Characterization of a NiTiCu shape memory alloy produced by powder technology

Purpose: The main aim of presented work was to find a sintering conditions (temperature and time) for manufacturing of a Ni(1-X)Ti50CuX alloy (where X = 2; 3; 5; 10; 15; 20 and 25at%.) by powder technology. Design/methodology/approach: Various conditions of sintering considering temperature and time were applied to compacted powders. Sintering temperature varied from 850°C to 1100°C and sintering time was chosen from a range of 5 to 50 hours, respectively. Microstructure, structure, chemical composition and thermal behavior of sintered blends were studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and X-ray diffraction. Findings: Homogenous alloys, containing lower addition of copper (less than 10 at%), were sintered at 940°C for 7 hours. For higher copper content (10-25at%) lower sintering temperature 8500C but longer sintering time was preferred (20 hours). The quality of the alloy was characterized by porosity and density. In sintered blends non-transformable phases Ti2(Ni,Cu) and (Ni,Cu)3Ti, which posses the crystal structure of Ti2Ni and Cu3Ti respectively, were found. Despite the fact that same sintering conditions lead to an increase of inhomogeneity all sintered alloys reveal the presence of the reversible martensitic transformation. Practical implications: Obtained results allowed to optimize sintering condition for NiTiCu shape memory alloy manufacturing

Effect of Annealing on the Transformation Behavior and Mechanical Properties of Two Nanostructured Ti-50.8at.%Ni Thin Wires Produced by Different Methods

Abstract. A Ti-50.8at.%Ni wire produced using a co-drawing method and a commercial Ti-50.8at.%Ni wire were annealed at different temperatures between 450°C and 700°C. Grains with diameter less than 100nm were revealed by transmission electron microscopy for both wires before annealing treatment. However, the microstructural heterogeneity of the co-drawn wire is more obvious than that of the commercial wire. Multi-stage martensitic transformation was observed in the co-drawn wire, compared with the one-stage A↔M transformation in the commercial wire after annealing at 600°C for 30min. The differences of total elongation, plateau strain and pseudoelastic recoverable strain between the commercial wire and the co-drawn wire were also observed. The differences of the transformation behavior and mechanical properties between the commercial wire and the co-drawn wire are attributed to the microstructural difference between these two wires

Preface to the viewpoint set on: shape memory alloys

status: publishe

A tribute to Prof. Dr. Ir. Luc Delaey, Dep. Metallurgy and Materials Science, Catholic University Leuven, Belgium

Biographical Itemstatus: publishe

Shape memory materials: State of the art and requirements for future applications

NiTi is only one of many alloy systems that exhibit the shape memory effect. The reason of his success is only the fact that it is the best in many aspects. His large market share created a significant price reduction so that in combination with its good properties, it became a preferential alloy even when compared with Cu-based alloys. Moreover NiTi alloys can be easily tuned to optimal performance by applying the proper combination of deformation and heat treatments. Its most successful applications are related to medical devices.status: publishe

Non-medical applications of shape memory alloys

The diversity of (potential) applications using shape memory alloys (SMA), apart from the medical field, becomes quite large. Classic categories such as free recovery, actuators, constrained recovery, pseudo-elasticity or damping require further specifications. For example, micro-actuators. smart materials or active damping, can be all classified as actuator applications, but each of those items demands specific functional performance, dimensions and processing. Furthermore, success for applications can only be realised in so far those materials offer also a price-competitive advantage relative to other functional materials or mechanical designs. This competition requires perfect control of the material performance. It is known that especially Ni-Ti alloys can be tuned relatively easy to some specific requirements of the envisaged application: hysteresis, transformation temperatures, damping capacity. At the other side little is known on recovery stresses, wear resistance, fracture mechanics, fatigue... In this paper we would like to stress the need for further exploration of the 4P-relation: principles-properties-processing-products as well in companies as in universities or other research laboratories. This will be illustrated by describing some actual applications indicating why they are successful, other applications why they failed and still others that can only be realised if some further, probably possible. material improvement can be realised. (C) 1999 Elsevier Science S.A. All rights reserved.status: publishe

Damping capacity of thermoelastic martensite in shape memory alloys

Shape memory alloys attract increasing interest as materials that can be used for passive as well as active damping applications. The passive high damping capacity finds its origin in the thennoelastic martensitic phase due to the hysteretic mobility of martensite variants or different phase interfaces. The damping capacity increases with increasing amplitude of the applied vibration. Special interest exists moreover for damping extremely large displacements. This is realised by applying the mechanical hysteresis occurring during pseudoelastic loading. This aspect is nowadays very strongly studied as a tool for protecting buildings against earthquakes in seismic active regions. Active damping can be obtained in hybrid composites by controlling the recovery stresses or strains of embedded shape memory alloy wires. This controls the internal energy of a structure which allows controlled modal modification and tuning of the dynamic properties of structural elements. But also impact damage, acoustic radiation, dynamic shape control can be actively controlled. As a consequence, improved fatigue-resistance, better performance and a longer lifetime of the structural elements can be obtained. This paper overviews the specific damping properties and damping functional behaviour of shape memory alloys, with special emphasis on NiTi. It is illustrated by actual applications and applications under development. (C) 2003 Elsevier Science B.V. All rights reserved.status: publishe

Cycling effects, fatigue and degradation of shape memory alloys

The stability and lifetime of a shape memory device is characterised by the changes of its transformation temperatures. its cold shape, its hot shape, its global two way shape memory effect, its recovery stress. This global behaviour is influenced by a complex combination of internal and external parameters. Internal parameters are: the alloy system, the alloy composition, the type of transformation and the lattice structure including defects. External parameters are: the thermomechanical treatments, the way of training, the applied stress, the imposed shape memory strain, the amplitude of temperature cycling, the absolute temperature of the environment. The possible physical mechanisms which are at the origin of the limited lifetime of any shape memory element and which are, to a greater or lesser extent, controlled by the above mentioned internal and external parameters, are: the stabilisation of specific martensite variants, the creation of lattice defects during transformation-cycling (transformation plasticity or defects that could trigger unwanted variants), changes in the order of the lattice, changes in defect density and/or defect configurations as a result of ageing. This paper will present an overview of reported and new observations related to those aspects of shape memory alloys.status: publishe