Characterization of Shape Memory Alloys by DSC and DMA, Part 1: DSC Analysis

An object made from a shape memory alloy recovers its original shape after deformation when it is heated. This phenomenon is known as the shape memory effect. Objects made from shape-memory alloys also exhibit superelasticity. This is the name given to the property that a strongly deformed object returns to its original shape when the load causing the deformation is removed. In Parts 1 and 2 of this article series, we will describe these properties and show how they can be investigated by DSC and DMA.

Introduction

Objects made from shape memory alloys (SMA) exhibit two interesting properties: the shape memory effect and superelasticity. In the shape memory effect, a plastically deformed object made from a shape memory alloy recovers its “original” shape when it is heated.

In superelasticity, an object is referred to as superelastic if it recovers its “original” shape after deformation of up to 10% when the load causing the deformation is removed.

For these two effects to occur, the material must have been previously conditioned in an “original” shape. The shape memory effect and superelasticity are illustrated in Figure 1.

Well-known shape memory alloys include nickel-titanium alloys and copper alloys such as CuZnAl (copperzinc-aluminum), CuAlNi (copperaluminum-nickel) or CuAlBe (copperaluminum-beryllium). The most widely used nickel-titanium alloys were developed in the early 1960s at the U.S. Naval Ordnance Laboratory and commercialized under the name nitinol (Nickel-Titanium Naval Ordnance Laboratory).

Nitinol combines high corrosion resistance, biocompatibility, the shape memory effect and superelasticity. Due to these properties, nitinol is used in the aerospace industry, automotive engineering, in electronics, and for medical applications.

Practical uses are for example for pipe connections (the collars are stretched in the cold state and shrink-fitted by heating), actuators, medicinal implants (stents) or dental braces.

The shape memory effect and superelasticity in shape memory alloys result from reversible solid-state transformations in the crystalline structure. At high temperatures, shape memory alloys are present in the austenite form, at low temperatures in the martensite form.

Nitinol-austenite has a simple body-centered cubic structure with a nickel atom in the middle and four titanium atoms in the corners of the cell (see Figure 2, left). The martensite form is less symmetrical and has a monoclinic lattice (see Figure 2, right).

In shape memory alloys, martensite can be present as normal martensite (detwinned martensite) or as a twinned structure (twinned martensite) (see Figure 3). If austenite is cooled, the twinned martensite structure is formed. Normal martensite occurs when austenite or the twinned structure of martensite is subjected to a mechanical stress [1].

These structural transformations can be investigated by both DSC and by DMA. In this first article, we want to focus on analysis by DSC. DMA measurements will be discussed in Part 2 of this article series.

Experimental Details

The DSC experiments were carried out using a METTLER TOLEDO DSC 1 equipped with an FRS5 sensor and an IntraCooler. The samples were nitinol wires with different compositions, diameters, and thermal pretreatment. In addition, measurements with a DSC-Microscopy system were performed to visually illustrate the shape memory effect.

Results and Discussion

Characteristic transformation temperatures

Figure  4 shows the DSC curves of a nitinol sample that consisted of equal numbers of nickel and titanium atoms (equiatomic nitinol). The martensitic sample was heated from 0 °C to 100 °C at 10  K/min and then cooled at the same rate. Before this, the sample was annealed at 540 °C for 10 hours.

Conclusions

The deformation of an object made of a shape memory alloy can be completely eliminated either by heating (shape memory effect) or by removing the stress that causes the deformation (superelasticity). These unusual properties are the result of reversible transformations of the crystal lattices. The transformation temperatures depend strongly on the composition and thermal history of the material. The different effects can easily be investigated by DSC.

The shape memory effect can be directly observed using a DSC-Microscopy system.

^{Characterization of Shape Memory Alloys by DSC and DMA, Part 1: DSC Analysis | Thermal Analysis Application No. UC 403 | Application published in METTLER TOLEDO Thermal Analysis UserCom 40}