Darrell Gaydosh

Microstructure and tensile properties of Fe-40 at. pct Al alloys with C, Zr, Hf, and B additions

The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe6Al6Zr and Fe6Al6Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions significantly improved high-temperature ductility by suppressing cavitation.

Room temperature flow and fracture of Fe-40at.%Al alloys

Abstract The room temperature tensile behavior of Fe-40at.%Al alloys was investigated. Extrusion of both prealloyed powders and castings was performed to produce a wide range of grain sizes for characterization of mechanical properties. In addition, directionally solidified single crystals were also studied. It was found that the influence of processing variables such as extrusion temperature or the form of the starting material (powder vs. cast ingot) on mechanical properties could be explained primarily by their effects on grain size. Grain refinement improved both ductility and strength, whereas rapid quenching after annealing resulted in increases in yield strength and decreases in ductility. The effects of quenching were explained by the evidence of large numbers of quenched-in vacancies. In the binary alloy, fracture was primarily intergranular irrespective of cooling rate, while alloys containing boron or Zr+B failed transgranularly and maintained their ductility in the rapidly quenched condition. Single crystals oriented in the [100] direction showed evidence of slip behavior yet did not exhibit significant tensile ductility. This lack of ductility was attributed to an early onset of cleavage failure.

Properties and Potential of Two (ni,pt)ti Alloys for Use as High-temperature Actuator Materials

The microstructure, transformation temperatures, basic tensile properties, shape memory behavior, and work output for two (Ni,Ti)Pt high-temperature shape memory alloys have been characterized. One was a Ni30Pt20Ti50 alloy (referred to as 20Pt) with transformation temperatures above 230 °C and the other was a Ni20Pt30Ti50 alloy (30Pt) with transformation temperatures above 530 °C. Both materials displayed shape memory behavior and were capable of 100% (no-load) strain recovery for strain levels up to their fracture limit (3-4%) when deformed at room temperature. For the 20Pt alloy, the tensile strength, modulus, and ductility dramatically increased when the material was tested just above the austenite finish (Af) temperature. For the 30Pt alloy, a similar change in yield behavior at temperatures above the Af was not observed. In this case the strength of the austenite phase was at best comparable and generally much weaker than the martensite phase. A ductility minimum was also observed just below the As temperature in this alloy. As a result of these differences in tensile behavior, the two alloys performed completely different when thermally cycled under constant load. The 20Pt alloy behaved similar to conventional binary NiTi alloys with work output due to the martensite-to-austenite transformation initially increasing with applied stress. The maximum work output measured in the 20Pt alloy was nearly 9 J/cm3 and was limited by the tensile ductility of the material. In contrast, the martensite-to-austenite transformation in the 30Pt alloy was not capable of performing work against any bias load. The reason for this behavior was traced back to its basic mechanical properties, where the yield strength of the austenite phase was similar to or lower than that of the martensite phase, depending on temperature. Hence, the recovery or transformation strain for the 30Pt alloy under load was essentially zero, resulting in zero work output.

Development of a Numerical Model for High-Temperature Shape Memory Alloys

A new thermomechanical hysteresis model for a high-temperature shape memory alloy (HTSMA) actuator material is presented. The proposed Brinson–Preisach model is capable of predicting the strain output of a tensile-loaded HTSMA when excited by arbitrary temperature–stress inputs for the purpose of actuator and control design. Quasistatic generalized Preisach hysteresis models available in the literature require large sets of experimental data for model identification at a particular operating point, and substantially more data for multiple operating points. The minor loop algorithm is an alternate approach to common Preisach methods that is better suited for research-stage alloys, such as recently developed HTSMAs, for which a complete identification database is not yet available. A detailed description of the minor loop hysteresis algorithm is presented in this paper and a methodology for determination of model parameters is introduced. The algorithm is assembled together with a modified form of the one-dimensional Brinson constitutive equation to provide a continuous thermomechanical response even within the characteristically wide detwinning region of the HTSMA. The computationally efficient algorithm is shown to demonstrate each of the unique characteristics of Preisach minor loop hysteresis over the usable actuation range in high-stress, high-temperature applications.

Development of a HTSMA-Actuated Surge Control Rod for High-Temperature Turbomachinery Applications

In recent years, a demand for compact, lightweight, solid-state actuation systems has emerged, driven in part by the needs of the aeronautics industry. However, most actuation systems used in turbomachinery require not only elevated temperature but high-force capability. As a result, shape memory alloy (SMA) based systems have worked their way to the forefront of a short list of viable options to meet such a technological challenge. Most of the effort centered on shape memory systems to date has involved binary NiTi alloys but the working temperatures required in many aeronautics applications dictate significantly higher transformation temperatures than the binary systems can provide. Hence, a high temperature shape memory alloy (HTSMA) based on NiTiPdPt, having a transformation temperature near 300 o C, was developed. Various thermo-mechanical processing schemes were utilized to further improve the dimensional stability of the alloy and it was later extruded/drawn into wire form to be more compatible with envisioned applications. Mechanical testing on the finished wire form showed reasonable work output capability with excellent dimensional stability. Subsequently, the wire form of the alloy was incorporated into a benchtop system, which was shown to provide the necessary stroke requirements of ~0.125 inches for the targeted surge-control application. Cycle times for the actuator were limited to ~4 seconds due to control and cooling constraints but this cycle time was determined to be adequate for the surge control application targeted as the primary requirement was initial actuation of a surge control rod, which could be completed in approximately one second.

Calibration of a three-dimensional multimechanism shape memory alloy material model for the prediction of the cyclic “attraction” character in binary NiTi alloys

As typically utilized in applications, a particular shape memory alloy device or component operates under a large number of thermomechanical cycles, hence, the importance of accounting for the cyclic behavior characteristics in modeling and characterization of these systems. To this end, the present work is focused on the characterization of the evolutionary, cyclic behavior of binary 55NiTi (having a moderately-high transformation temperature range). In this study, an extensive set of test data from recent cyclic, isobaric, tension tests was used. Furthermore, for the calibration and characterization of this material, a newly developed, multiaxial, material-modeling framework was implemented. In this framework, multiple, inelastic mechanisms are used to regulate the partitioning of energy dissipation and storage governing the evolutionary thermomechanical response.

Work production using the two-way shape memory effect in NiTi and a Ni-rich NiTiHf high-temperature shape memory alloy

The work output capacity of the two-way shape memory effect (TWSME) in a Ni50.3Ti29.7Hf20 (at%) high-temperature shape memory alloy (HTSMA) was investigated and compared to that of binary Ni49.9Ti50.1 (at%). TWSME was induced through a training procedure of 100 thermomechanical cycles under different tensile stresses. It was observed that TWSME in as-extruded and trained Ni50.3Ti29.7Hf20 could produce 0.7% strain against a compressive stress of 100 MPa, corresponding to a maximum work output of 0.08 J g−1, compared to a maximum value of 0.06 J g−1 for binary NiTi. A peak aging heat treatment of 3 h at 550 °C, which previously has been shown to result in near-perfect functional stability in Ni50.3Ti29.7Hf20 during isobaric thermal cycling, did not improve the TWSME and actually resulted in a decrease in the magnitude and stability of the TWSME and its work output capacity. Nevertheless, the magnitude of TWSM behavior of Ni50.3Ti29.7Hf20, in the absence of an aging heat treatment, renders it an attractive candidate for high-temperature TWSM actuation.

RAPIDLY SOLIDIFIED NiAl AND FeAl.

Melt spinning was used to produce rapidly solidified ribbons of the B2 intermetallics NiAl and FeAl. Both Fe-40Al and Fe-45Al possessed some bend ductility in the as spun condition. The bend ductility of Fe-40Al, Fe-45Al, and equiatomic NiAl increased with subsequent heat treatment. Heat treatment at approximately 0.85 T (sub m) resulted in significant grain growth in equiatomic FeAl and in all the NiAl compositions. Low bend ductility in both FeAl and NiAl generally coincided with intergranular failure, while increased bend ductility was characterized by increasing amounts of transgranular cleavage fracture.

Influence of Test Procedures on the Thermomechanical Properties of a 55NiTi Shape Memory Alloy

Over the past few decades, binary NiTi shape memory alloys have received attention due to their unique mechanical characteristics, leading to their potential use in low-temperature, solid-state actuator applications. However, prior to using these materials for such applications, the physical response of these systems to mechanical and thermal stimuli must be thoroughly understood and modeled to aid designers in developing SMA-enabled systems. Even though shape memory alloys have been around for almost five decades, very little effort has been made to standardize testing procedures. Although some standards for measuring the transformation temperatures of SMAs are available, no real standards exist for determining the various mechanical and thermomechanical properties that govern the usefulness of these unique materials. Consequently, this study involved testing a 55NiTi alloy using a variety of different test methodologies. All samples tested were taken from the same heat and batch to remove the influence of sample pedigree on the observed results. When the material was tested under constant-stress, thermal-cycle conditions, variations in the characteristic material responses were observed, depending on test methodology. The transformation strain and irreversible strain were impacted more than the transformation temperatures, which only showed an affect with regard to applied external stress. In some cases, test methodology altered the transformation strain by 0.005-0.01mm/mm, which translates into a difference in work output capability of approximately 2 J/cm3 (290 in•lbf/in3). These results indicate the need for the development of testing standards so that meaningful data can be generated and successfully incorporated into viable models and hardware. The use of consistent testing procedures is also important when comparing results from one research organization to another. To this end, differences in the observed responses will be presented, contrasted and rationalized, in hopes of eventually developing standardized testing procedures for shape memory alloys.

Transformation and Deformation Characterization of NiTiHf and NiTiAu High Temperature Shape Memory Alloys

NiTiHf and NiTiAu are exciting candidates amongst an emerging class of high temperature shape memory alloys (HTSMAs), exhibiting properties conducive to actuator applications in demanding automotive and aerospace environments. NiTiHf can be tailored to achieve a highly favorable balance of properties, including high strength, stability, and work output at temperatures up to 300°C, yet at a reduced cost compared to Pt, Pd, and Au containing counterparts [1]. NiTiAu shows potential for work output at much higher temperatures – where the benefits may offset cost – but additional research is needed. These investigations focus on developing a fundamental understanding of the inherent microstructure-property relationships for these NiTi-based HTSMAs. The attractive characteristics seen in many of these systems are strongly influenced by the formation of nano-scale precipitates. Advanced electron characterization techniques are used to explore the interaction of these precipitates with martensite at low temperature, and with dislocations at higher temperature. These insights are further incorporated into microstructural modeling frameworks with the aim of developing accurate simulations of polycrystalline functional response.