Sample size effects related to nickel, titanium and nickel-titanium at the micron size scale

Micron-sized compression specimens, fabricated using a focused ion beam (FIB), indicate a dramatic strengthening effect as sample dimensions are reduced from 20μm to sub-micron diameters in nickel and gold microcrystals. To understand this effect, novel microscopy techniques were utilized to study the mechanical properties and dislocation substructures from microcrystals of pure nickel, Ti-6wt.%Al, Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242) and Ti-50.8at.%Ni. The dislocation behavior that governs plasticity is quite different between each of these materials and as such produces different size effects at small sizes.

The nickel compression results indicate a dramatic increase in strength as sample dimensions are reduced. Quantitative dislocation density measurements performed on slip-plane TEM foils extracted from nickel microcrystals indicate an increase in stored dislocation density at smaller sizes. However, hardening contributions from forest-hardening and source truncation hardening were insufficient in explaining the high observed flow stresses. This result suggests that other hardening mechanism are operating in the nickel microcrystals.

The titanium alloys exhibit a much less dramatic strengthening effect compared to the nickel microcrystals. The titanium microcrystals, at all sample sizes tested (1-60μm), are stronger than bulk compression specimens. Even at the 60μm sizes bulk behavior is not observed, while at only 20 microns nickel microcrystals exhibit bulk properties. Transmission electron microscopy (TEM) investigations indicate several dislocation pile-ups of both screw and edge character at the microcrystal surfaces. These pile-ups appear to be related to ion damage induced by the fabrication of these samples, resulting in a strengthening effect that follows a Hall-Petch relationship.

Nickel-Titanium alloys deform through a phase transformation, as well as dislocation motion. The microcrystal compression results indicate no observable size effect related to the strength of the 5μm and 20μm crystals. TEM studies indicate an increase in dislocation activity, ⟨1 0 0⟩{1 1 0}, with the number of loading cycles. However, determining the relationship between plasticity and the martensitic transformation was inconclusive.

This work indicates a need for further investigations into the effects of dislocation junction formation at small sizes and the effects that gallium ion damage has on the mobility of edge and screw dislocation segments for microcrystals fabricated with the FIB. Each of these may contribute a strengthening effect in the nickel and titanium alloys, respectively. Coupling microcrystal compression experiments with TEM provides a unique methodology for studying a poorly understood relationship between plasticity and the martensitic phase transformation in NiTi alloys.