J. Malarría

Relationship between martensitic plate size and austenitic grain size in martensitic transformations

A systematic experimental analysis based on an assessment of the mean martensite plate size (hplate) in sub-grain domains was implemented to characterize the martensite morphology in polycrystalline Cu-based shape memory alloys. In the grain size range below 100 μm, a linear relationship between the average width of the martensite plates and the mean grain size was obtained for a thermal-induced martensitic transformation. This evaluation allows us to perform an analysis of how microstructural length scales affect the martensitic transformation.

Microstructural evolution in the pseudoelastic cycling of Cu–Zn–Al single crystals: behavior at a transition stage

Abstract Cu–Zn–Al single crystals have been pseudoelastically cycled through the bcc-18R martensitic transformation. The test temperatures varied from low temperatures (liquid nitrogen) to above room temperature. The microstructure evolution during cycling has been analyzed by optical and transmission electron microscopy. A description of defects formed after cycling both in the bulk and on the surface is presented. A transition in the type of formed defects is found corresponding to an increase in the test temperature. Dislocation tangles parallel to the basal plane of the martensite, with trapped martensite inside, give way to dislocation arrays parallel to the habit plane of the transformation, as the test temperature increases. A correlation between this transition behavior and the mechanical evolution was found and the mechanisms of generation of bulk and surface defects are discussed. A correlation between dislocation arrays parallel to the habit plane and intrusion–extrusion defects on the surface is also presented. Both type of defects appear at temperatures higher than 173 K. The effect of point defects introduced during cycling and their change in mobility with test temperature are considered.

Study of atomic displacement fields in shape memory alloys by high-resolution electron microscopy

Abstract The distortion of the atomic lattice is determined using a recently developed method of analysing high resolution electron microscope images. The analysis is carried out in terms of the individual lattice fringes which contribute to the image contrast. A perfect set of fringes has an amplitude and phase given by the corresponding Fourier component. A distorted lattice can be analysed by introducing the concept of a ‘local’ Fourier component, representing the local values of amplitude and phase as a function in position of the image. The local phase is used to determine the line of a perfectly coherent interface between two sets of lattice planes. The degree of self-accommodation of a series of martensitic plates in a Cu–Zn–Al shape-memory-alloy is studied by the phase analysis of an experimental high resolution image.

Heat Treatments of Fe-Mn-Si Based Alloys: Mechanical Properties and Related Shape Memory Phenomena

Heat treatments are usual means for modifying alloy microstructures and, consequently, to control mechanical properties. The aim of this work was to find suitable processes for improving the shape memory effect (SME) of Fe–Mn–Si-based alloys. In particular, we studied mechanisms that affect the plastic deformation of the austenite phase. A Fe–15Mn–5Si–9Cr–5Ni alloy was deformed by rolling at different temperatures and subsequently annealed at recovery- and recrystallization-temperature. The mechanical properties of the material after processing were evaluated by performing tension and flexure tests. The SME of room temperature deformed specimens was measured after heating them to 550°C for shape recovery. We found that the material rolled at 800°C followed by an annealing treatment at 650°C recovers nearly 95 % of a 3 % deformation. In this thermo-mechanical condition, the material has a yield stress of 450 MPa and an ultimate tensile strength of 880 MPa, corresponding to a total elongation of about 16 %. Optical and electron microscopy observations show that the matrix annealed at high temperature contains a low density of defects. As a consequence, there are fewer nucleation sites for martensite and the associated SME is low. On the other hand, annealing at intermediate temperatures (around 650°C) produces a favorable structure containing a large density of stacking faults.

Preparation by Twin Roll Casting and Characterization of TiNi Shape Memory Alloys Strips

The thermoelastic martensitic transformation which gives remarkable functional properties to Shape Memory Alloys (SMA) is very sensitive to the chemistry of the alloys and to its microstructure. In many cases, especially for alloys with high transformation temperatures, applications are limited by the poor ductility. To overcome the brittleness of SMA, one approach can be effective : the development of non conventional production technologies which enable to obtain materials in forms close to requirements: strips of SMA have been prepared by Twin Roll Casting (TRC). The formation of a solid sheet with plane surface is only possible for a limited combination of the different machine parameters and they will be discussed. For comprehension of the process parameters effect, the attention will be focused on TiNi. Microstructures and functional properties will be examined in relation with the microstructures induced by the technique or after specific thermal treatment.

Cryostat for fatigue tests down to liquid‐nitrogen temperatures

A cryostat was constructed and installed in a MTS 810 servohydraulic testing machine for studying fatigue properties associated with pseudoelastic cycling of shape memory alloys down to liquid‐nitrogen temperatures. Special attention was focused on eliminating temperature fluctuations during the fatigue experiments. In order to perform long time fatigue tests, an automatic controlled system which supplies the cryostat with liquid nitrogen was developed and described in detail.

Design, construction, and performance of a device for directional recrystallization of metallic alloys

A device was designed to apply the directional recrystallization method to Fe-based alloys in order to obtain bamboo-like microstructures. This microstructure is suitable for improving creep properties and resistance to fatigue in some alloys and for enhancing pseudoelastic properties in shape memory alloys. The design and construction of a flat coil are described in detail. In addition, we developed an electromechanical system to control the movement of a wire within the flat coil. The construction details and system performance are presented. Furthermore, metallographic studies taken from the directionally recrystallized low-carbon steel samples are shown. Nearly monocrystalline and bamboo-like microstructures were achieved in the steel wires.

The Shape Memory Effect in Melt Spun Fe-15Mn-5Si-9Cr-5Ni Alloys

At room temperature, Fe-15Mn-5Si-9Cr-5Ni alloys are usually austenitic and the application of a stress induces a reversible martensitic transformation leading to a shape memory effect (SME). However, when a ribbon of this material is obtained by melt-spinning, the rapid solidification stabilizes a high-temperature ferritic phase. The goals of this work were to find the appropriate heat treatment in order to recover the equilibrium austenitic phase, characterize the ribbon form of this material and evaluate its shape memory behaviour. We found that annealing at 1050°C for 60 min, under a protective argon atmosphere, followed by a water quenching stabilizes the austenite to room temperature. The yield stress, measured by tensile tests, is 250 MPa. Shape-memory tests show that a strain recovery of 55% can be obtained, which is enough for certain applications.