Physical metallurgy of Ti–Ni-based shape memory alloys

A review of shape memory alloy research, applications and opportunities

Medical devices which are currently proposed to use elements made from shape memory alloys may be improved by the use of stress-induced martensite alloy elements instead. The use of stress-induced martensite decreases the temperature sensitivity of the devices, thereby making them easier to install and/or remove.

Medical devices incorporating SIM alloy elements

Human tissue is structured mainly of self-assembled polymers (proteins) and ceramics (bone minerals), with metals present as trace elements with molecular scale functions. However, metals and their alloys have played a predominant role as structural biomaterials in reconstructive surgery, especially orthopedics, with more recent uses in non-osseous tissues, such as blood vessels. With the successful routine use of a large variety of metal implants clinically, issues associated with long-term maintenance of implant integrity have also emerged. This review focuses on metallic implant biomaterials, identifying and discussing critical issues in their clinical applications, including the systemic toxicity of released metal ions due to corrosion, fatigue failure of structural components due to repeated loading, and wearing of joint replacements due to movement. This is followed by detailed reviews on specific metallic biomaterials made from stainless steels, alloys of cobalt, titanium and magnesium, as well as shape memory alloys of nickel–titanium, silver, tantalum and zirconium. For each, the properties that affect biocompatibility and mechanical integrity (especially corrosion fatigue) are discussed in detail. Finally, the most critical challenges for metallic implant biomaterials are summarized, with emphasis on the most promising approaches and strategies.

Metallic implant biomaterials

At present, strong requirements in orthopaedics are still to be met, both in bone and joint substitution and in the repair and regeneration of bone defects. In this framework, tremendous advances in the biomaterials field have been made in the last 50 years where materials intended for biomedical purposes have evolved through three different generations, namely first generation (bioinert materials), second generation (bioactive and biodegradable materials) and third generation (materials designed to stimulate specific responses at the molecular level). In this review, the evolution of different metals, ceramics and polymers most commonly used in orthopaedic applications is discussed, as well as the different approaches used to fulfil the challenges faced by this medical field.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsif.2008.0151

Biomaterials in orthopaedics.

A summary of recent research on the nitinol alloys and their potential application in ocean engineering

Through single‐crystal x‐ray diffraction methods, the crystal structure of TiNi has been determined in the temperature range −70° to 900°C. Contrary to what has been assumed from previous work based on the powder pattern methods, the TiNi crystal structure is not a simple CsCl type. Rather, it has an a0=9A superlattice and an a0=3A sublattice with 54 atoms per unit cell complex structure.The 9A superlattice undergoes, at about 166°C, a ``martensitic'' pseudo order‐disorder transition which is not accompanied by a crystallographic transformation. Through the understanding of this unique transition the apparent contradicting observations made on TiNi by various past investigators can now be reconciled and the unusual physical properties associated with the alloy are explained qualitatively.

Crystal Structure and a Unique ``Martensitic'' Transition of TiNi

Through an investigation of the transport and related thermodynamic properties of TiNi at and around its ``martensitic'' transition temperature, the existence of a critical range extending over a 60°C interval is established. Within this critical range, the phase transition is second order. Irreversibility of various properties within the critical range is interpreted in terms of irreversible shear movement of atoms. On the basis of transport data, the band structure of TiNi is inferred to be a single or ``nearly'' single positive band within the temperature range investigated. The postulate that some of the valence electrons undergo a ``covalent''→``conduction'' electronic transformation in the course of the second‐order transition is consistent in large measure with experimental data.

The Irreversible Critical Range in the TiNi Transition

METAL ALLOYS OF THE FORMULA TINIXCO1-X, TICOXFE1-X, WHEREIN X IS AN INTEGER FROM 0 TO 1 UNDERGO MARTENSITIC TRANSITIONS OVER A VERY WIDE TEMPERATURE RANGE FROM LESS THAN 4*K. TO 1000*K. THE METAL ALLOYS ARE WORKED AND HEAT TREATED TO EFFECT A REVERSION BACK TO AN ORIGINAL CONFIGURATION AS THE RESULT OF THE MARTENSITIC TRANSITION OF THE ALLOYS DUE TO THE HEAT TREATMENT.

Method of treating variable transition temperature alloys

Some unique physical property changes related with the TiNi martensitic transition, such as ``memory'' effect, have been known for some time. In this letter we report some additional observations which are of interest and are related to the TiNi transition.

William J. Buehler

Papers

A summary of recent research on the nitinol alloys and their potential application in ocean engineering

Crystal Structure and a Unique ``Martensitic'' Transition of TiNi

The Irreversible Critical Range in the TiNi Transition

Method of treating variable transition temperature alloys

Additional unique property changes observed during TiNi transition