Osman Adiguzel

Martensite ordering and stabilization in copper based shape memory alloys

Stabilization behavior of the martensite in shape memory CuZnAl and CuAlMn alloys has been studied by x-ray measurements and electron microscopy. In these alloys influenced by both quenching and post-quench heat treatments, the degree of stabilization depends on quenching conditions. The stabilization process is mainly due to structural change and atomic arrangement in the martensitic lattice inherited from the parent phase. From x-ray results, it is suggested that stabilization and loss of memory are directly related to disordering in a martensitic state. In the present study, it was concluded that the spacing differences ({delta}d) between some selected pairs of diffraction planes of type (h{sub 1}k{sup 1}1) and (h{sub 2}k{sub 2}1) satisfying the relation (h{sub 1}{sup 2}-h{sub 2}{sup 2})/3 = (k{sub 2}{sup 2}-k{sub 1}{sup 2})/n, with n = 1 for 9R and n = 4 for 18R martensites reflect the degree of ordering in martensite, in addition to the lattice distortion parameters defined as {phi} = {vert_bar}sin{sup 2}{theta}{sub 202}-sin{sup 2}{theta}{sub 122}{vert_bar} and {phi}{prime} = {vert_bar}sin{sup 2}{theta}{sub 320}-sin{sup 2}{theta}{sub 040}{vert_bar}.

Effects of thermal treatments on transformation behaviour in shape memory Cu–Al–Ni alloys

Abstract The martensitic transformation behaviour, morphology and transition temperatures in copper-based shape memory alloys are strongly influenced by heat treatments. The effects of various quenching, up-quenching and step-quenching in a water bath 100 °C and in an oil bath at 200 °C have been studied on two shape memory Cu–Al–Ni alloys. The changes at entropy and enthalpy at the martensitic transition have been determined by means of differential scanning calorimeter (DSC) measurements. It was observed that quenching has a marked influence on the transformation temperatures.

The influence of ageing on martensite ordering and stabilization in shape memory Cu-Al-Ni alloys

Abstract The martensitic transformation and the associated mechanical shape reversibility in copper-based shape memory alloys is strongly influenced by quenching and ageing treatments. Ageing of martensite in as-quenched Cu-Al-Ni alloys can result in loss of memory behavior. Structural studies have been carried out to measure the changes in the degree of order that develop during martensitic ageing of two Cu-Al-Ni alloys. Stabilization is directly related to disordering in martensitic state and the spacing differences (Δd) between selected pairs of diffraction planes reflect the degree of ordering in martensite. The changes in degree of order are shown to be similar in asquenched and post-quenched β-phase annealed alloys, thereby leading to the conclusion that loss of memory in as-quenched alloys is not solely attributable to any extra changes in degree of order brought about by excess vacancies during martensitic ageing.

Martensitic Transformation and Microstructural Characteristics in Copper Based Shape Memory Alloys

Martensitic transformations are first order solid state phase transitions and occur in the materials on cooling from high temperature. Shape memory effect is an unusual property exhibited by certain alloy systems, and based on martensitic transformation. The shape memory property is characterized by the recoverability of previously defined shape or dimension when they are subjected to variation of temperature. The shape memory effect is facilitated by martensitic transformation, and shape memory properties are intimately related to the microstructures of the materials. Martensitic transformations occur as martensite variant with the cooperative movement of atoms on {110}β - type plane of austenite matrix. Martensitic transformations have diffusionless character, and the atomic movement is confined to interatomic lengths in the materials. The basic factors which govern the martensitic transformation are Bain distortion and homogeneous shears. Copper based alloys exhibit this property in metastable β-phase field.

Strain effects on the macroscopic behaviour and martensite morphology in shape-memory CuZnAl alloys

Abstract Shape-memory alloys exhibit super-elasticity when they are deformed at a temperature less than the transformation temperature. The specimen remains in the deformed shape on releasing the strain at this temperature but recovers the undeformed original shape on heating to temperatures over the reverse transformation temperature. As a result of their ability to generate spontaneous shape changes on thermal cycling, these alloys can convert heat directly into mechanical work due to the substantial recovery stresses generated during the martensite-to-parent transformation. These alloys are very sensitive to thermal effects and are influenced strongly by both quenching and post-quench heat-treatments. The effects of external stress on the character of the macroscopic shape in two shape-memory CuZnAl alloys are investigated. The effects of post-quench heat-treatment on martensite morphology were observed by means of optical microscopy. Thermal hysteresis occurring during the heating and cooling processes is attributed to internal friction. Furthermore, parallel-sided martensite plates which can be grown almost in one dimension have been observed in the post-quench heat-treated specimens. It is suggested that the equilibrium phase (α-fcc) formed in the aged specimens causes the loss of shape memory.

Phase Transitions and Microstructural Processes in Shape Memory Alloys

Shape memory alloys exhibit a peculiar property, shape memory effect that is the result from the structural changes in microscopic scale. These alloys return to previously defined shapes when they are subjected to variation of temperature after deformation of the low temperature phase. Shape-memory effect is based on martensitic transformation, with which the material changes its internal crystalline structure. The ordered structure or super lattice structure is essential for the shape memory effect of the material. Copper based alloys exhibit this property in the β-phase field, which possesses the simple bcc-structure at high temperature austenite phase. As the temperature is lowered, austenite phase undergoes martensitic transition following two ordering reactions, and microstructural changes in microscopic scale govern this transition. In the present work, Cu alloys were investigated by transmission electron microscope, TEM, and x-ray diffraction techniques.

Thermoelasticity, Superelasticity and Nanoscale Aspects of Structural Transformations in Shape Memory Alloys

Shape memory alloys have a peculiar property to return to a previously defined shape on heating after deformation in low temperature product phase region. These alloys take place in a class of functional materials due to the response to the variation of temperature, and they are used shape memory elements in a wide range of industry; in particular, they are used in the construction sector, aeronautical industry due to the energy dissipation properties. Shape memory effect is facilitated by martensitic transformation which is a solid state phase transformation and occurs in thermal manner in material on cooling from high temperature parent phase region. This transformation is governed by changes in the crystalline structure of the material. Thermal induced martensite occurs as martensite variants, twinned martensite, in self-accommodating manner on cooling from high temperature parent phase region. Mechanically deformation of these alloys in martensitic state proceeds through martensite variant reorientation by the detwinning process. Martensitic transition occurs as self-accommodated martensite with lattice invariant shears which occur in two opposite directions, -type directions on the {110}-type plane of austenite matrix. In addition, shape memory alloys can exhibit another property called superelasticity performed in only mechanical manner. These alloys can be deformed just over austenite finish temperature, and recover the original shape on releasing the stress in superelastic manner. Copper based alloys exhibit this property in metastable β-phase region, which has bcc-based structures at high temperature parent phase field and these structures martensiticaly turn into the layered complex structures following two ordered reactions on cooling.

Self-Accommodating Nature of Martensite Formation in Shape Memory Alloys

Shape memory effect is a peculiar property exhibited by certain alloy system. This behavior is facilitated by martensitic transformation, and shape memory properties are intimately related to the microstructures of alloys; in particular, the morphology and orientation relationship between the various martensite variants. Martensitic transformation occurs in thermal manner, on cooling the materials from high temperature parent phase region. Thermal induced martensite called self-accommodated martensite or multivariant martensite occurs as multivariant martensite in self-accommodating manner and consists of lattice twins. Shape memory alloys are deformed in low temperature martensitic phase condition, and deformation proceeds through a martensite variant reorientation. Copper based alloys exhibit this property in metastable β - phase region.

Internal Stresses in Martensite Formation in Copper Based Shape Memory Alloys

Shape memory alloys constitute a class of materials called smart materials which exhibit an unusual property, shape memory effect. The behaviour of these materials is evaluated by the structural changes caused by internal stresses in microscopic scale. Copper based alloys exhibit this property in β-phase field. Shape memory alloys undergo a solid state phase transition, martensitic transition by means of lattice invariant shears and Bain distortion on cooling from high temperatures. Both distortions are caused by the internal stresses in the material. The lattice invariant distortion involves the introduction of stacking sequences on one of the close packed {110}β planes of matrix called martensite basal plane. The formation and evolution of the layered structure in copper based ternary shape memory alloys consist of lattice invariant shears and shear mechanism. Shape memory elements cycle between the deformed and undeformed shapes against the temperature changes while using in the devices. These alloys can be used as actuator or sensor due to this property.

Premartensitic and martensitic transitions and crystallography in copper based shape memory alloys

Copper-based beta-phase alloys constitute a class of materials which exhibit shape memory effect within a certain range of composition. This effect in beta-phases of ternary alloys based on noble metal copper is evaluated by the structural changes in microscopic scale with a displacive transformation called martensitic transition which is first-order transformation, due to the metastable character and change their internal crystalline structures with changing temperature. Beta phases of copper based alloys have the disordered bcc structure, (β)-phase, at high temperatures and undergo two types of ordered reactions called premartensitic transitions with cooling. The first transition is a firstnearest neighbour (nn) ordering reaction and results in a B2-type superlattice. However, bcc to DO3 transition induces the next-nearest neighbour (nnn) ordering reaction. The martensitic transition occurs from the B2(CsCl) or DO3(Fe3Al) type ordered structures to the layered structures on further cooling. Martensitic transformations in these alloys occur occurs in a few steps. . In this kind of transformation, internal stress is effective and one of the steps is type shear mode occuring on close-packed planes of matrix phase called basal plane of martensite. Product martensite phase has the long period layered structures which consist of an array of close-packed planes with complicated stacking sequences called as 3R, 9R or 18R structures depending on the stacking sequences. It is determined that the basal plane of 9R (or 18R) martensites originates from one of the {110}planes of the parent, and a homogenous shear occurs on this plane in two opposite directions during the transformation. Also, the basal plane is subjected to the hexagonal distortion with martensite formation on which atom sizes have important effect. In case the atoms occupying the lattice sites have the same size, the hexagon becomes regular hexagon otherwise Otherwise deviations occur from the hexagonal arrangement. The knowledge of the associated distortion allows us to obtain information on the ordering degree in the martensite. In the present contribution, x-ray diffraction and transmission electron microscopy (TEM) studies were carried out on two copper based ternary alloys aged for long term, nearly 20 years.