The discovery of SMAs was monumental, affecting multiple industries such as aerospace, automotive, biomedical, and civil engineering (Stoeckel, 1990; Hartl and Lagoudas, 2007; Kapoor, 2017). This material group has extensively impacted the fields of active and elastocaloric materials, due to desired thermal and mechanical properties, cyclical abilities, while being lightweight and elementally diverse. The quintessential characteristics of SMAs are the shape memory effect, martensitic transformation, pseudoelastic effect, and actuation response due to exhibiting high actuation densities (Otsuka and Wayman, 1998; Hartl and Lagoudas, 2008). The shape memory effect is defined by the recoverability hysteresis that shows the phase transformations that occur from thermal cycling and/or applying load. The martensitic transformation is a sharp single-step transformation that occurs when the material shifts from austenite to twinned martensite. It is a solid–solid, diffusionless transformation resulting from a change in crystal structure but no diffusion of atoms. The pseudoelastic effect is defined by the complete shape recovery that occurs when the material is loaded in the high-temperature austenitic phase. A phase transformation from austenite to martensite occurs when loaded, which appears as additional elasticity, that is, “pseudoelasticity.” The material transforms back to the austenite phase upon unloading, allowing for a repeatable and fully recoverable shift between the two phases (Tadaki et al., 1988). While SMAs have been studied extensively (Tadaki et al., 1988; Stoeckel, 1990; Otsuka and Wayman, 1998; Miller and Lagoudas, 2001; Otsuka and Ren, 2005; Hartl and Lagoudas, 2007; Hartl and Lagoudas, 2008; Lobo et al., 2015; Kapoor, 2017; Ko et al., 2021), the effect of restricting the martensitic transformation (B2→B19′ transformation) to nanodomains has remained relatively unexplored. This group of SMAs are called strain glass alloys (SGAs) (Sarkar et al., 2005; Wheeler et al., 2018a).

SGAs are a recently discovered group of active metallic alloys derived from SMAs. Unlike SMAs, SGAs do not show any phase transformation from thermal cycling; however, a phase transformation occurs via mechanical loading (Wheeler et al., 2018a). Some of the improved functional properties of SGAs are operational stresses, dimensional stability, and higher actuation densities (Wheeler et al., 2019). SGAs represent a new method of processing SMAs to obtain desired characteristics with multifunctional properties.

The transformation from SMA to SGA occurs when the martensitic transformation is disrupted, and glass-like martensitic nanodomains are produced via induced strain. The martensitic transformation noticed in SMAs, which is a single-step sharp transition, is replaced by the continuous strain glass transition, along with highly strained austenite (Wheeler et al., 2018b; Wang M. et al., 2022). This disruption can occur by compositionally or processing-induced strain. A precise amount of strain is necessary in both types of SGAs, causing the interruption of the transformation and making the alloy harder. The increase in hardness inhibits the thermal phase transformation and increases the actuation stresses of the alloy, which ultimately increases the work output of the SGA (Wheeler et al., 2018b).

SGAs have been mostly studied in near-equiatomic NiTi systems (Sarkar et al., 2005; Zhang et al., 2010; Qin et al., 2017; Wheeler et al., 2018a; Wheeler et al., 2018b; Bag et al., 2019; Wheeler et al., 2019; Liang et al., 2022). Compositionally induced strain occurs from altering the alloy composition and/or incorporating additional elements to SMAs, causing lattice distortions. These lattice distortions break up the long-range order (LRO) martensitic twins noticed in SMAs and result in short-range order (SRO) martensite that forms amorphous nanodomains (Ren, 2011). Whenever the alloy system is above a critical concentration, the thermal phase transformation is no longer observed. Elemental additions in NiTi-based SMA systems such as Fe (Wang et al., 2010; Hou, 2012), Cu (Hao et al., 2019; Wang W. et al., 2022), Co, Cr, and Mn (Zhou et al., 2010) have also proven to have a strain glass transition at specific concentrations. Most studied SGA systems are compositionally induced (Sarkar et al., 2005; Wang et al., 2010; Zhang et al., 2010; Zhou et al., 2010; Hou, 2012; Qin et al., 2017; Hao et al., 2019; Chen et al., 2020; Xu et al., 2021; Wang W. et al., 2022); however, another method of achieving strain glass states called processing-induced SGAs was recently identified (Wheeler et al., 2018a; Wheeler et al., 2018b; Wheeler et al., 2019; Liang et al., 2022). In compositionally and processing-induced SGAs, strain is introduced into the lattice in the form of atomic size mismatch or dislocations, respectively. As the concentration of these strain-inducing defects increases, the domain size of martensite decreases (Wang M. et al., 2022). When the defect concentration surpasses a critical value, there is a transformation in microstructure from twinned LRO domains to fragmented nanodomains.

This study focuses on processing-induced SGAs, which are produced by introducing dislocations into SMAs via cold working. These dislocations disturb the periodic distribution of atoms, which disrupts the martensitic transformation and causes glassy SRO martensite by finely fragmenting the martensitic twins. For processing-induced SGAs, an initial SMA composition is cold worked in steps, or rolling passes, to the point at which the thermal response is fully suppressed. One unique aspect of processing-induced SGAs, as compared to compositionally induced SGAs, is that the transformation temperatures can remain fixed.

It is important to distinguish processing-induced SGAs from cold-worked SMAs (Miller and Lagoudas, 2001; Khelfaoui and Guénin, 2003; Chang et al., 2017) and amorphous SMAs (Koike et al., 1990; Sergueeva et al., 2003; Peterlechner et al., 2010; Jiang et al., 2013). Cold-working SMAs is a method of hardening the material that is commonly employed for processing and to obtain amorphous NiTi SMAs. While processing-induced SGAs may seem synonymous with cold-worked or amorphous NiTi SMAs, there are key characteristics that distinguish the three groups of alloys. SGAs are comprised of a crystalline matrix with amorphous nanodomains (Wheeler et al., 2018a), while amorphous SMAs are comprised of an amorphous matrix with a nanocrystalline phase (Koike et al., 1990). Generally, the intent with cold-worked SMAs is to create finer grains and introduce some dislocations into the material, which improves the ease of transformation (Miller and Lagoudas, 2001). Cold-worked SMAs tend to still exhibit thermal-phase transformation and recoverability (Chang et al., 2017). In contrast, amorphous SMAs are cold worked with the intent of achieving total SRO and exhibit degraded pseudoelastic behavior and shape memory effect, meaning the alloy shows very low or no recoverability but exhibits an increase in elasticity and strength (Ko et al., 2021). Conversely, SGAs show nearly linear pseudoelastic behavior (Wang et al., 2006). Complete amorphization, or merely cold working the material, is not the aim with processing-induced SGAs. Rather, the aim is to obtain the precise amount of strain required to suppress the thermal phase transformation and provide enhanced mechanical properties. One of the key distinctions of processing-induced SGAs is recognizing when the thermal transformation has been fully suppressed, meaning the martensite has become thoroughly distributed as amorphous nanodomains in a strained austenitic crystalline matrix. Beyond this critical point, the material becomes more amorphous and mechanically brittle, while below this critical point, the material still shows some LRO and only exhibits a strained thermal transformation. The enhanced mechanical properties of SGAs are a result of precisely controlling the amount of input strain, that is, dislocations worked into the material. The introduction of dislocations makes the material harder to transform, which in turn increases the amount of work generated by the SGA. The purpose and importance of SGAs, as compared to cold-worked and amorphous SMAs, lies in the ability of the material to have shape memory-like behavior with a fully suppressed thermal response and an enhanced mechanical response.

The aim of this study is to investigate and compare the two compositions of the commercially available SMA plates and explore where the SGA transition occurs. This was achieved by cold-working sections of the Ni_49.5Ti_50.5 and Ni_50.8Ti_49.2 (at. %) SMA plates in 5% increments until the thermal response was fully repressed. Differential scanning calorimetry (DSC) was performed on the cold-rolled samples to determine the critical amount of strain necessary to fully suppress the thermal transformation that indicates when a potential SGA has been created. Vickers hardness testing provided an elementary method to determine the heightened resistance to plastic deformation that is characteristic of potential SGAs. Both DSC and Vickers hardness testing served as preliminary characterization techniques to prove a potential SGA was achieved by showing the suppressed thermal transformation and proving an increase in resistance to plastic deformation. Transmission electron microscopy (TEM) and synchrotron radiation X-ray diffraction (SR-XRD) were further performed to determine the strain glass transition for both plates and provide detail regarding the micro- and nano-structural characteristics. This testing allows further elucidation of processing-induced SGAs and their associated properties.

SMA plates of Ni_49.5Ti_50.5 (SM495) and Ni_50.8Ti_49.2 (SE508) were supplied by ATI Specialty Alloys & Components. These compositions were specifically investigated for four reasons: a) highly researched; their behavior and properties are well-known, b) commercial availability, c) binary compositions make it easier to isolate the effects of cold work, and d) the presence of different phases result in different properties at room temperature. At room temperature, Ni_49.5Ti_50.5 is fully martensitic, while Ni_50.8Ti_49.2 is fully austenitic.

Although the compositions seem analogous, the 1.3 at. % Ni difference has a large impact on the transformation temperatures and associated properties. The Ni_49.5Ti_50.5 SMA plate is a Ni-lean NiTi composition that is known for its shape memory effect at room temperature, along with being in the martensitic phase. In similar studies by Wheeler et al. (2018a), Wheeler et al. (2018b), and Wheeler et al. (2019), a Ni_49.5Ti_50.5 SMA rod was cold worked to discover that the processing-induced strain glass state in this material occurred at a 44% thickness reduction. An in-depth analysis of the same Ni_49.5Ti_50.5 material via SR-XRD characterization was conducted by Ashmore et al. (2023). The Ni_50.8Ti_49.2 SMA plate is a Ni-rich NiTi composition that is known for its pseudoelasticity at room temperature as well as being in the austenitic phase.

The objective of this research is to further identify the critical defect concentration to induce a strain glass transition in existing SMA NiTi systems. Studies have shown that both Ni_49.5Ti_50.5 (Wheeler et al., 2018a; Wheeler et al., 2018b; Wheeler et al., 2019; Ashmore et al., 2023) and Ni_50.8Ti_49.2 (Liang et al., 2022) have a strain glass transition. These two SMA plates were chosen for this study to further investigate and compare the effects of disrupting the martensitic transformation on the material that starts in either martensite or austenite at room temperature.

The as-received SMA state (0% cold rolled) was fully characterized for comparison with SGA samples and to estimate the amount of cold rolling necessary to suppress the martensitic transformation.

Rolling in linear passes at room temperature was conducted using a Durston Rolling Mill to ensure a precisely controlled amount of input strain. DSC samples were prepared by cutting a small sample section, weighing between 14 mg and 25 mg, with a low-speed diamond saw. The sample was then placed and sealed in an Al crucible before being programmed and tested using a Netzsch DSC 204 F1 Phoenix. The samples were tested using a temperature profile between −100°C and 200°C at a heating rate of 10°C/min. The process of cold rolling in 5% increments and performing DSC continued in steps until the transformation temperatures were constant, meaning there was no associated enthalpy peak detected for both Ni_49.5Ti_50.5 and Ni_50.8Ti_49.2. The previous 5% increment samples were then revisited and rolled in smaller 1% increments to verify exactly when the SGA state was reached, that is, the precise amount of rolling required to completely remove the thermal response, or enthalpy peaks. Further characterization was employed and necessary to prove a strain glass transition.

Vickers hardness was performed to test the alloy’s ability to resist plastic deformation as it was processed. Samples were prepared for each 5% increment by cold mounting and polishing to a mirror finish. Ten indents, arranged in a 5 × 2 array with 0.5 mm spacing, were programmed using an automatic Vickers hardness tester at 0.5 HV. The resulting hardness values were averaged for each increment and graphed.

TEM was performed to visualize the nanostructure and nanodomains between the as-received SMA and cold-rolled SGA states. TEM samples were prepared using a focused ion beam (FIB) lift-out technique with an FEI Nova FIB/SEM. TEM was performed using an FEI Tecnai G2 F20S-Twin S/TEM operating at 200 keV. High-resolution TEM micrographs were collected in the high-angle annular dark field and bright-field Z-contrast configuration.

The as-received state, two intermediate cold-rolled states, and the potential SGA state were further characterized using SR-XRD to collect wide-angle X-ray scattering (WAXS) diffraction patterns and d-spacing plots. High-energy SR-XRD measurements were collected at the Sector 6-ID-D beamline of the Advanced Photon Source (APS) at Argonne National Laboratories. A beam energy of 60 keV and a beam size of 400 × 200 μm² were used. Samples were aligned with the long axis of the sample in the vertical direction normal to the beam. Each ex situ diffraction pattern represents 100 summed, 0.1 s exposures, for a total of 10 s per diffraction image. Full Debye–Scherrer diffraction rings were collected using a PerkinElmer amorphous Si detector at a distance of 36 cm from the sample and calibrated using a CeO₂ standard.

A recent method of processing SMAs was utilized to understand the effect of disrupting the martensitic LRO and forming amorphous nanodomains with martensitic SRO. Based on the obtained results, SGA states were analyzed and confirmed using various characterization methods to determine trends and compare Ni_49.5Ti_50.5 and Ni_50.8Ti_49.2. For Ni_49.5Ti_50.5, a 33% thickness reduction was required to obtain a cold work-induced strain glass state, while a 24% reduction was required for Ni_50.8Ti_49.2. This percentage difference is due to the dissimilar amounts of Ni and the resulting parent phase. Various characterization methods provided in-depth analysis and further evaluated the trends for obtaining these strain glass states, as well as the effect of having different parent phases for the starting as-received material.

DSC was the initial characterization method employed to ascertain the amount of cold rolling required for potential SGA states. These potential states were determined by the absence of an enthalpy peak, indicating fixed and constant transformation temperatures. A quintessential characteristic of SGAs is the absence of a thermal phase transformation, which is why DSC tends to be the preliminary characterization method. Vickers hardness testing was conducted to display the enhanced mechanical abilities of potential SGAs compared to SMAs in a straightforward manner. A relatively linear increase with the amount of cold rolling occurs until the SGA state, where the Vickers hardness peaks. The incorporated strain enhances the work output of the material. TEM was the first nano-characterization method to visualize the strain glass transition and verify potential SGA states. A variation in micro-/nano-structure was noticed across the SMA samples, such as lathes, triple points, and striations. Specifically, martensitic twins were noted for the Ni_49.5Ti_50.5 alloy, and large austenitic grains were present for the Ni_50.8Ti_49.2 alloy. A rather consistent profile of martensitic nanodomains is observed across the SGA samples. Different parent phases also resulted in microstructural differences between the two alloys. SR-XRD Debye–Scherrer 2D diffraction patterns further clarified the strain glass transition of these samples based on the increased amount of texture, a shift toward strong grain alignment, and nonuniform size and intensity distribution of the diffraction pattern rings. Peak broadening was a trend noticed for both d-spacing plots and 1D SR-XRD plots and is a common trend for the transition toward more amorphous-like states from a crystalline state; however, it is only a contributing factor for detecting the stain glass transition. The 1D SR-XRD data showed SGAs to be consistent across temperature profiles, which correlated with the obtained DSC data that indicated a retained austenitic phase and a suppressed martensitic phase.

Analysis using these techniques validated the potential strain glass state obtained from DSC and further defined the transition from SMA to SGA using process-induced methods while providing a comparative analysis of using alloys of differing parent phases. The evolution of mechanical ability, microstructure, and crystallographic features was observed under increasing strain.