Near-a titanium alloys are an integral part of aeroengines; however, since the 1970s, it has been recognized that laboratory and field components fail in a reduced number of cycles when a dwell at the peak stress is imposed. Research over the last few decades has shown that one of the primary reasons for the debit in fatigue life is related to the presence of microtexture in these alloys. Many aeroengine components were forged before the concept of microtexture, and its deleterious effects, had been realized. Thus, because of the increased potential for early failure of these components, a need exists for a nondestructive method to assess the degree of microtexture present in legacy hardware in order to separate those which are prone to dwell fatigue failure from those that are not. Hardware with a high degree of microtexture can be scheduled for more frequent inspections to reduce the risk of in-flight failure. The present work describes a methodology by which this can be achieved using ultrasonic attenuation measurements of the component in pulse-echo imaging mode. The results indicate nearly linear dependence of ultrasonic attenuation on microtextured region size in the d/k = 0.1 to 1.0 range, where d and k are the effective microtexture region size in the direction of wave propagation and the ultrasonic wavelength, respectively.

No abstract

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.

Beta-Ti alloys contain sufficient concentrations of b stabilizing alloy additions to permit retention of the metastable b phase after cooling to room temperature. Decomposition of the metastable b phase results in the formation of several possible phases, at least two of which are metastable. Concurrently, equilibrium a phase often forms first by heterogeneous nucleation at the a grain boundaries with an accompanying precipitate free zone observed adjacent to the grain boundary a. The grain boundary regions are softer than the precipitation hardened matrix. As a consequence, fracture follows the prior b grain boundaries, especially in highstrength conditions. This fracture mode results in low tensile ductility and/or fracture toughness. This article will describe methods of minimizing or eliminating grain boundary a formation by using metastable transition precipitates to nucleate a more rapidly. The effects on fracture behavior also will be described.

No abstract