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Results of in-situ neutron diffraction studies of the B2↔R transformation and R-phase reorientation processes in thermal, mechanical and thermomechanical loads on NiTi alloy polycrystals are presented. It is discussed how the crystallographic features and structure instability of R-phase affects the macroscopic thermomechanical σ - e - T response of NiTi when the R-phase is involved.

The atom positions in the structure of the R-phase in a TiNi(Fe) intermetallic compound have been refined using the multi-slice least-squares method based on electron diffraction data obtained from 50 nm sized regions in a CM30 FEG TEM instrument. The refinement accounts for dynamic scattering including the specimen thickness and crystal misorientation as refining parameters. Compared with the original positions in the (111) planes of the parent B2 phase, each third Ti and Ni plane in the R-phase separates into three layers with different z-coordinates. The final data reveal a centre of symmetry in the R-phase structure leading to the P3 space group in contrast to earlier suggestions of a P3 or P3 lm group. Finally, it is suggested that the fine scale antiphase-like domains observed in the R-phase appear due to mirror planes over which the above mentioned atomic shifts in the structure are reversed. This phenomenon is expected to cause an artificial symmetry increase in the space group determination when using CBED with a probe size close to the size of the domains.

We investigated the phase composition of (i) solution annealed and subsequently aged Ti-50.7 at.-\%Ni alloy and (ii) solution annealed Ti-50.1 at.-\%Ni alloy cycled 30 times between 100°C and -196°C by x-ray diffraction through the martensitic phase transformation. Both samples show \"two-step\" transformation behaviour with two exothermic differential scanning calorimetry (DSC) peaks on cooling and one endothermic DSC peak on heating. For the aged Ni-rich material the x-ray data, which reflect the conditions at the sample surface only, indicate that in the presence of the Ni4Ti3 precipitates the B2 phase transforms to the R-phase, which then transforms to B19′ phase in the second step on cooling. The transition to B19′ starts well above the second peak. In the cycled, precipitate-free material, the R-phase coexists with B2, and/or B19′ throughout its limited temperature range of stability.

Phase transformation behavior of Ti-50Ni and Ti-49Ni(at%) alloy ribbons fabricated by melt spinning, have been investigated. The two-stage B2-R-B19' transformation appeared in as-spun alloy ribbons, while the one-stage B2-B 19' transformation appeared in alloy ribbons heat treated at 873 K. At the linear velocity higher than 36 m/s, the B2-R-B19' transformation was observed in as-spun Ti-49Ni alloy ribbons. In as-spun Ti-50Ni alloy ribbons, the B2-R-B 19' transformation appeared even at the linear velocities of 21 m/s. From TEM observations, it was found that very fine spherical Ti 2 Ni particles with the size less than 20 nm were formed in Ti-49Ni alloy ribbons. The Ti 2 Ni particles had an orientation relationship to the B2 phase and had coherency strains around them, which was thought to play an important role to induce the R phase transformation.

It was first found by the present authors that a two-stage R phase transformation appears in a Ti-50.9at%Ni alloy aged at 473K. The two-stage R phase transformation is characterized by two DSC peaks prior to the martensitic transformation during cooling (the first R phase transformation on cooling is denoted by R 1 and the second by R 2 ). Transformation temperatures of both the R-phases, R 1 and R 2 , increase with increasing aging time. According to thermal cycling tests under various constant stresses, transformation strain of R 2 is larger than that of R 1 . With increasing applied constant stress, the latter strain increases and the former one decreases. However, total strain of both R 1 and R 2 is almost constant irrespective of stress.

In TiNi alloy systems, there are three transformation paths. Fe added alloy, TiNi(Fe), transforms into the R phase whose crystal structure is similar to AuCd ζ phase. Cu added alloy, TiNi(Cu), transforms into the orthorhombic phase which is called B19. They transform into monoclinic martensite called B19' when temperature is further lowered. Without third element addition, quenched Ti 50 Ni 50 transforms into B 19' directly. The crystal structure of the monoclinic phase can be described with a combination of alternative [011] movements on the (Oil) plane and [100] movements on the (011) plane. Phonon behavior in martensite materials is attractive for the transformation mechanism. Alternative motion of [011] on (011) in TiNi corresponds to [110]TA 2 phonon softening at the Brillouin zone boundary. Phonon dispersion behavior of [110]TA 2 in equi-atomic Ti 50 Ni 50 was measured and softening around q∼1/3 is reported rather than at the zone boundary.

TiNi alloy system has been widely used for industrial application and fundamental research. Many studies were performed under circumstance that R phase was not certainly established. Recently R phase was established and structure of R phase was determined. Present study reports diffuse scattering study as a precursor phenomenon to R-phase by using four-circle diffractometer under circumstance that the structure was determined. Diffuse scattering was observed around 1/3 of [110] * incommensurate position at various temperatures in parent phase. Shift from commensurate position broke symmetry and temperature dependence looked random. FWHM of 1/3 reflections and integrated intensities were measured also in parent phase. FWHM decreased and integrated intensities increased with approaching transformation temperature.

The crystal structure of the R-phase in Ti-Ni alloy is determined by the X-ray powder diffraction method. The Pawley method (whole powder pattern decomposition method) and the Rietveld method were used for the analysis. The results of the analysis indicate the space group of the R-phase is P3 rather than P31m. The atomic positions obtained are compared with those of Au-Cd. The transformation mechanism is also discussed in comparison with that for Au-Cd alloy.

Morphology and martensitic transformation temperatures of sputter-deposited Ti 51.5 Ni 48.5 thin films were investigated by carrying out transmission electron microscopy (TEM) of those films before and after annealing at several different temperatures. The thin films were prepared by RF magnetron sputtering. TEM observations showed that the as-deposited thin films were all amorphous and contained no crystalline phases even after annealing at lower temperatures than 370°C. However, they were crystallized into a B2 type order phase by annealing at higher temperatures than 400°C. The grain size was about 1-2 μm and not sensitive to annealing temperature. Subgrain boundaries and plate-shaped defects were observed in the specimens annealed at 400°C and 500°C, respectively, and Ti 2 Ni precipitates were in the specimen annealed at higher temperatures than 600°C. M s temperature for the B2 → B19' martensite transformation decreases with decreasing annealing temperature. On the contrast to M s , T R temperature for the B2 → R phase transformation was hardly affected by annealing temperature.

Acoustic emission (AE) signals generated during phase transformations in a Ti 50.6 Ni 48.3 Fe 1.5 shape memory alloy have been measured, and the AE parameters have been correlated with the phase transformation events. The AE count rate curve during cooling of the specimen was found to have two distinct peaks at temperatures of approximately 8 and -30 °C. These peaks were confirmed by both optical microscopy and differential scanning calorimetry to correspond to the B2 to R phase transformation (at 8 °C) and the R to B19' transformation (at -30 °C) respectively. This is the first detection of the AE events associated with the R-phase transformation in Ti-Ni shape memory alloys. Although the amplitude distributions during both transformations were almost identical, both the duration and the rise time of AE events during the B2 to R phase transformation were larger than those during the R to B19' transformation. These observations suggest that the transformation velocity of the R-phase transformation is slower than that of the martensitic transformation, and are consistent with the nature of both these transformations. It is concluded that the AE technique can be applied to the determination of transformation temperatures of Ti-Ni alloys on cooling as well as DSC and electrical resistivity measurement.

The influence of cold work (CW) and heat treatment (HT) on the austenite to R-Phase (A

The characteristics of R-phase transformations in aged Ti-49.83 at% Ni (Nitinol) shape memory alloys at ���u&�ZHUHH(SODLQHGZLWKWKHKHOSRI�'L

Strain fields introduced by coherent Ni4Ti3 precipitates in austenitic Ni-Ti are believed to be a possible origin of why the R-phase transformation is introduced as an extra step before transforming to the B19'. The presence of this strain field was already confirmed in the past by conventional transmission electron microscopy (TEM) techniques and measured quantitatively by high resolution TEM (HRTEM). This time the geometrical phase method is applied on HRTEM micrographs to measure the full 3D strain tensor of the strain fields. Since each atomic resolution micrograph only results in a 2D measurement of the strain, observations in two different zone orientations are combined to retrieve the 3 x 3 strain tensor. In this work observations in a [1-1 1](B2) and [1 0-1](B2) zone orientation are used and this in case of precipitates with a diameter of around 50nm. In a next step the measured strain tensor is compared to the calculated eigenstrain of the R-phase in reference to the B2 matrix. This comparison shows that the introduced strain is very similar to the eigenstrain of one R-phase variant. Since for both structures, Ni4Ti3 and R-phase, four orientation variants are possible, each variant of the R-phase is thus able to accommodate the strain field of one of the Ni4Ti3 variants.

European Symposium on Martensitic Transformations, ESOMAT 2015, Antwerb, Belgium, 14 Sep 2015 - 18 Sep 2015 ; MATEC web of conferences 33, 03016 (2015). doi:10.1051/matecconf/20153303016, The purpose of this study was to evaluate the effect of heat treatment on the transformation temperatures ofK3, K3XF and MTwo endodontic files. DSC was used to investigate the domains of transformation temperatures andhow the transformation temperatures vary along the file. The files were analysed as received and subjected to heattreatment. X-ray diffraction analysis was also used to investigate the stress-induced martensite transformation on thefiles under flexion. The results show that heat treatment influences the transformation temperatures and that underflexion, the conditions and situations in which files present a stress-induced martensite transformation can beidentified., Published by EDP Sciences, Les Ulis

Microstructural instability involving the formation of topologically close-packed (TCP) phases is restricted during the alloy development of Ni-based single crystal superalloys. In this study, the effects of alloying elements including Co, Cr, Mo and Ru on the formation of different TCP phases were investigated in a series of single crystal superalloys. Experimental results showed that more additions of Cr and Mo promoted the TCP phase formation, while Co and Ru played a positive role in improving microstructural stability. It is indicated that σ, P and R phases existed with various morphology and compositions in different experimental alloys during thermal exposure at 1100 ∘ C. The content of Co, Cr and Mo in those alloys affected the types of TCP phases significantly, while Ru additions showed a negligible effect. R phase was prone to form in alloys containing high level of Co addition. Meanwhile, the ratio of Cr and Mo content had strong influence on the formation of σ and P phases in alloys containing low level of Co addition. The effects of alloying additions on the elemental partitioning ratios between γ and γ ′ phases contributed to their corresponding influence on TCP phase formation.