B19 Martensitic Transformation Research Articles

Martensitic transformation behaviors of Ti-rich Ti–Ni alloy fibers fabricated by melt overflow

Ti50Ni50, Ti50.5Ni49.5, Ti51Ni49 and Ti51.5Ni48.5 fibers were fabricated by melt overflow process. The rapid solidification can increase the solubility above the equilibrium solubility. The effects of the rapid solidification of Ti-rich Ti–Ni alloys on the microstructure, transformation temperatures and shape memory characteristics are investigated. The addition of 0.5at.% Ti in Ti50Ni50 alloy greatly increases the transformation temperature. However, the transformation temperatures decrease again for Ti content exceeding 51at.%. Results of thermal cycling tests under various constant stress levels reveal that the recoverable elongation associated with B2–B19 martensitic transformation of Ti50.5Ni49.5 fibers is two times larger than that of Ti51.5Ni48.5 alloy fiber.

Structure, Phase Transformations and Properties of Rapidly Quenched Ti<sub>2</sub>NiCu Alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase and chemical compositions and structure of Ti2NiCu alloys. The alloys of the quasi-binary section TiNi–TiCu, which exhibit in the initial as-cast state thermoelastic martensitic transformations B2↔B19 and related shape memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti2NiCu alloys was varied with respect to titanium and nickel within x ≤ ±1 at. %. The mechanical properties of the alloys have been measured in the initial state and after subsequent heat treatment. The kinetics of the crystallization from the amorphous state, devitrification processes and the forward and reverse thermoelastic martensitic transformations were investigated. Their characteristic temperatures have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition has been constructed.

Martensitic transformation behaviors of rapidly solidified Ti–Ni–Mo powders

For the fabrication of bulk near-net-shape shape memory alloys and porous metallic biomaterials, consolidation of Ti–Ni–Mo alloy powders is more useful than that of elemental powders of Ti, Ni and Mo. Ti50Ni49.9Mo0.1 shape memory alloy powders were prepared by gas atomization, and transformation temperatures and microstructures of those powders were investigated as a function of powder size. XRD analysis showed that the B2–R–B19 martensitic transformation occurred in powders smaller than 150μm. According to DSC analysis of the as-atomized powders, the B2–R transformation temperature (TR) of the 25–50μm powders was 18.4°C. The TR decreased with increasing powder size, however, the difference in TR between 25–50μm powders and 100–150μm powders is only 1°C. Evaluation of powder microstructures was based on SEM examination of the surface and the polished and etched powder cross sections and the typical images of the rapidly solidified powders showed cellular morphology. Porous cylindrical foams of 10mm diameter and 1.5mm length were fabricated by spark plasma sintering (SPS) at 800°C and 5MPa. Finally these porous TiNi alloy samples are heat-treated for 1h at 850°C, and then quenched in ice water. The bulk samples have 23% porosity and 4.6g/cm3 density and their TR is 17.8°C.

Effect of the deviation of the chemical composition from the stoichiometric composition on the structural and phase transformations and properties of rapidly quenched Ti50 + x Ni25 − x Cu25 alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and electron diffraction have been used to study phase composition and structure of an almost stoichiometric alloy Ti50Ni25Cu25. The alloys of the quasi-binary section TiNi-TiCu to be studied, which exhibit in the initial ascast state thermoelastic martensitic transformations B2 ↔ B19 and related shape-memory effects, have been produced by rapid quenching of the melt (melt spinning technique). The chemical composition of the Ti50 + xNi25 − xCu25 alloys was varied with respect to titanium and nickel within x ≤ ±1% (from Ti49Ni26Cu25 to Ti51Ni24Cu25). It has been shown that the rapid quenching from the melt at a cooling rate of 106 K/s provides amorphization for all the alloys under consideration. Heating to 723 K or higher temperatures leads to the devitrification of the amorphous alloys with the formation of a polycrystalline structure of the B2 austenite. The mechanical properties of the alloys have been measured in the initial amorphous state and after subsequent heat treatment. It has been established that, depending on the degree of deviation of the alloy from the stoichiometric composition, which leads to solid solution decomposition in the process of nanocrystallization upon heat treatment, there occur regular changes in the mechanical properties and shape-memory effects of the alloys. The characteristic temperatures of the onset and finish of the process of crystallization from the amorphous and amorphous-crystalline states and the critical temperatures of the onset and finish of the forward and reverse thermoelastic martensitic transitions have been determined by measuring temperature dependences of the electrical resistivity of the alloys. The diagram of the dependence of the critical temperatures on the chemical composition of the alloy has been constructed.

Thickness-dependence of the B2–B19 martensitic transformation in nanoscale shape memory alloy thin films: Zero-hysteresis in 75 nm thick Ti 51Ni 38Cu 11 thin films

The influence of film thickness on the B2–B19 martensitic transformation properties of nanoscale Ti 51Ni 38Cu 11 thin films with thicknesses ranging from 750 to 50 nm is reported. For these films an unexpected behavior of the phase transformation temperatures was observed: A f and O s initially decrease with decreasing film thickness but increase sharply again for thicknesses <100 nm. The phase transformation temperatures and thermal hysteresis width range from 58 to 35 °C ( A f ) and 14 to ∼0 K, respectively. For the first time we can show that substrate-attached Ti–Ni–Cu thin films as thin as 50 nm show reversible B2–B19 phase transformations. Furthermore, it is shown that with decreasing film thickness a change in the tetragonality of the B19 martensite phase occurs. This leads to fulfilling the so-called λ 2 criterion, causing a vanishing hysteresis for a film thickness of 75 nm.

Structure of domains formed upon martensitic transformations B2 → B19 and B2 → B19′ in alloys with a B2 superstructure

An analysis of symmetry changes upon the structural B2-ISS2 phase transition with subsequent martensitic transformations into the B19 and B19′ phases has been performed. An interrelation between the atomic displacements and the lattice deformation has been shown to exist. It has been established that the structures with longitudinal and transverse displacements have the same symmetry and can pass into one another via a twinning operation. A sequence of transformations of the tensor of lattice deformations consistent with the transformations of the atomic displacements has been obtained. It has been found that the operations that provide these transformations are the operations of twinning.

PHASE FIELD SIMULATION OF EFFECTS OF PORES ON B2-R PHASE TRANSFORMATION IN NiTi SHAPE MEMORY ALLOY

Recently, porous NiTi shape memory alloys (SMAs) have drawn great interest in vari- ous engineering fields, in particular for biomedical applications as one of the promising biomaterials for hard-tissue replacements and orthopedic implants. It is well known that the porous NiTi SMAs exhibit three transformations, B2-B19 � , B2-R and R-B19 � . Among these phase transformations B2-B19 � and R-B19 � involve high lattice distortion and large transformation hysteresis. Consequently, these distortion and transformations usually introduce structural defects which may result in degradation of mechanical stability for the functional application. On the contrary, B2-R transformation is governed by small lattice distortion which indicates less damage to the microstructure and lower sensitivity to the defects, and thus generates higher reversibility and mechanical stability. Due to these unique virtues, it is of great importance to study the R-phase transformation behavior in porous NiTi SMAs, since the pore has a significant influence on B19 � martensitic transformation as well as the R-phase transforma- tion. In this paper, a three-dimensional phase field model aiming at accounting for the pore effect on phase transformation in NiTi SMAs was developed to study the B2-R phase transformation behavior in porous NiTi SMAs. The model was applied to characterize the microstructure evolution of B2-R phase

Martensitic transformation behaviors of Ti49+xNi21−xCu30 (x=0,1,2,3) shape memory alloy strips

Four batches of Ti–Ni–Cu strips (Ti49Ni21Cu30, Ti50Ni20Cu30, Ti51Ni19Cu30 and Ti52Ni18Cu30) were prepared by an arc melt overflow technique. The microstructure of as-cast strips exhibited columnar grains normal to the strip surface. XRD analysis showed that B2–B19 martensitic transformation occurred in all alloy strips. The martensitic transformation start temperature (Ms) of the Ti49Ni21Cu30 strip was 54.6 °C and continued to increase with increasing Ti-content. During cycle deformation with an applied stress of 120 MPa, transformation hysteresis and elongation associated with the B2–B19 transformation were observed to be 4.1 °C and 1.84% in the Ti49Ni21Cu30 strip. The transformation hysteresis associated with the B2–B19 transformation increased with increasing Ti-content, while the elongation decreased with increasing Ti-content. Even though it is known that Ti–Ni–Cu alloys with high Cu-content (more than 13 at % Cu) are too brittle to deform plastically, the rapidly solidified Ti–Ni–Cu alloy strips, which contain 30 at % Cu, have excellent shape memory characteristics and mechanical properties.

Internal Friction of Ti-Rich Ti-Ni-Cu Melt-Spun Ribbon

Martensitic transformation (MT) and the internal friction (IF) behavior of the initially amorphous Ti-rich Ti-Ni-Cu melt-spun ribbon annealed at 400°C for 10 h were investigated by using DSC and dynamic mechanical analysis (DMA). The annealed ribbon shows B2 structure with strong preferential (110) orientation and exhibits B2↔B19 MT with the temperature hysteresis of 13°C. DMA tests show that the internal friction is frequencies dependence at 1-50 Hz. The tanδ values decrease with the increasing frequency, but it does not meet the 1/f dependence. Isothermal DMA tests show that the tanδ value at a temperature of 15°C in the MT zone increases quickly from 0.005 to 0.045 with the increasing isothermal intervals and keep stable subsequently. The isothermal time to complete the MT can be determined by the DMA tanδ curve directly.

Martensitic Transformation of Ti&lt;SUB&gt;50&lt;/SUB&gt;Ni&lt;SUB&gt;25&amp;minus;&lt;I&gt;X&lt;/I&gt;&lt;/SUB&gt;Pd&lt;SUB&gt;25&amp;minus;&lt;I&gt;Y&lt;/I&gt;&lt;/SUB&gt;Cu&lt;SUB&gt;&lt;I&gt;X&lt;/I&gt;+&lt;I&gt;Y&lt;/I&gt;&lt;/SUB&gt; Quaternary Shape Memory Alloys with &lt;I&gt;X&lt;/I&gt;, &lt;I&gt;Y&lt;/I&gt;&amp;le;10 at%

Ti 50 Ni 25-X Pd 25-Y Cu X+Y high temperature shape memory alloys (HTSMAs) with X, Y ≦ 10 at% have been studied by DSC, XRD, TEM/ EDS, hardness and cold-rolling workability tests. Experimental results show that all these HTSMAs exhibit B2 ↔ B19 martensitic transformation with Ms temperature can be predicted by the linear regression as: Ms (K) = 424.2 + 2.7X — 9.7Y. The substitution of Ni/Pd by Cu affects the lattice constants of B 19 martensite, the hardness and the cold-rolling workability which can be explained by the different atomic radii of Ni, Cu and Pd. Ti 50 Ni 15 Pd 25 Cu 10 HTSMA annealed at 523 K up to 9 weeks and thermal-cycled up to 100 times still has quite good thermal stability. However, Ti 50 Ni 15 Pd 25 Cu 10 HTSMA annealed in between 723 K∼923 K shows obvious decrease for both Ms temperature and ΔHc value, especially at 823 K due to Ti 2 Pd and Ti(Cu,Pd) 2 precipitated during annealing.

Relationship between grain size and martensitic transformation start temperature in a Ti–30Ni–20Cu alloy ribbon

The relationship between the parent phase grain size and the B2–B19 martensitic transformation start temperature (Ms) was investigated in a Ti–30Ni–20Cu alloy ribbon fabricated by melt spinning. The average grain size of the ribbons increased from 0.7 to 1.2 μm with increasing crystallization temperature from 748 to 823 K without significant compositional change and precipitation. Ms increased from 308 to 317 K with increasing crystallization temperature from 748 to 823 K, which was ascribed to the increase in grain size.

Microstructure modifications of Ti–Ni–Cu shape memory alloy strips fabricated by arc melt overflow process

Six batches of Ti50Ni50-xCux alloy (x=0–25) were prepared by a melt overflow process and strips of about 550 μm thickness were fabricated by rapid solidification. Microstructures and shape memory characteristics were investigated by means of x-ray diffraction (XRD), optical microscopy, differential scanning calorimetry and thermal cycling tests under a constant load. The microstructures of the as-cast strips exhibited columnar grains normal to the strip surface. XRD showed B2–B19′ martensitic transformation in the Ti50Ni50 binary alloy strip, while B2–B19 martensitic transformation in all the ternary alloy strips. The transformation temperature (Ms) increased with Cu-content. During cycle deformation with an applied stress of 60 MPa, the smallest transformation hysteresis and the largest elongation associated with the B2–B19 transformation were observed to be 3.5% in Ti50Ni25Cu25 alloy strip and 3.02% in Ti50Ni40Cu10 alloy strip.

Martensitic transformation of annealed Ti 50Ni 25Cu 25 melt-spun ribbons

Martensitic transformation of annealed melt-spun Ti 50Ni 25Cu 25 ribbons is investigated. Experimental results reveal that Ti 50Ni 25Cu 25 ribbons annealed at a temperature above or below T x have identical crystallization behavior and exhibit B2 ↔ B19 martensitic transformation. Ti 50Ni 25Cu 25 ribbons can have good shape memory effect and damping capacity when annealed under appropriate conditions to acquire abundant crystallized grains with low Cu content. In order to obtain the optimal shape memory effect, it is better to anneal crystallized ribbons at temperature between T g and T x because crystallization of ribbons is slower and it is easier to control their properties.

Shape memory behavior of Ti–Ni–Cu thin films

Three kinds of Ti-rich Ti–Ni–Cu thin films (Ti 51.9Ni 41. 6Cu 6.5, Ti 51.6Ni 36.8Cu 11.6, Ti 51.5Ni 33.1Cu 15.4) and three kinds of (Ni, Cu)-rich Ti–Ni–Cu thin films (Ti 48.9Ni 44.9Cu 6.2, Ti 48.5Ni 40.0Cu 11.5, Ti 48.6Ni 35.9Cu 15.5), numbers indicate at.%, were prepared by sputtering and annealed at 773, 873 and 973 K for 1 h. X-ray diffraction patterns showed that the Ti-rich thin films contain Ti 2Ni and Ti 2Cu phases, while the (Ni, Cu)-rich thin films contain a TiNiCu phase. The B2 → B19′ martensitic transformation occurred below 10 at.% Cu, while the B2 → B19 martensitic transformation occurred above 10 at.% Cu. The transformation temperature increased with increasing Cu content and annealing temperature. The temperature hystereses were small compared with those of Ti–Ni binary alloy thin films. The maximum recoverable strain decreases with increasing annealing temperature. It also decreases with increasing Cu content when the Cu content is more than 10 at.%. The critical stress for inducing plastic deformation increases with decreasing annealing temperature and increasing Cu content.

Effect of Hydrogen on the Crystallization of Amorphous Ti50Ni25Cu25 Titanium Alloy

We describe effects unknown earlier in the physics of interaction between hydrogen and amorphous metallic alloys: an increase in the crystallization temperature of hydrogen-containing Ti50Ni25Cu25 alloy in the course of its heating and suppression of the inverse martensitic transformation B2 → B19 after crystallization.

Effect of Hydrogen on the Properties of Rapidly Quenched Tini-Ticu Alloys with Shape Memory

We show that, after the introduction of hydrogen into rapidly quenched Ti50Ni25Cu25 alloys in the crystalline, amorphous, or amorphous-crystalline state, their shear modulus and electric resistance change appreciably. Subsequent thermocycling of these alloys is evidence of the suppression of direct and inverse martensitic transformations B2 ↔ B19 (B19′) due to hydrogen.

Neutron diffraction effects before the B2 ↦ B19 martensitic transformation in single crystal of binary TiNi based alloy

Experimental results of the microstructure changes of the B2 phase before the B2↔B19' martensitic transformation are presented in the paper. The neutron diffraction patterns from the B2 phase of the Ti 49 Ni 51 single crystal were obtained in the wide range of the reciprocal space. The neutron diffraction features in the [110] B2 direction and perpendicular to it are in details investigated in the work.

Structural anelasticity of NiTi during two-stage martensitic transformation

The two-staged thermoelastic martensitic transformation (TMT) B2→R→B19′ in polycrystalline equiatomic NiTi has been studied by means of measurements of strain amplitude-independent and amplitude-dependent internal friction (ADIF), Young’s modulus and amplitude-dependent modulus defects. The internal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent on the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelasticity. Based on the data obtained, the anelasticity is associated with the dislocations inside the martensitic variants, not with the interfaces or interface dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in the B19′ martensitic phase. This observation is explained by a much higher density of the dislocations inside the variants of the R phase as compared with that of the B19′ phase.

The Effect of Specimen Size on the B2⇆B19 Martensitic Transformation under Quasi-Static Condition in 49.5Ti-40Ni-10Cu-0.5Al(at%) Alloy

The necessity of revision of the conventional thermodynamic scheme for martensitic transformation(MT) is presented by deducing specimen mass(size) dependence of MT chracteristics under quasi-static condition(QSC). The B2⇔B19 MT characteristics in specimens with various mass w in 49.5Ti-40Ni-10Cu-0.5Al(at%) alloy have been investigated as a function of scanning rate T through DSC measurements. With decreasing T, experimental equilibrium temperatures, To * =(Ms+Af)/2, have shifted downwards and transformation temperature intervals, Ms-Mf, have reduced. By extrapolating T dependencies of MT characteristics to T=0, the MT characteristics under infinitesimally slow T, i.e., under QSC, may be deduced. Even under QSC, the MT characteristics have shown w dependencies in To *QS and in Ms QS -Mf QS , but not in the supercooling for the onset of transformation, To *QS -Ms QS .

Influence of Deformation Speed on Fatigue and Tensile Properties of a Ti-Ni-Cu Shape Memory Alloy

AbstractThe mechanical fatigue life and tensile property of a Ti-41at.%Ni-8.5at.%Cu alloy, which was solution-treated after some thermo-mechanical treatments, were investigated at 370±1 K as a function of deformation speed. The tensile properties were also investigated at 295±1 K as a function of deformation speed. The B2→B19 martensitic transformation start temperature, Ms, of the alloy was determined to be 338 K by a differential scanning calorimetry, while that of the as-rolled alloy could not be determined. Two types of fatigue tests were carried out by using plate-shaped specimens of 3.5 mm width and 1.5 mm thickness with sinusoidal waveform stresses of 20 Hz and 0.5 Hz frequencies being applied respectively. The fatigue life obtained from the 20 Hz tests was superior to that from the 0.5 Hz tests, the former being about 2 times longer than the latter at the same stress level. Tensile tests were performed at three tensilem speeds of 8.3 × 106, 8.3 × 10−5 and 8.3 × 10−3 m/s. In all the stress - strain curves obtained, an apparent yielding was observed after the liner elastic deformation. The apparent yielding is due to the occurrence of martensitic transformation. The critical stress for inducing martensite and tensile fracture stress increased with increasing tensile speed. Therefore, it is clear that fatigue and tensile properties of Ti-Ni-Cu shape memory alloys are strongly affected by not only test temperature but also deformation speed.