Monoclinic Martensite Research Articles

Elastic constants of Ti50Ni30Cu20alloy prior to martensitic transformation

Abstract Elastic constants of Ti50Ni30Cu20 alloy prior to the martensitic transformation B2 → B19 was measured for the first time by using the rectangular parallelepiped resonance method. The softening in both shear modulus ć and c44 was found with approaching transformation temperature. As a result of soft c 44, the anisotropy factor shows a low value of 2.8 in the vicinity of the transformation temperature. c 11, c 12, and bulk modulus were found to show little temperature dependence in the vicinity of the transformation temperature. Compared with the binary Ti50Ni50 alloy, Ti50Ni30Cu20 alloy exhibits a 40% higher and increasing anisotropy with approaching transformation temperature. Based on the experimental results, we explained why Ti50Ni30Cu20 transforms into B19 martensite, while Ti50Ni50 transforms into a strange monoclinic B19 martensite.

Temperature dependence of the second-order elastic constants of Cu-Zn-Al shape-memory alloy in its martensitic andβphases

The temperature dependence of the sound velocities for 13 propagation modes has been measured in a single crystal of Cu-Zn-Al monoclinic 18R martensite, using the pulse-echo method. By numerical procedure the complete set of nine second-order adiabatic elastic constants ( C ij ) of the closest orthorhombic reference phase, their relative thermal variation ( G ij ), and the Debye temperature (u D) have been obtained. The values found in the martensitic phase have been compared to data avalaible for the high-temperature bcc b-phase in the same alloy system. The velocity surfaces in the corresponding crystallographic directions of both phases have also been computed at different temperatures. It has been shown that the mechanical stability of the lattice for some particular distortions decreases as the transformation temperature is approached in both the martensitic as well as in the b phase. @S0163-1829~97!07033-1#

Shock-induced martensitic transformations in near-equiatomic NiTi alloys

Shock-impact generated tensile-stress pulses were used to induce B2-to-monoclinic martensitic transformations in two near-equiatomic NiTi alloys having different martensite transformation start (M s ) temperatures. The NiTi-I alloy (M s ≈+27 °C) impacted at room temperature at 2.0 and 2.7 GPa tensile stress-pulse magnitude, showed acicular martensite morphology. These martensite needles had a substructure containing microtwins, typical of “stress-assisted” martensite. The NiTi-II alloy (M s ≈−45 °C) showed no martensite formation when shocked with tensile-stress pulses of 2 GPa. For tensile stresses of 4.1 GPa, the alloy showed spall initiation near the region of maximum tensile-stress duration. In addition, monoclinic martensite needles, with a well-defined dislocation substructure, typical of “strain-induced” martensite, were seen clustering around the spall region. No stress-assisted martensite was formed in this alloy due to its very low M s temperature. The present article documents results of the use of a metallurgical technique for generating large-amplitude tensile stress pulses of finite duration for studies of phase transformations involving changes from a high density to a low density state.

Evolution of the crystal structure during phase working of titanium nickelide

Using x-ray diffraction we have measured the dynamic and static atomic displacements and the unit cell parameters of monoclinic martensite in the intermetallide 50.1 at. %Ti-Ni, during thermal cycling through the interval of the martensitic transition (280 to 400 K). It is shown that along with a change in the sequence of transitions from B2→B19′ to B2→R→B19′, there is also a change in the atomic displacements from predominantly dynamic on the first cycles to static displacements during the later ones. The martensite unit cell has a larger volume on cooling than on heating, and cycling leads to a lowering of its monoclinic angle.

The influence of post-bond heat treatment on microstructural development in martensite-forming NiAl-NiTi transient liquid phase bonds employing copper interlayers

An investigation is presented of the influence of post-bond heat treatment on microstructural development in transient liquid phase bonds between L10 martensite-forming Ni-24 at% Al-16 at% Cr and nominally stoichiometric NiTi which transforms to monoclinic (distorted B19 type) martensite. The joints discussed in this article were prepared using 50 μm thick commercial purity copper (99.8 wt% Cu) interlayers and were bonded at 1150°C for 1 h. The resulting bonds were post-bond heat treated at 1000°C for times varying between 4 and 12 h and then characterized by edge-on transmission electron microscopy.

The R-Phase Transformation in the Ti-Ni Shape Memory Alloy and its Application

ABSTRACTWe discuss a comprehensive design approach of Ti-Ni alloy coil springs and introduce a new application of the R-phase transformation. In order to attain high cyclic performance, one must understand the two relationships between design parameters and material characteristics and between material characteristics and cyclic performance. Metallurgical parameters and coil spring dimensions play an important role as design parameters in the former relationship. High cyclic performance of an actuator is closely related to the suppression of the monoclinic martensite. Transformation temperatures and their stress dependence is of primary importance as material characteristics in the latter relationship. A thermostatic mixing valve, which is the latest application of the R-phase transformation in Japan is then discussed as a new type of a shape memory alloy actuator. The R-phase transformation is employed to achieve not only a long cycle life but a linear operation with the set temperature to continuously control the mixing ratio of hot and cold water. This is achieved by changing the total length of the two-way actuator in a linear manner with the set temperature. The linear characteristic is satisfied between 35–50°C by optimizing thermomechanical treatment and the dimensions of Ti-Ni and biasing coil springs.

Mechanism of B2-B19-B19′ Transformation in Shape Memory Ti–Ni–Cu Alloys

Martensitic transformations which occur in Ti-45.5Ni-5.0Cu and Ti-44.0Ni-5.0Cu-1.0Al (at%) alloys were investigated by means of electrical resistivity measurements, X-ray diffraction analysis and transmission electron microscope observation. It was found that both the two alloys transform in two steps on cooling, i.e., from the high temperature cubic phase (B2-type structure) to the orthorhombic martensite (B19-type structure), and then to the monoclinic martensite (B19'-type structure). The two-step transformations in these alloys proceed as follows. On cooling, plates of orthorhombic martensite nucleate at certain sites in the cubic matrix. While the plates of orthorhombic martensite are in growth, plates of the monoclinic martensite nucleate in the orthorhombic martensite and grow consuming it. The transformation from the orthorhombic martensite to the monoclinic martensite occurs by shearing on (001) o plane to [100] o direction. A plate of the monoclinic martensite which have grown in this way consists of thin plates of (001) m compound twin as a result of alternative shearing to [100] o and [100] o direction. It is difficult to differentiate the successive two transformations of these alloys described above by electrical resistivity measurement or X-ray diffraction because the two transformations overlap and the amount of the orthorhombic martensite in the specimen through the two-step transformation is very small.

Electronic and Crystal Structure of the ZrCu Intermetallic Compound Close to the Point of Structural Transformation

AbstractIt is found that cooling of the B2 high‐temperature phase of ZrCu above the martensitic transition (MT) point is not accompanied by essential changes of its electronic structure. MT in ZrCu is realized generally by the formation of two monoclinic martensites with positive volume effect. The electronic structure of the B2 phase of ZrCu demonstrates a sharp peak of the electron density of states just above the Fermi level, which plays a stabilization role related to the shear deformation above MT. This stabilization effect rapidly weakens on cooling and can be the possible origin of the lattice instability of the high‐temperature B2 phase of the ZrCu intermetallic compound.

The diffraction patterns of crystals with layer defects

A method for the calculation of X-ray and neutron diffraction profiles of crystals with layer defects is described. Averages over a computer model of a configurational ensemble of crystallites with defects are used to define the scattering cross section. The method easily accommodates the modeling of complex defect geometries or correlations. Diffracted intensity profiles for the NixAl1−x (5, \overline 2) monoclinic martensite, trigonal polytypes and body-centered-cubic crystals are presented as examples.

In situ high resolution electron microscopy observation of electron-irradiation-induced martensitic transformation in a Ni 63Al 37 alloy

An electron-irradiation-induced transformation from cubic β 2 austenite to monoclinic 7M martensite was observed in situ under high resolution electron microscopy conditions in a thin foil of Ni 63Al 37 β 2 phase (B2, CsCl structure), with the start temperature of the martensitic transformation below but very close to room temperature. The structure of the martensite is consistent with thermoelastic or stress-induced martensite at this composition, including the existence of multiple stacking faults. The transformation is described by a gradual increase in the shear and shuffle amplitudes already existing in the precursor domains. Possible origins of the transformation are discussed.

TEXTURE DEVELOPMENT AND TEXTURE INHOMOGENEITIES IN A HOT-ROLLED CuZnAl SHAPE MEMORY ALLOY

The texture development in a hot rolled Cu-14.2wt.%Zn-5,6wt.%Al in the surface and centre of the sheet was studied. The observed texture in the centre of the sheet can be understood in terms of the {001)(011) and {211)(011) hot rolling texture in the austenite phase and the reported crystallographic orientation relationships between monoclinic martensite and b.c.c. austenit. In the surface of the sheet the texture was developed with the influence of shear deformation. Therefore in the martensitic phase only the orientation components were observed which were originated from the {001)(011) of the b.c.c. phase. Besides the variant selection of the crystal orientation was observed during the pahse transformation in the surface of the sheet.

In situ X-ray investigation of deformation martensite in titanium nickelide

X-ray structural analysis was used to carry out an investigation of the martensitic transformation in titanium nickelide during mechanical loading. A diffractometer with filtered cobalt radiation was used, along with an attachment that allowed the sample to be loaded by bending. No R-martensite was observed during loading. The structure of deformation martensite was identified as monoclinic B19′. When deformation is increased, the martensite has a more complicated structure and was identified as a mixture of monoclinic B19′ and triclinic B19″ phases. When the load is removed, the triclinic martensite disappears without hysteresis, whereas a small amount of monoclinic martensite remains and disappears only when heated to 40–60‡C.

Thermal Hysteresis in Ni-Ti and Ni-Ti-X Alloys and Their Applications

AbstractThe first practical shape memory alloy, NITINOL, has a transformation thermal hysteresis(Hs) of about 30K and a monoclinic martensite structure. The first application of this alloy was a pipe coupling. Although various ideas were proposed on thermal actuators, they were not easily industrialized at least partly because the amount of Hs of the binary Ni-Ti alloy was too large. While the reduction of Hs is not so easy as transformation temperature control, the alloy was improved in terms of Hs through the discovery of new martensite phases. The first Hs reduction was realized in a Ni-Ti-Cu alloy. The copper addition changes the martensite structure to orthorhombic and reduces Hs to 10-15K. The alloy was first applied to a thermal actuator in automobile fog-lamps. Subsequently the R-phase transformation was identified in the binary alloy under the combination of cold working and a heat treatment at relatively low temperatures. Hs was further reduced to 2K in the R-phase transformation. The R-phase transformation is also featured by an excellent fatigue life which reaches one million cycles. The small Hs and long repetition life rapidly widened the application field of shape memory actuators. An expansion of Hs up to 100K was also achieved by a combination of fine dispersion of niobium and cold working. At present, the Hs of the Ni-Ti base alloys ranges from 2K to 100K. An appropriate alloy system can be selected according to applications. The binary alloy has found a wider application field in the super elastic application. In this paper, we summarize the progress in the modification of Hs and present recent applications of each transformation.

Orthorhombic Martensite, Intermetallic Precipitates and Retained Austenite in Ti-Rich Ti(Ni+Cu) Sputtered thin Films

AbstractPeriodic multilayered titanium-rich Ni-Ti thin films were prepared by magnetron sputtering from alternating Ni45Ti50Cu5 alloy and pure titanium targets, with an alloy-layerfl'i-layer thickness ratio of 9:1. The microstructure, martensite transformation behavior, and precipitate and defect structures were studied in films which had been annealed at 923K for one hour and furnace cooled at <20 K/min. Energy dispersive X-ray fluorescence measurements showed that the resulting films had a hyperstoichiometric titanium content of approximately 51 atomic percent. Ti2Ni precipitates were found in the annealed structures which were oriented with [1 1 0 ]Ti2Ni parallel to [110]B2 and (001)Ti2Ni parallel within +/− 10 to (001)B2. Differential scanning calorimetry (DSC) revealed an unusually low transformation enthalpy for the martensite reaction in the film (9.1 J/g as opposed to 20.7 J/g for the alloy sputtering target), and a significant fraction of residual B2 austenite was found at temperatures well below the nominal Mf. The martensite transformation was found to occur in two steps involving, on cooling, the initial formation of an orthorhombic martensite prior to transformation to the monoclinic martensite phase at low temperature.

Twinning in ternary Ti50Ni(50−x)Mx alloys

Twinning in materials results from one of two different effects. Firstly, thermal changes may give rise to growth twins during solidification, annealing twins during recrystallization, or martensitic twins during solid-state phase transformations. Secondly, shear stresses can activate mechanical twinning as a deformation mode other than dislocation slip. TiNi alloys are unusual in that they exhibit both types of twinning. Due to the submicron dimension of both types of twins, transmission electron microscopy (TEM) is used to characterize the resulting structures.TiNi is well-known for its shape memory properties resulting from a martensitic transformation of the cubic B2 structure to the monoclinic B19' structure upon cooling. Minor concentrations of various third elements can be added to modify different shape memory properties. In the case of TiNiCu ternary alloys, up to 30% Cu is substituted for Ni while retaining the shape memory effect. For small Cu concentrations the B2 austenite phase transforms, upon cooling, to the B19' monoclinic martensite typical of the binary alloy.

Polytypic structures and morphology of martensite in alloy Ti50Ni40CU10

The methods of x-ray diffraction analysis and optical metallography were used to study the crystallography and morphology of martensite in alloy Ti50Ni40Cu10. It was found that polytypic structures of monoclinic martensite B19′ with 2H- and 4H-packings are formed in forward and reverse martensite-martensite transformations, while the morphology of martensite B19′ is pyramidal. Martensite transformation into a B2-superstructure develops through the formation of new martensite B19 pyramids near previously formed pyramids, each of them being formed by the burst mechanism.

Electron microscopy studies of martensitic transformations in Ti 50Ni 50 − xCu x alloys. Part II. Morphology and crystal structure of martensites

A transmission electron microscopy study has been made of the different martensites that form in Ti 50Ni 50 − x Cu x shape memory alloys in the composition range x = 1 to 19. The main product in alloys where x ≲ 8 was a monoclinic martensite involving ( 11 1 ) transformation twins, similar to the martensite found in classical binary TiNi alloys, and in agreement with previous reports. However, in addition, two different types of martensite were also frequently observed, especially in the composition range x ⋍ 4 to 8 . One of these had a monoclinic structure similar to the preceding but involving (001) twins [rather than ( 11 1 )], and the other had a 4H-type structure involving (001) stacking faults. In the 19 at.% Cu alloy a unique martensite was found with an orthorhombic structure but involving none of the characteristics internal defects usually associated with a lattice invariant deformation. It is thus concluded that the crystallography of martensitic transformations in Ti 50Ni 50 − x Cu x alloys changes considerably with increasing cooper content, in contrast to previous reports.

The structure of NiTiCu shape memory alloys

A combined electron microscopy and X-ray diffraction study has been made of NiTiCu shape memory alloys where Cu is substituted for Ni in binaryβ NiTi. It is shown that the high temperature phase with the CsCl structure is retained for copper contents of up to 30 wt pct, and that this phase transforms on cooling to a monoclinic martensite with similar morphology and lattice parameters to those previously reported for the binary NiTi alloy. Initial substitution of Cu for Ni produced slight changes in the lattice parameters of both the high and low temperature phases, then these remain remarkably constant on further addition of Cu. The chief effect of the Cu substitutions is to reduce the distortion needed to form martensite from the parent austenite. An attempt to explain the effect of the Cu is made in terms of a few simple alloying theories, but it is concluded that an effective treatment must consider the interplay of several material parameters.

Martensitic transformation in near-equiatomic TiNi alloys

The crystal structure and morphology of the composition have been studied by utilizing electron martensitic phases in TiNi alloys of near-equiatomic diffraction and transmission electron microscopy. Two monoclinic martensites, β′ and β″, are observed to form as a result of the low temperature instability of the parent β phase. These transformations are similar to the 2H modulation in the AuCd system. The 2H modulation in the AuCd system has an orthorhombic symmetry whereas in the present case the symmetry is monoclinic. The β′ and β″ phases form as alternating platelets, occasionally showing dislocations at the common interface.

Crystal structure and internal defects of equiatomic TiNi martensite

The crystal structure and internal defects of equiatomic TiNi martensites have been studied using the techniques of electron microscopy, electron diffraction, and X-ray diffraction. The crystal structure may be described as of distorted B19 type with a monoclinic unit cell, the lattice parameters of which are a = 2.889 A, b = 4.120 A, c = 4.622 A, and β = 96.8°. The structure is derived from the matrix B2 in two steps. Namely, the matrix B2 transforms into a B19 first, and then a monoclinic martensite is derived by shearing the B19 in [100]B19 direction on (001)B19 plane. As an internal defect of the martensite, transformation twins with (111) twinning plane have been found without ambiguity, whose presence is essential for the understanding of the unique memory effect in this alloy. The orientation relationship between the matrix and martensite, and some characteristics of a pre-martensitic transition are also described.