Marcus L. Young

Giant magnetostriction in annealed Co 1−x Fe x thin-films

Chemical and structural heterogeneity and the resulting interaction of coexisting phases can lead to extraordinary behaviours in oxides, as observed in piezoelectric materials at morphotropic phase boundaries and relaxor ferroelectrics. However, such phenomena are rare in metallic alloys. Here we show that, by tuning the presence of structural heterogeneity in textured Co(1-x)Fe(x) thin films, effective magnetostriction λ(eff) as large as 260 p.p.m. can be achieved at low-saturation field of ~10 mT. Assuming λ(100) is the dominant component, this number translates to an upper limit of magnetostriction of λ(100)≈5λ(eff) >1,000 p.p.m. Microstructural analyses of Co(1-x)Fe(x) films indicate that maximal magnetostriction occurs at compositions near the (fcc+bcc)/bcc phase boundary and originates from precipitation of an equilibrium Co-rich fcc phase embedded in a Fe-rich bcc matrix. The results indicate that the recently proposed heterogeneous magnetostriction mechanism can be used to guide exploration of compounds with unusual magnetoelastic properties.

Stress-Induced Martensite in Front of Crack Tips in NiTi Shape Memory Alloys: Modeling Versus Experiments

NiTi-based shape memory alloys (SMAs) exhibit an unusual stress distribution at the crack tip as compared to common engineering materials, due to a stress-induced martensitic transformation resulting from highly localized stresses. Understanding the fracture mechanics of NiTi-based SMAs is critical to many of their applications. Here, we develop an analytical model, which predicts the boundaries of the transformation region in the crack tip vicinity of NiTi-based SMAs. The proposed model is based on a recent analytical approach which uses modified linear elastic fracture mechanics concepts to predict the crack tip stress distribution and transformation region in SMAs but, unfortunately, it applies only to the plane stress condition. To overcome this limitation, the proposed model accounts for stress triaxiality, which plays an important role in restricting crack tip plastic deformations in common ductile metals as well as the stress-induced martensite in NiTi SMAs. The effects of triaxial stress at the crack tip are taken into account by including a new parameter, the transformation constraint factor, which is based on the plastic constraint factor of elasto-plastic materials. The predictions of the model are compared with synchrotron x-ray micro-diffraction observations and satisfactory agreement is observed between the two results. Finally, the evolution of crack tip transformation boundaries during fracture tests of miniature compact tension specimens is predicted and the effects of applied load and crack length are discussed.

Archaeometallurgy using synchrotron radiation: a review

Archaeometallurgy is an important field of study which allows us to assess the quality and value of ancient metal artifacts and better understand the ancient cultures that made them. Scientific investigation of ancient metal artifacts is often necessary due to their lack of well-documented histories. One important requirement of analytical techniques is that they be non-destructive, since many of these artifacts are unique and irreplaceable. Most synchrotron radiation (SR) techniques meet this requirement. In this review, the characteristics, capabilities, and advantages and disadvantages of current and future SR facilities are discussed. I examine the application of SR techniques such as x-ray imaging (radiography/microscopy and tomography), x-ray diffraction, x-ray fluorescence, x-ray spectroscopy, Fourier transform infrared spectroscopy, and lastly combined SR techniques to the field of archaeometallurgy. Previous case studies using these various SR techniques are discussed and potential future SR techniques are addressed.

Biological Responses and Mechanisms of Human Bone Marrow Mesenchymal Stem Cells to Zn and Mg Biomaterials

Zn biomaterials attract strong attentions recently for load-bearing medical implants because of their mechanical properties similar to bone, biocompatibility, and degradability at a more matched rate to tissue healing. It has been shown previously that Zn alloys are beneficial for bone regeneration, but the supporting mechanisms have not been explored in detail. Here, we studied the biological responses of human bone marrow mesenchymal stem cells (hMSC) to Zn and the underlying cellular signaling mechanisms. Typical Mg material AZ31 was used as a comparative benchmark control. Direct culture of cells on the materials revealed that cell adhesion, proliferation, and motility were higher on Zn than on AZ31. Significant cytoskeletal reorganizations induced by Zn or AZ31 were also observed. Mineralization of extracellular matrix (ECM) and hMSC osteogenic differentiation, measured by Alizarin red and ALP staining and activities, were significantly enhanced when cells were cultured with Zn or AZ31. Quantitative PCR also showed the increased expression of bone-related genes including ALP, collagen I, and osteopontin. Using small RNA interference to knockdown related key molecules, we illustrated the mechanisms of Zn-induced cellular signaling. TRPM7 and GPR39 appear to be the major cellular receptors facilitating Zn2+-entry into hMSC. The intracellular Zn2+ then activates the cAMP-PKA pathway and triggers intracellular Ca2+ responses, leading to activation of MAPK. In addition, Zn2+ activates the Gαq-PLC-AKT pathway as well. Eventually, all of this signaling would lead to enhanced differential regulation of genes, cell survival/growth and differentiation, ECM mineralization, osteogenesis, and other cellular activities.

Influence of heat treatment and microstructure on the tensile pseudoelastic response of an Ni-rich NiTi shape memory alloy

Abstract The influence of microstructure on the stress–strain behavior of an Ni-rich NiTi shape memory alloy is examined. Specimens cut from a large-diameter bar of Ni50.7Ti49.3 shape memory alloy were analyzed in two states: (i) annealed and (ii) annealed and aged. The annealed state shows a fully austenitic structure with no precipitates and no distortions caused by residual stresses. The annealed and aged state has coherent Ni4Ti3 particles precipitated in the proximity of the austenite grain boundaries. The size of the precipitates increases moving away from the grain boundaries toward the grain interiors. The evolution of the two states in the stress–strain–temperature space has been analyzed using tensile specimens with special geometry. Due to the complex effects of the coherent precipitates, the specimens in the aged state exhibited lower stress plateaus in the tensile loading–unloading curves, which enabled the occurrence of transformation pseudoelasticity from room temperature to 333 K.

Nanoindentation of pseudoelastic NiTi containing Ni4Ti3 precipitates

Abstract Depending on the processing method, pseudoelastic NiTi alloys can have small, lenticular Ni4Ti3 precipitates; however, the mechanical properties of these precipitates are not well understood. By performing nanoindentation with a spherical indenter, Ni4Ti3 precipitates within a pseudoelastic NiTi alloy were examined. Scanning electron microscopy was used to examine the indents after nanoindentation. After unloading, the hardness and remnant depth ratios of the indents in the Ni4Ti3 precipitates, the NiTi matrix, and the “average” NiTi alloy were compared. To decouple the effects of elasticity from those of pseudoelasticity, similar nanoindentation experiments were performed on an NiAl sample and compared with results from the NiTi sample.

SHAPE MEMORY ALLOY ACTUATOR DESIGN: CASMART COLLABORATIVE BEST PRACTICES

Upon examination of shape memory alloy (SMA) actuation designs, there are many considerations and methodologies that are common to them all. A goal of CASMART’s design working group is to compile the collective experiences of CASMART’s member organizations into a single medium that engineers can then use to make the best decisions regarding SMA system design. In this paper, a review of recent work toward this goal is presented, spanning a wide range of design aspects including evaluation, properties, testing, modeling, alloy selection, fabrication, actuator processing, design optimization, controls, and system integration. We have documented each aspect, based on our collective experiences, so that the design engineer may access the tools and information needed to successfully design and develop SMA systems. Through comparison of several case studies, it is shown that there is not an obvious single, linear route a designer can adopt to navigate the path of concept to product. SMA engineering aspects will have different priorities and emphasis for different applications.Copyright

Synthesis of Al0.5CoCrCuFeNi and Al0.5CoCrFeMnNi High-Entropy Alloys by Laser Melting

High-entropy alloys (HEAs) are a blend of 5+ metallic elements that can form solid solution disordered structures. Two HEAs were synthesized using laser melting in an argon atmosphere. The relatively well-studied Al0.5CoCrCuFeNi alloy was first produced to establish the feasibility of laser melting. Manganese was then substituted for copper to potentially lower both cost and density, producing a novel Al0.5CoCrFeMnNi alloy system. These samples were analyzed with XRD, SEM, EDS, and Vickers microhardness to determine the effects of the manganese substitution, as well as the effects of laser melting in comparison to the more traditional arc melting.

Low-Pressure and Low-Temperature Hydriding–Pulverization–Dehydriding Method for Producing Shape Memory Alloy Powders

Shape memory alloys (SMAs) are of high interest as active, adaptive “smart” materials for applications such as sensors and actuators due to their unique properties, including the shape memory effect and pseudoelasticity. Binary NiTi SMAs have shown the most desirable properties, and consequently have generated the most commercial success. A major challenge for SMAs, in particular, is their well-known compositional sensitivity. Therefore, it is critical to control the powder composition and morphology. In this study, a low-pressure, low-temperature hydriding–pulverization–dehydriding method for preparing well-controlled compositions, size, and size distributions of SMA powders from wires is presented. Starting with three different diameters of as-drawn martensitic NiTi SMA wires, pre-alloyed NiTi powders of various well-controlled sizes are produced by hydrogen charging the wires in a heated H3PO4 solution. After hydrogen charging for different charging times, the wires are pulverized and subsequently dehydrided. The wires and the resulting powders are characterized using scanning electron microscopy, differential scanning calorimetry, and X-ray diffraction. The relationship between the wire diameter and powder size is investigated as a function of hydrogen charging time. The rate of diameter reduction after hydrogen charging of wire is also examined. Finally, the recovery behavior due to the shape memory effect is investigated after dehydriding.

Processing-induced strain glass states in a Ni49.5Ti50.5 shape memory alloy

Shape memory alloys (SMAs) represent a revolutionary and innovative class of active materials which can provide potential solutions to many of todays engineering problems due to their compact form, high energy densities, and multifunctional capabilities. While many applications in the biomedical, aerospace, and automotive industries have already been investigated and realized for Nickel-Titanium (NiTi) based SMAs, the effects of restricting the ferroelastic transformation to nanosized domains is not well understood and the potential remains untapped. In binary NiTi, the martensitic transformation, which is characterized by long-range strain ordering (LRO), can be replaced with a strain glass transition, which consists of an LRO parent phase and a short-range strain ordered glassy phase. Such alloys have been named strain glass alloys (SGAs) due to the fact that they exhibit a glassy state which results from compositionally- or processing-induced strain. While SGAs do not exhibit a stress-free, temperature-induced macroscopic phase change, they still exhibit the strain recovery and actuation capabilities intrinsic to near equiatomic NiTi and other SMAs. It has been shown in the available literature that certain compositions, for example 51.5 at. % Nickel in binary NiTi, can create a strain glass; however, these compositionally-induced NiTi SGAs generally have transformation temperatures below 173 K and this will restrict their practical applications. In the present study, a new method for producing a strain glass phase in Ti-rich NiTi through sufficient plastic deformation via cold work is reported; the resulting SGA exhibits a temperature-induced ferroelastic recovery above room temperature. Additionally, the macroscopic actuation capabilities are improved when compared to both compositionally-induced SGAs and the base material due to the increased functional stresses of the SGA. To better understand the transition from an SMA to an SGA, Ni49.5Ti50.5 (at. %) rods were processed to several degrees of cold work and characterized via scanning and transmission electron microscopy, differential scanning calorimetry, thermomechanical testing, and synchrotron radiation x-ray diffraction. The experimental results indicate that twin size decreases with additional cold work and, around 45% thickness reduction, stress-free thermal cycling no longer results in a measurable phase transformation; however, mechanically-induced phase transformation is still possible, where fully recoverable strains in these SGAs were observed to be above 4.5% when loaded at room temperature and recovered at 150 °C.Shape memory alloys (SMAs) represent a revolutionary and innovative class of active materials which can provide potential solutions to many of todays engineering problems due to their compact form, high energy densities, and multifunctional capabilities. While many applications in the biomedical, aerospace, and automotive industries have already been investigated and realized for Nickel-Titanium (NiTi) based SMAs, the effects of restricting the ferroelastic transformation to nanosized domains is not well understood and the potential remains untapped. In binary NiTi, the martensitic transformation, which is characterized by long-range strain ordering (LRO), can be replaced with a strain glass transition, which consists of an LRO parent phase and a short-range strain ordered glassy phase. Such alloys have been named strain glass alloys (SGAs) due to the fact that they exhibit a glassy state which results from compositionally- or processing-induced strain. While SGAs do not exhibit a stress-free, temperatur...