R. Delville

Transmission electron microscopy study of phase compatibility in low hysteresis shape memory alloys

Recent findings have linked low hysteresis in shape memory alloys with phase compatibility between austenite and martensite. To investigate the evolution of microstructure as phase compatibility increases and hysteresis is reduced, transmission electron microscopy was used to study the alloy system Ti50Ni50 − x Pd x , where the composition is systemically tuned to approach perfect compatibility. Changes in morphology, twinning density and twinning modes are reported, along with special microstructures occurring when compatibility is achieved. In addition, the interface between austenite and a single variant of martensite was studied by high-resolution and conventional electron microscopy. The low energy configuration of the interface detailed in this article suggests that it plays an important role in the lowering of hysteresis compared to classical habit plane interfaces.

Functional twin boundaries

Functional interfaces are at the core of research in the emerging field of ‘domain boundary engineering’ where polar, conducting, chiral, and other interfaces and twin boundaries have been discovered. Ferroelectricity was found in twin walls of paraelectric CaTiO3. We show that the effect of functional interfaces can be optimized if the number of twin boundaries is increased in densely twinned materials. Such materials can be produced by shear in the ferroelastic phase rather than by rapid quench from the paraelastic phase.

Tem Investigation of Microstructures in Low-Hysteresis Ti50Ni50-xPdx Alloys with Special Lattice Parameters

NiTi is one of the most popular shape-memory alloys (SMA) in medical applications because of its biocompatibility and its remarkable properties that allow recoverable mechanical energy to be stored in a compact delivery system. Still, its undesirable fatigue properties exemplified by the occurrence of medical-device fracture, along with large temperature/stress hysteresis and the narrow temperature range of operation translate to a tight margin of error for engineering design of the devices.

Dedicated TEM on domain boundaries from phase transformations and crystal growth

Investigating domain boundaries and their effects on the behaviour of materials automatically implies the need for detailed knowledge on the structural aspects of the atomic configurations at these interfaces. Not only in view of nearest neighbour interactions but also at a larger scale, often surpassing the unit cell, the boundaries can contain structural elements that do not exist in the bulk. In the present contribution, a number of special boundaries resulting from phase transformations or crystal growth and those recently investigated by advanced transmission electron microscopy techniques in different systems will be reviewed. These include macrotwins between microtwinned martensite plates in Ni–Al, austenite-single variant martensite habit planes in low hysteresis Ni–Ti–Pd, nanotwins in non-textured nanostructured Pd and ferroelastic domain boundaries in CaTiO3. In all discussed cases these boundaries play an essential role in the properties of the respective materials.

In Situ Synchrotron X-Ray Diffraction Investigation of the Fast Recovery of Microstructure during Electropulse Treatment of Heavily Cold Drawn Nanocrystalline Ni-Ti Wires

Recovery processes responsible for evolution of microstructures in 0.1mm thin cold-drawn Ni-Ti shape memory alloy wire heat treated by DC electric pulse were investigated by combination of in-situ tensile stress - strain, electrical resistance and X-ray diffraction measurements. The X-ray data were used to obtain direct experimental information on the evolution of the phase fractions, internal strain and defects in the microstructure evolving through activation of a sequence of recovery processes during the short time electropulse treatment. It is shown that superelastic functional properties of the treated Ni-Ti wire can be precisely set by controlling the progress of the recovery processes by prescribing the time evolution of temperature T(t) and tensile stress s(t) (displacement control) in the treated wire.

In-Situ Investigation of the Fast Lattice Recovery during Electropulse Treatment of Heavily Cold Drawn Nanocrystalline Ni-Ti Wires

Shape memory alloys (SMA) such as the near equiatomic Ni-Ti alloy [1] have attracted considerable attention for their unique functional thermomechanical properties as superelasticity or shape memory effect deriving from the martensitic transformation. Ni-Ti wires are being produced from extruded bars by multiple hot working passes finished by a final cold drawing. In this so called “cold worked” (as-drawn, hard, etc.) state, the alloy possesses a heavily deformed microstructure resulting from severe plastic deformation [2] consisting of mixture of austenite, martensite, and amorphous phases with defects and internal strain [3].

TEM investigations on novel shape memory systems with Ni-depletions

Many of today’s shape memory systems contain a substantial amount of Ni. The most typical system is of course Nitinol® based on the binary Ni-Ti alloy used near to its equiatomic composition. Another well known system is Ni-Al, which has been very thoroughly studied more for its fundamental physical characteristics rather than its practical applications. Recently, however, new ternary systems have attracted a lot of attention attempting to, e.g., introduce magnetic transitions and driving forces, such as in Ni-Mn-Ga or Co-Ni-Al or lower the hysteresis and increase the transformation temperatures in Ni-Ti-(Au,Pd,Pt), replacing mainly Ni with the ternary compound. At the same time, new studies have revealed unwanted Ni release in commercial wires for medical use. Focusing on Ni thus remains important for understanding the behaviour of martensitic transformations and shape memory applications.