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High-resolution, in situ, sub-surface observations of microstructure evolution during martensitic phase transformation
The images above show changes in the internal microstructure of a metal while it is cooled. This particular material undergoes a diffusionless, reversible phase transformation from a high-temperature phase called "austenite" to a low-temperature phase called "martensite." Taken 100s of μm under the surface, these measurements show the austenite phase "disappearing"--this is the austenite transforming to martensite (martensite phase not shown). The measurements (lattice rotation, elastic strain, and dislocation density) reveal surprising internal material behavior both near to and
far from the transformation. These measurements were made using a new X-ray
diffraction technique called Dark-Field X-Ray Microscopy (DFXM). This
experiment is the first-ever DFXM experiment on the class of materials called
shape memory alloys, and the results demonstrate the utility of DFXM in
studying embedded microstructures and interfaces in advanced and functional
materials in situ and in operando.
Ferroelastic twin reorientation in shape memory alloys elucidated with 3D X-ray microscopy
The reversible rearrangement of crystallographic twins enables a variety of functional material technologies but is also one of the most challenging material behaviors to observe. Here, we use three-dimensional (3D) X-ray diffraction methods to resolve sequential snapshots of the load-induced rearrangement of monoclinic twin
microstructures through bulk specimens in 3D and across three orders of magnitude
in length scales. Using these techniques, we are able to track the volumes of each
crystallographic variant through loading and reconstruct portions of strain localization
bands in situ and in 3D. Analyses of the data elucidate the sequence of twin
rearrangement mechanisms that occur within the propagating strain localization
bands, connect these mechanisms to the texture evolution, and reveal the effects of
geometrically necessary lattice curvature across the band interfaces. These findings
will guide future researchers in studying, modeling, and employing twin
rearrangement in novel multiferroic technologies.
3D in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium
Near-field and far-field high-energy diffraction microscopy (nf- and ff-HEDM) and microcomputed tomography (mCT) X-ray techniques were used to study the formation of subgrains as a result of constant force thermal cycling actuation cycles applied to a bulk single crystal nickel-titanium (NiTi) shape memory alloy (SMA) sample. We present 3D reconstructions of the microstructure under load and at high temperatures, including the formation, size, location, and structure of emergent high-angle and low-angle grain boundaries and distributions of the boundary misorientation angles. These results are used to understand the prevalence of new high angle grain boundaries within a bulk sample relative to low-angle subgrains, as well as the expected representative volume for observing the different subgrain mechanisms. This microstructure evolution understanding is then connected to functional fatigue, the predominant mode of failure for SMA actuators.
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Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel-titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. 3D in situ reconstructions of the microstructure are presented, including the structure and distribution of emergent grain boundaries. After just one cycle, the subgrain structure is significantly refined, and heterogeneous Σ3 and Σ9 grain boundaries emerge. The low volume and uneven dispersion of the emergent Σ boundaries across the volume show why previous transmission electron microscopy investigations of Σ grain boundary formation were inconsistent.
3D X-ray diffraction methods were used to analyze the evolution of the load-induced rearrangements of monoclinic twin microstructures within bulk nickel–titanium specimens in 3D and across six orders of magnitude in length scales. These findings will guide future researchers in employing twin rearrangement in novel multiferroic technologies, and they demonstrate the strength of 3D, multiscale, in situ experiments to improve our understanding of complicated material behaviors and to provide opportunities to advance our abilities to model them.
The image to the left shows three-dimensional X-ray diffraction (3DXRD) data for a shape memory alloy (SMA) before (top) and during (bottom) deformation (red = austenite, green = martensite); the results reveal both strengths and weaknesses of long-accepted micromechanical theories of martensitic transformations. The 3DXRD techniques and analyses presented in this work provide non-destructive means to study the mysteries of complicated microstructure evolution occurring underneath the surface of SMAs, as well as other crystalline materials.
The properties of shape memory alloys (SMAs) can be designed for specific engineering applications by tuning the composition and through thermomechanical processing. In this paper, a simple procedure for expounding the relationships between engineering performance metrics and composition and processing is introduced. The results reiterate known trends between micromechanical properties and engineering performance, as well as show important departures from such trends that can improve the performance of SMAs in several ways simultaneously.
Myths and truths of Nitinol mechanics: elasticity and tension–compression asymmetry
The unusual and desirable properties of nickel-titanium shape memory alloys, or Nitinol--the shape memory effect, superelasticity, and actuation--stem from a complex system of underlying micromechanics. Historically, it has been difficult to understand the causal relationship between micromechanics and mechanical performance, and a number of myths were born as a result. Over the past few decades, research has collectively shed light on many of the true mechanics of Nitinol, as well as what we do not yet know. In this article, two of the more prominent myths of Nitinol mechanics are surveyed under the spotlight of truths elucidated through modern research.
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