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The temperature dependence of the critical stress of R→B19' transformation in NiTi shape memory alloys remains unclear due to the difficulties in precisely identifying the phase constitution and acquiring sole R phase. Here we investigate the thermally- and stress-induced martensitic transformation in a near-equiatomic nanocrystalline NiTi alloy. In situ synchrotron X-ray diffraction is used to identify the phase evolution such that the temperature window for sole R phase is firmly determined. We find that the temperature dependence of R→B19' is negative (−0.2 MPa/K) in our alloy, in sharp contrast to 3–5 MPa/K reported. Possible reasons for the discrepancies are discussed. Negative temperature-dependence of stress-induced R→B19' transformation was discovered for the first time in NiTi alloys, being in sharp contrast to previous reports. This calls for a revisit of such transformation. [ABSTRACT FROM AUTHOR]

A Cu/carbon-steel nanocomposite is fabricated by pressing, rolling and wire drawing. The average layer thickness is about 69 nm for steel and 67 nm for Cu. Cu suppresses the growth of prior austenite at elevated temperature, and thus the steel layer thickness barely increases during austenization. Subsequent quenching results in a large amount of metastable retained austenite within the steel layers. The martensitic transformation behavior of the composite during plastic deformation is investigated by in situ tensile test using synchrotron X-ray diffraction at room temperature. It is found that the retained austenite, rather than Cu, dominates the plastic deformation of the nanocomposite. In situ results verify that the transformation of retained austenite initiates in a manner of Lüders-like band due to stress-induced martensitic transformation, followed by strain-induced martensitic transformation until fracture. [ABSTRACT FROM AUTHOR]

This paper reports a study on non-linear temperature dependences of the critical pseudoelastic stress and stress hysteresis for stress-induced martensitic transformation deformation of a nanocrystallined Ni 47 Ti 50 Fe 3 alloy. Such non-linear temperature dependences were found over a wide low temperature range from -196 °C to 20 °C and they defy the predictions of the Clausius-Clapeyron law for the transformation. The pseudoelastic behaviour exhibited two unusual characteristics, including a progressively decreasing d σ / d T slope and a rapidly widening stress hysteresis with decreasing the temperature. In-situ X-ray diffraction analysis indicated that the starting microstructure of the alloy for pseudoelastic deformation was a mixture of the B2 and a crystallographically suppressed R phase (denoted R*) within -8 °C to -196 °C. The volume fraction, the rhombohedral lattice distortion and the internal atomic shuffle of the suppressed R phase increased upon cooling. These continued evolutions of the crystal structure of the R* phase and the microstructure of the alloy upon cooling are ascribed to be responsible for the abnormal non-linear temperature dependences of the pseudoelastic stresses and the widening of the stress hysteresis. [ABSTRACT FROM AUTHOR]

The mechanism of low thermal expansion phenomenon in antiferromagnetic FeMn alloys needs to be revealed. In this work, the evolution of lattice thermal expansion and internal stress in FeMn alloys was investigated by in-situ synchrotron X-ray diffraction during thermal cycling. Preceding the martensitic transformation, the coefficient of lattice thermal expansion (CLTE) decreased and the full width at half maximum (FWHM) of the diffraction peaks broadened abnormally. The evolution of the lattice thermal expansion and the FWHM were both reversible and isotropic. The additional reflection spots around the fundamental diffraction spots in the electron diffraction patterns of austenite and the dispersed nano-scale martensite embryos in austenite observed in the dark field TEM images proves the existence of premartensitic transformation. Similar phenomena were found in FeMn Invar alloy, which indicate that the Invar effect is closely related to the internal stress evolution caused by premartensitic transformation. • Both Fe27Mn and Fe24Mn exhibited low CTE and broadening of diffraction peaks upon cooling. • The evolution of thermal lattice strain and FWHM of diffraction peaks were reversible and isotropic. • Martensite embryos were observed in austenite phase during Invar effect of Fe27Mn and premartensitic transformation of Fe24Mn. • These similar phenomena imply a close relationship between Invar effect and the premartensitic transformation in FeMn alloy. [ABSTRACT FROM AUTHOR]

The lattice strain evolution is essential for the understanding of the invar effect. In this letter, the transformation behavior and lattice strain of austenite in a Fe 66 Ni 29 Co 5 alloy is studied by in situ synchrotron X-ray diffraction during cooling. An abnormal increase of the lattice strain is observed upon martensitic transformation. The strain inhomogeneity is found to increase monotonically before transformation during cooling. The anomalies are suggested to be related to the restricted nature of the transformation, which is further evidenced by the abnormal lattice strain evolution in a laminated Cu/steel nanocomposite. [Display omitted] [ABSTRACT FROM AUTHOR]

Large thermal hysteresis (T hys) is favorable in shape memory alloys (SMAs) because it renders wide service and storage temperature range of the SMA-made parts. In this letter, we show that large intrinsic T hys up to 100 K is achieved in a low Nb content (2 at.%), β-Nb-free and single-phase NiTiNb SMA fabricated by casting, forging, wire-drawing and annealing. Such a large T hys is comparable to that of the widely used dual-phase NiTiNb SMAs in which high Nb content and β-Nb are required. By exploiting the grain-size-dependence of T hys in our single-phase NiTiNb SMA and that in an equiatomic NiTi SMA, we attribute the large T hys to the enhanced kinetic resistance of transformation by the small grains. The connection between the T hys and the kinetic resistance is demonstrated by a modified dislocation-based kinetic model. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper reports a study of multilayer nanolamellar Mo/Nb/TiNi composites designed by the principle of martensitic transformation enabled lattice strain matching. The composite was fabricated by hot packaged accumulative roll bonding (PARB) and wire drawing. The composite showed an apparent yield strength of ~1200 MPa and a recoverable superelastic strain of ~7%, with a mechanical damping of ~62 MJ/m3. Both the Nb and Mo nanolamellae sandwiched between the TiNi layers exhibit large elastic lattice strains of ~2.5% and ~3.0% respectively, benefited from the lattice strain matching and load transfer enabled by the martensitic transformation of the TiNi matrix. The Nb and Mo nanolamellae in the composite exhibited high component stresses of 2.4 GPa and 9.5 GPa, and each carries ~31% of the applied load even though with only 4% and 18% volume fractions, respectively. [ABSTRACT FROM AUTHOR]

This study explores a new concept of the synergy between two constituent shape memory alloys (SMAs) in a composite to achieve a wide superelastic temperature window. In this work, a nanocrystalline NiTi/NiTiNbFe dual-SMA composite is designed, in which two constituent SMAs are both capable of stress-induced martensitic transformation, but within different temperature ranges. Based on the principle of lattice strain matching between the two SMAs, superelastic martensitic transformation in one induces superelastic transformation in the other at temperatures the latter will not when standing alone. Via this synergy the composite is able to achieve an ultra-wide superelastic temperature window of 336 °C, from −196 to 140 °C, with a ∼5% tensile strain recovery and high superelastic stresses in the range of 700–1400 MPa. Such properties appear superior to all NiTi-based shape memory alloys to date. Image 1 • Lattice strain matching was used to design a new dual-SMA lamellar composite with more than 300°C superelasticity window. • The composite exhibits ultra-high superelastic stresses ranging from 700 to 1400 MPa over the temperature window. • Strong synergy between the two SMA components is responsible for the superior superelastic properties. [ABSTRACT FROM AUTHOR]

Taking full advantage of intrinsic high strength of nano-reinforcements has been proven difficult in a composite until the concept of strain matching was experimental verified in a nanowire Nb/TiNi in-situ composite, which was obtained via solidifying eutectic Nb/TiNi and subsequent severe wire drawing. However, the volume fraction of the nano-reinforcement was dictated by eutectic reaction, which severely limited the development of high-performance composites. In this paper, the martensitic transformation nanocomposites were successfully acquired with increased volume fraction and changeable morphology of the nano-reinforcement through hot packaged ARB (PARB) and subsequent wire drawing. The nano-reinforcement with lamellar configuration in the composite could exhibit a large lattice strain of ~3.8%, which demonstrated the applicability and effectiveness of the strain matching principle in nano-lamellar composite. The obtained composites show a yield strength over 2.0 GPa and a quasi-linear elastic strain as larger as ~6.5% with adjustable apparent elastic modulus from 30 to 50 GPa. [ABSTRACT FROM AUTHOR]