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.     

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.     

Influence of Aging Treatment on In-Situ Electrical Resistance Variation During Aging of Nickel-Rich NiTi Shape Memory Wires

Electrical resistance variations of Ni_50.9Ti_49.1 shape memory wires were studied during aging treatment at different temperatures via in-situ electrical resistance measurement. The results showed that during aging treatment, a cyclic behavior was observed in the electrical resistance variations, which could be related to the precipitation process. The evaluation of transition temperatures was conducted, after aging, using differential scanning calorimetry (DSC) analysis. The precipitation process is found to occur in four different stages. The results show that depending on the stress level around precipitates, two-, three-, or four-step martensitic transformation could be observed in DSC curves. In the points with maximum stress level (during precipitation process), four-step martensitic transformation is observed.     

Comparison of shape memory characteristics of a Ti-50.9 At. Pct Ni alloy aged at 473 and 673 K

The effect of aging on transformation and deformation behavior, i.e., the transformation temperatures, shape memory behavior, and multistage martensitic and R-phase transformations, was investigated for a Ti-50.9 at. pct Ni alloy aged at a low temperature (<600 K) rarely used for practical applications and at a high temperature (>600 K) conventionally used for practical applications. It was found that there are many differences between aging at 473 and 673 K. The martensitic and R-phase transformation temperatures significantly varied depending on aging time and temperature. It is found that two-stage R-phase and multistage martensitic transformations appear in both the specimens aged at 473 and 673 K, respectively. The two-stage R-phase transformation appeared by aging at 473 K over 36 ks, while the multistage martensitic transformation (MSMT) appeared by aging at 673 K in the range of aging times between 1.2 and 36 ks. It is found that the critical stress for slip increases with increasing aging time in specimens aged at 473 K, while that of specimens aged at 673 K increases with increasing aging time until reaching a maximum, then it decreases with a further increase in aging time. It is also found that the critical stress for slip is superior for specimens aged at 473 K than that for specimens aged at 673 K. It was confirmed that dense and fine lenticular precipitates of about 10 nm in length were formed through aging, resulting in superior shape memory characteristics.     

Mechanical properties and structure of age-hardened Ti-Cu alloys

The room temperature tensile properties of age-hardened Ti-Cu alloys, 0.9 and 4 at. pct Cu, were investigated. The results were correlated to the microstructure and to the observed interaction mechanism between moving dislocations and precipitated particles. Two types of precipitates were observed upon aging between 400° and 500°C. One type was identified as a metastable, ordered precipitate coherent with the matrix. The other type was the stable Ti₃Cu phase which was partially coherent with the matrix. Both types of particles were precipitated from a martensitic microstructure which resulted from the β → α transformation. Although the martensitic microstructure contributed to the high flow stress, the elongation to fracture was principally determined by the dislocation-particle interaction mechanism. The maximum elongation to fracture was obtained by inducing a dislocation by-pass mechanism in a structure containing homogeneously distributed, partially coherent Ti₃Cu particles. The tendency of the Ti₃Cu particles to precipitate preferentially on boundaries was minimized by low temperature aging. Formerly Assistant Professor Materials Research Laboratory, Rutgers University, New Brunswick, N.J.     

<Emphasis Type="Italic">In-Situ</Emphasis> Synchrotron Characterization of Transformation Sequences in TiNi-Based Shape Memory Alloys during Thermal Cycling

In-situ synchrotron radiation has been used to provide direct analysis of the transformation sequences in TiNi-based shape memory alloys during thermal cycling. The high resolution, narrow peak width Debye–Scherrer diffraction spectra enabled positive identification and quantification of the phase transformation sequences, which is not possible through normal laboratory studies. The results facilitate a clearer understanding of the development and influence of intermediate phases such as R or B19 on sequential martensitic transformations. Ti_50.2Ni_49.8 transformed predominately via a single-step B2 ↔ B19′ transformation, although evidence of the R phase was found during cooling in every cycle. The martensitic start temperature was depressed by ~0.6 °C per cycle, while the R-phase start temperature was found to be unaffected. Ti₅₀Ni₄₁Cu₉ transformed through a two-step B2 ↔ B19 ↔ B19′ sequence, with the B2 → B19 transformation reaching completion prior to the formation of any B19′. The transformation temperatures of Ti₅₀Ni₄₁Cu₉ were found to be insensitive to thermal cycling, remaining constant over the studied cycle range.     

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.     

<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.     

Multiscale structure and properties of cast and deformation processed polycrystalline NiTi shape-memory alloys

The objective of this study is to examine fundamental processing-structure-property relationships in polycrystalline NiTi bars. Three different polycrystalline Ti-50.9 at. pct Ni (Ti-55.7 wt pct Ni) materials were examined: (1) cast, (2) cast then hot rolled, and (3) cast, hot rolled, then cold drawn. The structure of the materials was investigated at various scales ranging from nanometers to micrometers. The cast materials contained random crystallographic textures along the loading axis of the extracted samples. The hot-rolled and cold-drawn materials contained a strong 〈111〉 texture parallel to the deformation-processing direction. The high-temperature hot-rolling process facilitated recrystallization and recovery, and curtailed precipitate formation, leaving the hot-rolled and cold-drawn materials in near solutionized states. The cold-drawn material contained a high density of dislocations and martensite. After a mild aging treatment, all three materials contained distributed coherent Ti₃Ni₄ precipitates on the order of 10 nm in size. The cast material was capable of full shape-memory transformation strain recovery up to approximately 5 pct strain at room temperature under both tension and compression. The hot-rolled and cold-drawn materials demonstrated significant tension-compression stress-strain asymmetry owing to their strong crystallographic texture. Under compression, the deformation-processed materials were only capable of 3 pct transformation strain recovery while under tension they were capable of nearly 7 pct transformation strain recovery. Based on the present results, the presence of small coherent Ti₃Ni₄ precipitates is determined to be the driving force for the favorable strain transformation strain recovery properties in all three materials, despite drastically different grain sizes and crystallographic textures. The unique dependence of elastic modulus on stress-state, temperature, and structure is also presented and discussed for the deformation-processed materials. In addition, we demonstrate that the appearance of a Lüders band transformation under tensile loading can be controlled by material structure. Specifically, the presence of significant martensite and dislocations in the cold-drawn materials was shown to mitigate the Lüders band propagation and result in a more gradual transformation.     

The enhancement of strengthening dislocated martensite

The basis of this work was the investigation of improving the tensile properties of dislocated martensites by dispersion of precipitates in the austeniteprior to the martensite transformation. Two types of precipitation-hardenable austenitic alloys were used. One is based on Fe-22 Ni-4 Mo-0.28 C where the precipitates are Mo₂C and are obtained by ausforming and aging, and the other is Fe-28 Ni-2 Ti where the precipitates are the coherent fccγ’ (Ni₃Ti) ordered phase obtained by ausaging. After the austenitic dispersion treatment both alloys were transformed to martensite by quenching to liquid nitrogen and the properties measured and compared to martensites obtained by conventional heat treatment (i.e. no precipitates in austenite). The results show that prior dispersions increase the strength of martensite and this is interpreted as being due to an increase in dislocation density resulting from dislocation multiplication at the particles during the γ →M _s transformation. In addition, the stabilities of the austenitic alloys are such that upon certain aging treatments, the alloys transform partially to martensite (due to precipitation) and “composite” materials are obtained whose strength depends on the volume fraction and yield strengths of the phases present. Formerly Graduate Student, Department of Materials Science and Engineering, University of California, Berkeley, Calif.     

Phase transitions and shape memory in NiTi

The “premartensitic” transformation of the B2 phase in NiTi was studied using electrical resistance measurement, X-ray diffraction, and shape-change effects. The degree of rhombohedral distortion of that transformation and the electrical resistance change show similar temperature dependence. In particular, whenT _R » M_s, this encompasses a narrow temperature region of rapid increase just belowT _R, followed by an extended range of levelling off. The onset of the martensitic transformation is not necessarily preceeded by the rhombohedral distortion, and the latter proceeds continuously belowT _R and persists throughout the martensite transformation range. Thus three kinds of phase transformation can occur: B2 ai R, B2 ai M, R ai M; B2 ai R is not a precursory phenomenon. Observations on the two-way memory (TWM) and its correlation to resistance changes in NiTi leads to the conclusion that whenT _R >M _s, the TWM is comprised of two stages. On cooling, the B2 → R transformation is responsible for the initial stage, which is thermally reversible without hysteresis. The second stage of the TWM, which is due to martensite formation, is reversible in shape change, but with a thermal hysteresis. It is also suggested that on heating, the R → B2 transformation can contribute to the primary shape memory effect in NiTi.     

In-Situ High-Energy X-Ray Diffuse-Scattering Study of the Phase Transition of Ni2MnGa Single Crystal under High Magnetic Field

The defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a high magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] crystallographic direction of the Heusler phase were studied by the in-situ high-energy X-ray diffuse-scattering experiments on the high energy synchrotron beam line 11-ID-C of APS and thermomagnetization measurements. Our experiments show that a magnetic field of 50 KOe applied along the [ 1[`1]0 ] \left[ {1\bar{1}0} \right] direction of the parent Heusler phase can promote the premartensitic transition of Ni₂MnGa single crystal, but puts off martensite transition and the reverse transition. The premartensitic transition temperature (T _PM ) increases from 233 to 250 K (−40 to −23 °C). The martensite transition start temperature (M _s ) decreases from 175 to 172 K (−98 to −101 °C), while the reverse transition start temperature (A _s ) increases from 186 to 189 K (−87 to −84 °C). The high magnetic field leads to a rapid rearrangement of martensite variants below the martensite transition finish temperature (M _f ). The real transition process of Ni₂MnGa single crystal under the high magnetic field was in-situ traced.     

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.     

Transformation relaxation and aging in a CuZnAl shape-memory alloy studied by modulated differential scanning calorimetry

The reverse martensitic transformation and aging processes in a polycrystalline Cu-23.52 at. pct Zn-9.65 at. pct Al shape-memory alloy have been studied using the recently developed modulated differential scanning calorimetry (MDSC) technique, and some new findings are obtained. By separating the nonreversing heat flow from the reversing heat flow, MDSC can better characterize the thermodynamic, kinetic, and hysteretic features of thermoelastic martensitic transformations. Two kinds of exothermal relaxation peaks have been identified and separated from the endothermal reverse martensitic transformation: one is associated with the movement of twin interfaces or martensite-parent interfaces, and another is due to the atomic reordering in the parent phase via a vacancy mechanism. The martensite aging processes have been examined, and two stages of the aging process been distinguished: the first stage of aging is characterized by the stabilization of martensite, as manifested in the increase in the reversing enthalpy of the reverse martensitic transformation and in the transformation temperatures, and the second stage is, in fact, the decomposition of the martensite on prolonged aging, accompanied by a decrease in the transformation enthalpy. The results suggest that the mechanisms of the relaxation in the martensite and in the parent phase may be quite different.     

A study of preferred orientation of martensite and shape change during phase transformation in Cu-Al-Ni-Mn alloy

The metallography, stress, electric resistance and temperature in Cu-12Al-5Ni-3Mn mass % alloy have been measured simultaneously during the direct and inverse martensitic transformation. The preferred orientation of martensite in a hot-rolled sample is along the rolling direction. When the sample is cooled under a tensile stress from a high temperature (>A_f) to room temperature (< M_f), the preferred orientation of martensite is along the direction which has an angle of 45° to the direction of tensile stress. On cooling, the orientation of some secondary martensite has an angle of 45° with the primary martensite. The shift of zero point of the internal friction torsion pendulum i.3. the shape change during martensitic transformation is discussed in terms of the preferred orientation of martensite.     

Effect of thermal cycling on the R-phase and martensitic transformations in a Ti-rich NiTi alloy

The effect of thermal cycling on transformation temperature was studied on a Ti-rich NiTi alloy. The study was carried out by determining the electrical resistance, the internal friction, and the elastic modulus vs temperature. This study shows that the martensite microstructure is modified by the successive cycling transformation. In addition, we established that both the martensite internal friction and the transition peak are sensitive to the transient effect (the vibration frequency lies around 300 Hz). But the major results concern the behavior associated with the R phase occurrence and its evolution. We have stated that the premartensitic phase becomes stable following the diminishment of the beginning of the martensite formation (M _s ). Interrupted cooling has also shown that, contrary to the martensite, the R phase exhibits no hysteretic behavior.     

Kinetics of martensitic transformation induced by a tensile stress pulse

The kinetics of martensitic transformation induced by a tensile stress pulse (generated by the reflection of a compressive shock wave at a free surface) in time durations in the microsecond range, were studied in an Fe-32wt%Ni-0.035wt%C alloy. The tensile hydrostatic component of stress interacts with the dilatational strain (~0.04) of the martensitic transformation and increases the M_s temperature. Shock waves were produced by normal impact of a projectile on a target in a one-stage gas gun. Impact experiments were conducted by varying either the temperature (−10 to −50°C), or pulse duration (0.22−1.76 μs) at a constant pressure. The martensitic transformation, normally considered to be athermal in Fe-Ni-C alloys, exhibits an isothermal nature in the microsecond regime. The fraction transformed increases with decrease in temperature at a constant pulse duration, and increase in pulse duration at a constant temperature. The mean volume of the lenticular martensite was found to be constant throughout the progress of the transformation, consistent with the autocatalytic spreading of clusters. The activation energies for γ→α' transformation in the Fe-32wt%Ni-0.035wt%C alloy, calculated with a modified version of Pati and Cohen's kinetic equation [Acta metall. 17, 189 (1969)], range from 38,000 J/mol at −10°C to 25,000 J/mol at −60°C. The activation energies are linearly related to the total driving force (chemical free energy change + mechanical work due to the transformation). The activation volume for the transformation was calculated and found to be equal to approximately 60 atoms (0.7nm³). This indicates that the martensitic nucleation in this alloy, and under the imposed stress conditions, is interface-mobility controlled.     

Effect of aging on shape memory behavior of Ti-51.3 At. pct ni thin films

Thin films of Ti-51.3 at. pct Ni were prepared by sputtering. The sputter-deposited thin films were solution treated at 973 K for 1 hour and then aged at various temperatures between 573 and 773 K for 3 different times of 1, 10, and 100 hours. After the heat treatment, the shape memory behavior was examined with a thermomechanical tester. The aging effects on the shape memory characteristics, such as the critical stress for inducing slip deformation, the maximum recoverable strain, and the R phase and martensitic transformation temperatures, were discussed based on transmission electron microscopic (TEM) observation of the microstructure in the age-treated thin films. In all the age-treated thin films, the presence of Ti₃Ni₄ precipitates was confirmed. When the precipitate diameter was less than 100 nm, the shape memory characteristics were very sensitive to the microstructure. The aging effects on the shape memory characteristics of the Ni-rich Ti-Ni thin films were found to be almost consistent with those reported in bulk specimens, and, thus, the shape memory behavior of the sputter-deposited thin films of Ni-rich Ti-Ni can be controlled in the same manner as that of bulk specimens.     

Characterization of the Martensitic Transformation in NiPtAl Alloy Using Digital Holographic Imaging

Surface reliefs due to phase transformations in a 56.8Ni-5.6Pt-37.6Al at. pct alloy were characterized in situ using digital holographic imaging during thermal cycling from room temperature up to 405 K (132 °C). The 3D images of the surface revealed that the austenite plates formed during heating are exactly the same for each cycle, which is not the case for the martensite plates formed during cooling. The martensite start temperature was found to vary by up to ~ 20 K from one grain to another within the same specimen. The absence of Ni₃Al γ′ precipitates, due to the relatively high Al content, results in the propagation of the martensitic transformation over grains up to a millimeter in size. Bright-field optical imaging showed the formation of large martensite plates in some grains, with cracks perpendicular to these plates, upon cycling. Cracks were also observed at grain boundaries and could be related to the height variations across the grain boundaries.