous mechanically alloyed powders exhibited a wide supercooled liquid region of more than 60 K before crystallization. The glass-transition and crystallization temperatures of mechanically alloyed samples were different from those prepared by squeeze casting. It is suspected that different thermal properties arise from the introduction of impurities during the MA process. The amorphization behavior of Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ was examined in detail. The X-ray diffraction and extended X-ray absorption fine structure (EXAFS) results show the fully amorphous powders formed after 5 hours of milling. A kinetically modified thermodynamic phase transformation process was observed for the glass-transition behavior in the Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ amorphous powder.     

Acoustic emission produced by the delta-to-alpha phase transformation in Pu-Ga alloys

Detailed measurements are reported of acoustic emission (AE) produced during theδ → α phase transformation in plutonium-gallium alloys. During thermal cycling, the number of AE signals produced decreased with each thermal cycle more rapidly than the amount of additional material transformed. Thus, some portion of the transformation does not produce detectable AE. The average amplitude of the signals produced during quenching was not appreciably dependent on gallium content or cooling cycle. The amplitude of the signals is greater for rapid cooling than for slow cooling. The AE behavior is consistent with an isothermal, nonthermoelastic martensitic transformation.     

Defects and Plastic-Deformation Modes of Bulk-Metallic Glasses

Results of the investigation of the point-defect manifestation in the recovery kinetics of Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 and Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk-metallic glasses (BMGs) irradiated with 2.5 MeV electrons at 80 K (–193.15 °C) are presented. An observation of the pronounced annealing stages at 155 K and 130 K (–118.15 °C and –143.15 °C), and 225 K (–48.15 °C), shows that irradiation generates stable point defects in BMGs. The ultrasonic vibrations (USVs) of different amplitudes were used to investigate their effects on the cluster boundaries. The Kaiser effect is chosen as a tool for examining the boundary-slip initiation and impact of vibrations on the intercluster-boundary structure. Both the acoustic-emission activity and strength decrease due to the specimen pretreatment by USV. This effect is interpreted as a result of boundary softening under the USV. The inherent tensile strength of a Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 BMG (in atomic percent) in the as-cast state was determined by means of high-field mechanical loading using the field-ion microscopy. It was revealed that the strength is characterized by a strong size effect in a nanometer-scale range as a result of the manifestation of the structural nanoheterogeneities and, in part, the existence of the cluster boundaries.     

Pseudoelastic behavior of a CuAlNi single crystal under uniaxial loading

In order to study the basic properties of pseudoelasticity of a CuAlNi single crystal, an investigation was carried out to observe and analyze the orientation dependence of the stress-induced martensitic transformation. The transformation is the β ₁ to β′₁ stress-induced transformation in a Cu-13.7 pct Al-4.18 pct Ni (wt pct) alloy. From the uniaxial tension of three groups of differently oriented flat specimens, we obtained a series of stress-strain curves. In addition, the micrograph of martensitic evolution was observed by utilizing a long-focus microscope. It is found that martensite appears in the shape of bands or thin plates on the surface of the specimen. The formation of martensite is a very quick process, and martensite “jumps” out until the specimen is completely transformed into a single variant. The experimental results are analyzed and compared to a constitutive model proposed recently. It is found that the constitutive model cannot describe transformation hardening, since the model ignores the surface-energy change. Nevertheless, the proposed constitutive model cannot only precisely predict the forward and reverse transformation, but can also characterize the stress-strain hysteresis behavior during pseudoelastic deformation under uniaxial tension loading.