ADVANCED MATERIALS & PROCESSES | MARCH 2025 37 F2004 and F2082 were created to determine transformation temperatures by differential scanning calorimetry (DSC) and bend and free recovery, respectively, and ASTM F2005 defines standard terminology for NiTi-based SMAs. These specifications and test methods are currently maintained by ASTM task groups F04.12—Metallurgical Materials and F04.15.15—Nitinol Test Methods. Over the past two decades, nonmedical industries, including automotive, aerospace, and oil and gas, have seen successful commercial application of SMA actuation without standards[14]. In many cases, these applications relied on internal qualification procedures or, in some cases, the standards for superelastic SMAs developed primarily for medical applications. Figure 1 shows the shared usage of the core medical SMA standards as indicated by the overlap in the medical and actuation communities. The nonmedical industries and actuator applications could experience more widespread use of SMA applications with optimized manufacturing processes and quality assurance guided by standards developed specifically for actuation. Around 2020, with an initial focus on certification of aerospace SMA actuator application[15] the first set of test methods for SMA actuation were published[16]. This included E3097— Standard Test Method for Uniaxial Constant Force Thermal Cycling (UCFTC)[17], E3098—Standard Test Method for Uniaxial Pre-strain and Thermal Free Recovery (UPFR)[18], and E3414—Standard Test Method for Constant Torque Thermal Cycling[19]. These test methods are currently maintained by task group ASTM task group E08.05.10—SMA for actuation with a scope that includes specifications and test methods for alloys that undergo a thermally induced phase transformation. The overview provided here is focused primarily on ASTM SMA standards, however the VDI-German Guidelines[20] and JIS-Japanese Industrial Standards[21] also develop and maintain standards and best practices for both SMA medical devices and actuation. DEVELOPMENT ACTIVITIES Additional standards and revisions are needed. There are ongoing standards development activities, several of which are described here. Contact information for each ASTM work item (WK) and elastocaloric activities can be found in Table 1. WK 84104 Determining the Particle and Inclusion Size, and Count in Nitinol Products. In the context of multiple failed interlaboratory studies in ASTM committee F04.12 on Metallurgical Materials to determine a precision and bias statement for F2063 inclusion content, it was realized that existing test methods for nonmetallic inclusions were inadequate for NiTi-based SMAs. A main application in industry is writing Nitinol material specifications and the testing thereof between suppliers and purchasers. Therefore, a current effort (ASTM WK 84104) is developing a new Standard Test Method for Determining the Porosity and Inclusion Content of Nickel Titanium Shape-Memory Alloys Using Image Analysis. The new test method, under the responsibility of ASTM committee E04.14 on Quantitative Metallography, should be applicable across industries and materials (not limited to medical or pseudoelastic). It will cover sample preparation techniques, imaging procedures, and digital image analysis. The initial focus of the new ASTM test method is on testing hot-worked product, per F2063, using scanning electron microscopy (SEM) in backscatter mode and optical microscopy. A one-lab repeatability study will be conducted upon initial publication to satisfy ASTM Form and Style Manual requirements for a new standard test method. A full international inter- laboratory study (ILS) on the test method will be conducted in the future. The working group plans to expand the scope of the standard in future revisions to also cover semi-finished products (e.g., bar, wire, and tube), as well as final devices or components. It is also envisioned to add another section later, elaborating on the application of ASTM E2283[22] on extreme value analysis of steels. It will advise on how to use this technique for a better understanding of the fatigue performance of Nitinol materials (e.g., mean free path between inclusions, nearest neighbor concept, volume at risk in an application). A first round of balloting was completed in January 2025, with courtesy ballots to committees E08 on Fatigue and Fracture and F04.12 Metallurgical Materials and F04.15 Test Methods, both for Medical Devices and Surgical Materials and Devices. These courtesy ballots were intended to reach the many Nitinol users that reside in these two major ASTM committees, allowing the gathering of broad feedback from the global Nitinol community on the new draft test method. WK 90888 Revision of F2063, Wrought NiTi-based SMA for Medical Devices and Surgical Implants. For decades, this specification has been the cornerstone of Nitinol material standards and testing methods, supporting its commercialization in medical applications. The latest initiative involves forming task group WK 90888 to revise the specification, focusing on material grading based on micro-cleanliness and expanding considerations beyond binary Nitinol. A key aspect of this revision is refining terminology to ensure consistency and precision, particularly in product classifications. One critical update is distinguishing between “hollow” and “tube.” While “hollow” is a form of mill product, the current F2063-18 specification incorrectly classifies “tube” as such, whereas it should be categorized as a semi-finished product. Additionally, the terms “billet” and “coil” are introduced to provide a more comprehensive product classification system, addressing gaps in the current version. 7 FEATURE
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