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Materials Science Forum

Dept. of Metallurgy and Materials Engineering, Catholic University of Leuven, de Croylaan 2, 3001Heverlee, Belgium(Received 16 June 1997; accepted in revised form 20 October 1997)Abstract—Experimental results have shown that, during mechanical cycling under tension–compressionload within 24% strains, the NiTi shape memory alloy is cyclic strain-hardened. The maximum stressesunder both tension and compression increase with increasing number of cycles and tend to stabilize withfurther cycling. The present work is focused on the martensite microstructure developed as a result ofmechanical cycling. TEM observations show that, before cycling, the martensite variants are well self-accommodated to each other with the h011itype II twinning as the main lattice invariant shear. Aftermechanical cycling, the martensite plates are still self-accommodated and the (111) type I twinning is mostfrequently observed. In addition to the stress-induced re-orientation of martensite and twin boundarymovement within the martensite plate, various lattice defects have been developed both in the junctionplane areas of martensite plates and within the martensite twins. # 1998 Acta Metallurgica Inc.1. INTRODUCTION

MICROSTRUCTURE OF NiTi SHAPE MEMORY ALLOY DUE TO TENSION-COMPRESSION CYCLIC

The microstructure evolution and softening processes during high-temperature deformation of a 21Cr-10Ni-3Mo duplex stainless steel

A novel high-strain-rate ferrite dynamic softening mechanism facilitated by the interphase in the austenite/ferrite microstructure

Many established, but also potential future applications of NiTi-based shape memory alloys (SMA) in biomedical devices and solid-state refrigeration require long fatigue life with 107–109 duty cycles1,2. However, improving the fatigue resistance of NiTi often compromises other mechanical and functional properties3,4. Existing efforts to improve the fatigue resistance of SMA include composition control for coherent phase boundaries5–7 and microstructure control such as precipitation8,9 and grain-size reduction3,4. Here, we extend the strategy to the nanoscale and improve fatigue resistance of NiTi via a hybrid heterogenous nanostructure. We produced a superelastic NiTi nanocomposite with crystalline and amorphous phases via severe plastic deformation and low-temperature annealing. The as-produced nanocomposite possesses a recoverable strain of 4.3% and a yield strength of 2.3 GPa. In cyclic compression experiments, the nanostructured NiTi micropillars endure over 108 reversible-phase-transition cycles under a stress of 1.8 GPa. We attribute the enhanced properties to the mutual strengthening of nanosized amorphous and crystalline phases where the amorphous phase suppresses dislocation slip in the crystalline phase while the crystalline phase hinders shear band propagation in the amorphous phase. The synergy of the properties of crystalline and amorphous phases at the nanoscale could be an effective method to improve fatigue resistance and strength of SMA. Increasing the fatigue life of shape memory alloys often compromises other mechanical properties such as yield strength and plastic deformation behaviour. Introducing a mixed nanostructure of crystalline and amorphous phases can enable superelasticity in NiTi micropillars with recoverable strain of 4.3%, yield strength of 2.3 GPa and 108 reversible-phase transition cycles under a stress of 1.8 GPa.

Nanocomposite NiTi shape memory alloy with high strength and fatigue resistance.

A newly developed, austenitic lightweight steel, containing a low-density element, Al, exhibits tensile elongation up to 50% as well as high ultimate-tensile stress (tensile fracture at 1800 MPa) without necking behavior. Electron backscatter diffraction analysis is carried out to investigate the orientation dependence of the martensitic transformation in tensile testing to 30% strain at 323 K (25 °C). A pronounced γ→e→α′ transformation is observed in and ∥TD (TD: tensile direction) γ-grains. The α′-transformation textures is analyzed. Large misorientation spreads is seen in the ∥TD γ-grains. Interestingly, twin-assisted martensitic transformation is detected in the ∥TD followed by the twin boundary directly moving to a γ/α′ phase boundary. These phenomena are related to a change of Schmid factor for different orientations of grains.

Grain-orientation-dependent of γ–ε–α′ transformation and twinning in a super-high-strength, high ductility austenitic Mn-steel

A rapid texture measurement system has been developed on the time-of-flight neutron diffractometer iMATERIA (beamline BL20, MLF/J-PARC, Japan). Quantitative Rietveld texture analysis with a neutron beam exposure of several minutes without sample rotation was investigated using a duplex stainless steel, and the minimum number of diffraction spectra required for the analysis was determined experimentally. The rapid measurement scheme employs 132 spectra, and by this scheme the quantitative determination of volume fractions of texture components in ferrite and austenite cubic phases in a duplex stainless steel can be made in a short time. This quantitative and rapid measurement scheme is based on the salient features of iMATERIA as a powder diffractometer, i.e. a fairly high resolution in d spacing and numerous detectors covering a wide range of scattering angle.

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Yusuke Onuki

Papers

Nanocomposite NiTi shape memory alloy with high strength and fatigue resistance.

Grain-orientation-dependent of γ–ε–α′ transformation and twinning in a super-high-strength, high ductility austenitic Mn-steel

Rapid measurement scheme for texture in cubic metallic materials using time-of-flight neutron diffraction at iMATERIA

Texture development in Fe–3.0 mass% Si during high-temperature deformation: Examination of the preferential dynamic grain growth mechanism

Elinvar property of cold-rolled NiTi alloy