Shape Memory Alloys

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Introduction to Shape Memory Alloys

Shape memory alloys (SMAs) are specialized metals exhibiting two unique properties: pseudo-elasticity and the shape memory effect. Arne Olander first observed these unusual properties in 1938, but significant research advances only emerged in the 1960s.

The fundamental phenomenon of the shape memory effect was extensively documented a decade after Olander's discovery by Russian metallurgist G. Kurdjumov and by Chang and Read. However, these properties gained wider recognition following the development of nickel-titanium alloy (Nitinol) by Buehler and Wang. Since then, research in this field has intensified, with numerous alloys investigated, including Ag-Cd, Au-Cd, Cu-Zn, Cu-Zn-Al, Cu-Al-Ni, Cu-Sn, Cu-Au-Zn, Ni-Al, Ti-Ni, Ti-Ni-Cu, Ni-Ti-Nb, Ti-Pd-Ni, In-Ti, In-Cd, and others. Currently, the most effective and widely utilized alloys include Ni-Ti, Cu-Zn-Al, and Cu-Al-Ni.

Crystallography and Engineering Challenges

The crystallographic properties of shape memory alloys have been studied for four decades. Because these materials are relatively recent innovations, some engineering aspects remain incompletely understood. Traditional engineering descriptors such as Young's modulus and yield strength have limited applicability to SMAs due to their strong temperature dependence. Instead, researchers must employ new descriptors such as stress rate and amnesia to characterize these materials properly.

SMAs function effectively as actuators—materials that "change shape, stiffness, position, and other mechanical characteristics in response to temperature or electromagnetic fields." Their potential applications as actuators have expanded possibilities across numerous scientific disciplines. The diverse applications have increased both the importance and visibility of these metals globally.

Phase Transformations: The Science Behind Shape Memory

The alloys demonstrating "shape memory" can undergo surprisingly large strains and then, upon temperature increase or unloading, revert to their original shape. Ni-Ti based SMAs currently provide the best combination of material properties for most commercial applications, though they are considerably more expensive than Cu-based alternatives. In many applications, Cu-based alloys offer a more economical option. Cu-Zn based SMAs have already been implemented in practical applications, while Cu-Al based SMAs show considerable promise.

The two unique properties of SMAs are made possible through a solid-state phase change—a molecular rearrangement occurring within the alloy. The two phases in shape memory alloys are martensite and austenite. Martensite, the relatively soft and easily deformed phase, exists at lower temperatures. Its molecular structure exhibits twinning, as shown in Figure 1 (middle). Upon deformation, this phase transforms to the configuration shown in Figure 1 (right). Austenite, the stronger phase, occurs at higher temperatures and has a cubic structure, as shown in Figure 1 (left). The undeformed martensite phase appears identical to the cubic austenite phase at the macroscopic level, meaning no visible size or shape change occurs in SMAs until the martensite is deformed.

Figure 1: Microscopic and macroscopic views of the two phases of shape memory alloys

The temperatures at which these phases begin and finish forming are represented by Ms, Mf, As, and Af. The load applied to an SMA increases these four variables as illustrated in Figure 2. The initial values of these variables are significantly affected by the alloy's composition (the proportions of each element present).

Figure 2: The dependency of phase change temperature on loading

The Shape Memory Effect: Mechanism and Applications

The shape memory effect becomes apparent when an SMA is cooled below the Mf temperature. At this stage, the alloy consists entirely of martensite, which can be easily deformed. After distortion, the original shape can be recovered simply by heating the material above the Af temperature. The heat transferred to the alloy drives the molecular rearrangement, similar to how heat melts ice into water, though the alloy remains solid. The deformed martensite transforms into the cubic austenite phase, which assumes the original shape of the material.

The shape memory effect has been implemented in various applications, including:

• The space shuttle
• Hydraulic fittings for airplanes
• Thermostats
• Vascular stents
• Coffeepots

Nitinol: The Premier Shape Memory Alloy

Nickel-titanium alloys have proven to be the most useful of all SMAs. Other shape memory alloys include copper-aluminum-nickel, copper-zinc-aluminum, and iron-manganese-silicon alloys. The generic name for nickel-titanium alloys is Nitinol. In 1961, William J. Buehler, a researcher at the Naval Ordnance Laboratory in White Oak, Maryland, discovered that Nitinol (Nickel Titanium Naval Ordnance Laboratory) possessed shape memory properties.

Nitinol undergoes phase changes while remaining solid. These phase transitions, martensite and austenite, "involve the rearrangement of particle positions within the crystal structure of the solid." Below the transition temperature, Nitinol exists in the martensite phase.

The transition temperature varies between approximately -50°C and 166°C depending on the specific composition. In the martensite phase, Nitinol can be bent into various shapes. To establish the "parent shape," the metal must be held in position and heated to approximately 500°C. This high temperature "causes the atoms to arrange themselves into the most compact and regular pattern possible," resulting in a rigid cubic arrangement known as the austenite phase. Above the transition temperature, Nitinol reverts from martensite to austenite, returning to its parent shape. This cycle can be repeated millions of times.

Pseudo-elasticity and Comparative Analysis of SMA Types

Pseudo-elasticity occurs in SMAs when the alloy consists entirely of austenite (temperature exceeds Af). Unlike the shape memory effect, pseudo-elasticity occurs without temperature change. The load on the SMA increases until the austenite transforms into martensite solely due to the applied force.

Figure 3: Microscopic diagram of the shape memory effect

Among the developed shape memory alloys, Au-Cd, Cu-Zn-Al, Cu-Al-Ni, and similar materials have demonstrated properties comparable to Ti-Ni alloys. The latter two Cu-type alloys are generally considered prominent due to their lower cost. When comparing these Cu-type alloys, Cu-Al-Ni is superior in both shape memory performance and heat stability (heat resistance). However, Cu-Al-Ni's poor cold workability limits its applications. Researchers have discovered that adding Ti to Cu-Al-Ni can improve workability by producing finer crystal grains.

Advantages and Challenges of Shape Memory Alloys

Some key advantages of shape memory alloys include:

• Biocompatibility
• Diverse application potential
• Excellent mechanical properties (strength, corrosion resistance)

Despite these benefits, challenges remain before SMAs can reach their full potential. These alloys remain relatively expensive to manufacture and machine compared to conventional materials like steel and aluminum. Most SMAs also exhibit poor fatigue properties, meaning that under identical loading conditions (twisting, bending, compressing), a steel component might endure more than one hundred times more cycles than an SMA element.

Conclusion

Shape memory alloys represent an innovative class of materials with unique properties that enable numerous specialized applications. As research continues and manufacturing techniques improve, these materials will likely find even more applications across various industries. The combination of their unique mechanical properties with ongoing advances in material science suggests that shape memory alloys will play an increasingly important role in future technological developments.