R. Gotthardt

Interaction between microstructure and multiple-step transformation in binary NiTi alloys using in-situ transmission electron microscopy observations

Abstract The multiple-step transformation is a special case of martensitic transformation observed in NiTi alloys. It is characterized by the appearance of several transformation steps during the transformation of the R phase into martensite. The present study shows that this effect can be observed after the sample has been solution treated at 1173K and further aged at 793 K and is accompanied by a special microstructure which consists in a well recrystallized matrix containing small coherent precipitates of the Ni4Ti3 type. These precipitates have been observed to be the source of a local stress field which influences both the nucleation and the growth of the R and martensitic phases. Based on the present observations, a model has been developed which explains the basic of the mechanisms involved in this phenomenon. According to this model, the additional transformation step can be considered to be a locally stress-induced martensitic transformation. This model is extended to interpret some results found...

Characterization of shape-memory alloy thin films made up from sputter-deposited Ni/Ti multilayers

Abstract A novel fabrication process for Ni–Ti shape-memory alloy thin films is presented. This process is based on the appropriate annealing of sputter-deposited Ni/Ti multilayers. X-ray diffraction shows that interdiffusion of the two constituents results either in the amorphization of the multilayer structure after annealing at 330°C or in the recrystallization as a Ni–Ti intermetallic compound after annealing at temperatures above 400°C. A single 30 min annealing step in the temperature range from 400 to 800°C is sufficient to obtain Ni–Ti films showing martensitic phase transformations and the shape-memory effect. The influence of increasing annealing temperature on the transformation behavior is investigated by differential scanning calorimetry. The evolution of the transformation temperatures is found to be qualitatively similar to conventional sputter-deposited Ni–Ti films. The corresponding microstructure is studied by transmission electron microscopy. A very fine-grained structure is observed even after annealing at 800°C. The film composition can be varied by adjusting the thickness ratio of the individual Ti and Ni layers. Transformation curves of films with nominal compositions of 49.5 and 54.0 at.% Ti are compared. It is demonstrated that Ni–Ti films made up from multilayers may possess an intrinsic “two-way” shape-memory effect, which is a very interesting feature in view of the development of thin film micro-actuators.

A new fabrication process for Ni–Ti shape memory thin films

Abstract A new fabrication method for Ni–Ti thin films showing martensitic phase transformation is presented. A multilayer of up to 100 alternating pure Ni and pure Ti layers is deposited in an automated DC magnetron sputter system optimized for homogeneous deposition on large substrates. The single layer thickness is in the range of 10–20 nm. The multilayer is subjected to different heat treatments in order to allow for interdiffusion of Ni and Ti and subsequent recrystallization as a Ni–Ti intermetallic compound. The martensitic and austenite phase in the resulting films could be identified by means of X-ray diffraction. Differential scanning calorimetry (DSC) measurements monitor the transformation behavior of films with different composition and indicate the existence of an R-phase. The microstructure of the annealed films is investigated by transmission electron microscopy (TEM). The composition and transformation temperatures can be adjusted by varying the thickness ratio of the initial Ni and Ti layers. These films show a well pronounced shape memory effect for certain annealing procedures.

Transformation properties and microstructure of sputter-deposited Ni-Ti shape memory alloy thin films

The influence of annealing parameters on the martensitic phase transformation in sputter-deposited Ti rich Ni-Ti films is systematically studied by differential scanning calorimetry and by transmission electron microscopy. The annealing temperature range extends from the crystallization temperature of the films up to 900°C. For increasing temperature, multiple phase transformations, transformations via an R-phase or direct martensite/austenite transformations are observed. A similar behavior is found for increasing annealing time. Related changes of the film microstructure, such as the strongly varying distribution of round Ti2Ni precipitates in the grains, are analyzed. Transformation temperatures could be shifted over a wide range by adjusting the film composition from 48 to 54 at.% Ti. The corresponding transformation curves, grain structure as well as nature and amount of precipitates were investigated. No subsequent annealing process is required for films deposited on substrates heated above about 500°C. In this case, the as-deposited films have a very fine-grained and homogeneous microstructure.

Latent strain in titanium-nickel thin films modified by irradiation of the plastically-deformed martensite phase with 5 MeV Ni2+

Lattice damage brought on by heavy ion irradiation is able to alter the displacive transformation characteristics of near equiatomic titanium-nickel. Irradiation of sputtered TiNi thin films can modify thermomechanical response to a depth of more than a micron, and may thus be used to create a perfectly bonded heterophase that deploys materials of sharply differing latent thermal strain on opposite sides of a thin sheet. If the alloy film is first martensitized, and then deformed in tension prior to partial-depth exposure to ion beam damage at temperatures well below A{sub s}, a novel active-passive bilayer results that expresses pronounced bending displacements on subsequent heating. In the present paper, describing experiments on stretched 6-{micro}m thick sputtered Ti{sub 50.2}Ni{sub 49.7} Films irradiated with 5 MeV Ni{sup 2+}, the authors show that ion-induced latent bending can be cyclically reversed in temperature-displacement space, and that appreciable mechanical work can be extracted. Marked effects are observed at doses as low as 5 x 10{sup 13} Ni{sup 2+} cm{sup {minus}2}. The approach, in which nominally planar processing is used, derives mechanical robustness from a naturally diffuse interface between the beam-damaged stratum and the adjacent unmodified shape-memory layer.

Active stiffening of composite materials by embedded shape-memory-alloy fibres

The use of shape-memory-alloy (SMA) fibres to actively changethe stiffness of a composite beam is investigated on a model system composed of an epoxy matrix with a series of embedded pre-strained NiTi fibres. Stiffness changes are detected through shifts in the natural vibration frequency of the beam. When electrically heated, the pre-strained NiTi fibres undergo a phase transformation. Since the shape recovery associated with the transformation is restrained by the constraints of both the matrix and the clamping device, a force is generated. This force leads to an increase in the natural vibration frequency of the composite beam. Depending on the degree of fibre pre-strain, either ordinary martensite, R-phase or a mixture of the two can be stress-induced. It is found that the R-phase gives rise to the largest change in vibration frequency for a given temperature increase and the most reversible behaviour. Its low transformation strain is also more favourable for fibre-matrix adhesion. The effect of stress relaxation in the polymer matrix on the composite response is discussed.

Structural anelasticity of NiTi during two-stage martensitic transformation

Abstract The two-staged thermoelastic martensitic transformation (TMT) B2→R→B19′ in polycrystalline equiatomic NiTi has been studied by means of measurements of strain amplitude-independent and amplitude-dependent internal friction (ADIF), Young’s modulus and amplitude-dependent modulus defects. The internal friction measurements were performed at a frequency of about 100 kHz, rendering negligible the transient internal friction component and allowing one to investigate the structural internal friction, much less dependent on the external parameters such as the heating/cooling rate or the frequency of vibrations. Attention is focussed on the amplitude-dependent anelasticity. Based on the data obtained, the anelasticity is associated with the dislocations inside the martensitic variants, not with the interfaces or interface dislocations, as is traditionally done. The ADIF and anelastic strain in the R phase have been found to be an order of magnitude higher than in the B19′ martensitic phase. This observation is explained by a much higher density of the dislocations inside the variants of the R phase as compared with that of the B19′ phase.

Shape Memory Alloy Wires Turn Composites into Smart Structures Part II : Manufacturing and Properties

The manufacturing route and resulting properties of adaptive composites are presented in the second part of this European project report. Manufacturing was performed using a specially designed frame to pre-strain the SMA wires, embed them into Kevlar-epoxy prepregs, and maintain them during the curing process in an autoclave. Composite compounds were then tested for strain response, recovery stress response in a clamped-clamped configuration, as well as vibrational response. Through the understanding of the transformational behavior of constrained SMA wires, interesting and unique functional properties of SMA composites could be measured, explained and modeled. Large recovery stresses and as a consequence, a change in vibrational response in a clamped- clamped condition, or a reversible shape change in a free standing condition, could be generated by the SMA composites in a controllable way. These properties were dependent on composite design aspects and exhibited a reproducible and stable behavior, provided that the properties of the matrix, of the wires and the processing route were carefully optimized. In conclusion, the achievements of this effort in areas such as thermomechanics, transformational and vibrational behavior and durability of SMA based composites provide a first step towards a reliable materials design, and potentially an industrial application.

Changes in stacking fault sequences during the martensitic phase transformation in CuZnAl shape memory alloys

Abstract The movement of an A/D-type intervariant twin boundary, of the internally faulted M18R martensite, was studied during deformation. Transmission electron microscopy in situ observations of CuZnAl single crystals show that changes in the stacking sequence of close-packed planes occur in the converting A- and D-variants. A simple mechanism is proposed which explains the origins of stacking fault changes across the A/D-interface and which accounts for the absence of a pure shear type stacking fault as has been reported in the literature. This mechanism consists of the transformation of one type of random stacking fault into another fault type by the motion of a partial dislocation on the basal plane. Glissile defects on the martensitic basal planes were observed experimentally, when high internal stresses are built up during the retransformation process.

Irradiation-induced phase transformation in undeformed and deformed NiTi shape memory thin films by high-energy ion beams

Irradiation-induced transformations in sputter-deposited Ti-rich NiTi thin films have been investigated with special attention given to the effects of prior deformation on the response of martensite phase. Irradiation at low fluence was carried out at room temperature with Au ions at 350 MeV, such that the projected ion range greatly exceeded the film thickness and lattice damage was attributable mainly to electronic stopping effects (43 keV nm−1). Ion-beam modified microstructures were investigated using transmission electron microscopy, X-ray diffraction, and differential scanning calorimetry. Amorphous tracks were observed in both Ti2Ni precipitates and martensite. The track diameter and the amount of amorphization were larger in the pre-deformed sample as compared to the undeformed sample. In both cases the displacive transformation temperatures and transformation enthalpies were depressed by irradiation, but to a greater degree in the pre-deformed material. Similarly, the extent of austenitic and R-phase regions observed to surround individual ion tracks was substantially greater for the pre-deformed films. These observations are discussed in terms of differences between the deformed and undeformed cases with respect to microstructure.