David S. Grummon

Microscopic superelastic behavior of a nickel-titanium alloy under complex loading conditions

The microscopic superelastic behavior of a nickel-titanium (NiTi) alloy has been studied by instrumented indentation experiments using both spherical and pyramidal (e.g., Berkovich) diamond indenters. The indentation load–displacement curves for a superelastic NiTi and an annealed copper were obtained under a range of indentation conditions. We show that indentation-induced superelasticity exists under both spherical and pyramidal indenters, which may be exploited for many applications, ranging from microelectromechanical systems to surface engineering.

Recovery of microindents in a nickel–titanium shape-memory alloy: A “self-healing” effect

The thermally induced recovery of microscopic deformation in a nickel–titanium shape-memory alloy was examined. Surface deformation was simulated by indenting the alloy in the martensite phase at room temperature using both spherical and pyramidal indenters. We show that deformation in spherical microindents can be almost completely reversed by moderate heating. Partial recovery was observed for pyramidal impressions formed by a Vickers indenter and the recovery ratio was independent of the indentation depth. The observations were rationalized using the concept of representative strain and maximum stress under the spherical and pyramidal indenters.

Effects of the ratio of hardness to Young's modulus on the friction and wear behavior of bilayer coatings

We present a study of the effects of the ratio of hardness to Young’s modulus on the friction and wear behavior of layered composite coatings. Layered coating structures with the same surface coating but different interlayers were prepared by physical vapor deposition. We found that the ratio of hardness to Young’s modulus plays an important role in determining the friction coefficient and wear resistance of layered composite coatings. A low friction coefficient and high wear resistance can be achieved in structures with high ratio of hardness to Young’s modulus and moderately high hardness.

Microscopic shape memory and superelastic effects under complex loading conditions

Abstract The microscopic shape memory (SM) and superelastic (SE) effects of martensitic and austenitic NiTi alloys were probed by instrumented indentation techniques. Both spherical and pyramidal indenters (i.e. Berkovich and Vickers) were used to determine the mechanical response of the NiTi alloys over a wide range of indentation loads and depths. The magnitude of any SM effects was quantitatively characterized by the thermally activated depth recovery ratio of the residual indentation depth. The magnitude of SE was quantitatively characterized by the depth and work recovery ratios obtained from the load–displacement curves. We show that (1) microscopic SM and SE effects exist under complex loading conditions, (2) the magnitude of the SM and SE effects can be rationalized using the concept of the representative strain and maximum strain and (3) instrumented indentation techniques are useful in quantifying SM and SE effects in the micro- and nano-meter length scales.

Shape memory surfaces

Temperature-controlled reversible surface protrusions can be made on NiTi shape memory alloys and thin films as a result of indentation induced two-way shape memory effect. First, spherical indents or scratches are made on the surface of a NiTi alloy in its martensite phase. Second, the indented or scratched surface is planarized to restore a flat surface. Reversible circular and line protrusions are produced by altering the temperature to drive the martensite to austenite phase transformation. This phenomenon can be exploited for a wide range of optical, tribological, and microelectromechanical device applications.

Superelastic NiTi honeycombs: fabrication and experiments

In this paper we demonstrate a new class of superelastic NiTi honeycomb structures. We have developed a novel brazing technique that has allowed us to fabricate Nitinol-based cellular structures with relative densities near 5%. Commercially available nickel-rich Nitinol strips were shape-set into corrugated form, stacked, and bonded at high temperature by exploiting a contact eutectic melting reaction involving pure niobium. After heat treatment to restore transformational superelastic response, prototype honeycomb structures were subjected to severe in-plane compression loading at room temperature. The specimens exhibited good specific strength, high specific stiffness, and enhanced shape recovery compared to monolithic shape memory alloys (SMAs). Compressive strains of over 50% could be recovered upon unloading. The demonstrated architectures are simple examples of a wide variety of possible built-up topologies, enabled by the bonding method, that can be engineered for customizable net section properties, arbitrary shape, and kinematically enhanced thermomechanical shape-memory and superelastic response.

Low-density open-cell foams in the NiTi system

It is shown that open-cell metallic foams having very low density, and that display martensite transformations required for shape memory and superelastic behavior, can be fabricated using a powder-metallurgy technique. Results are presented on experiments in which a polymeric precursor foam was coated with an equiatomic NiTi powder slurry and subsequently sintered to yield foams with relative densities as low as 0.039. Although contaminated with interstitial impurities, they displayed unambiguous calorimetric signature of the B2→B19′ transformation. The results are of considerable significance to potential applications requiring ultralightweight structures with the unusual dissipative and strain-recovery properties of NiTi shape-memory materials.

Effect of temperature on the devitrification kinetics of NiTi films

The effect of isothermal devitrification temperature on the reaction kinetics and the microstructure of freestanding NiTi films have been studied using differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The minimum temperature for complete crystallization of amorphous sputtered films was found to be 400 °C, and analysis of the nucleation kinetics yielded Avrami exponents between 2 and 3 for all films. Crystalline grains always nucleated first at the surface, grew laterally until impingement, then grew inward to form columnar grains. Surface roughness delayed the onset of surface nucleation. In very smooth surfaced films, multiple DSC exotherms heralded repeated nucleation of new wide, flat grains within the film interior. This secondary microstructure exhibited different kinetics than the columnar grains, and Avrami exponents consistent with a continuous nucleation mode.

Indentation stress dependence of the temperature range of microscopic superelastic behavior of nickel-titanium thin films

The microscopic superelastic behavior of thin-film NiTi is investigated by instrumented indentation experiments conducted at different temperatures. The indentation-induced superelastic effect is found to be persistent to about 100K above the austenite transformation finish temperature (Af). In contrast, the upper temperature where superelastic effect exists is only around Af plus 40K in uniaxial tension and compression tests, beyond which the plasticity of the austenite phase overwhelms the transformation-induced superelasticity. By combining the Clausius-Clapeyron equation and spherical cavity model for indentation, we show that the high hydrostatic pressure under the indenter is capable of elevating the transformation temperatures and increase the upper temperature limit of indentation-induced superelastic behavior.

Latent strain in titanium-nickel thin films modified by irradiation of the plastically-deformed martensite phase with 5 MeV Ni2+

Lattice damage brought on by heavy ion irradiation is able to alter the displacive transformation characteristics of near equiatomic titanium-nickel. Irradiation of sputtered TiNi thin films can modify thermomechanical response to a depth of more than a micron, and may thus be used to create a perfectly bonded heterophase that deploys materials of sharply differing latent thermal strain on opposite sides of a thin sheet. If the alloy film is first martensitized, and then deformed in tension prior to partial-depth exposure to ion beam damage at temperatures well below A{sub s}, a novel active-passive bilayer results that expresses pronounced bending displacements on subsequent heating. In the present paper, describing experiments on stretched 6-{micro}m thick sputtered Ti{sub 50.2}Ni{sub 49.7} Films irradiated with 5 MeV Ni{sup 2+}, the authors show that ion-induced latent bending can be cyclically reversed in temperature-displacement space, and that appreciable mechanical work can be extracted. Marked effects are observed at doses as low as 5 x 10{sup 13} Ni{sup 2+} cm{sup {minus}2}. The approach, in which nominally planar processing is used, derives mechanical robustness from a naturally diffuse interface between the beam-damaged stratum and the adjacent unmodified shape-memory layer.