Marc Zupan

Deformation mechanisms and microtensile behavior of single colony Ti-6242Si

The constant strain rate deformation behavior of individual a ‐b colonies of the titanium aeroengine alloy Ti‐6Al‐2Sn‐4Zr‐ 2Mo‐0.1Si (compositions in wt.%) has been investigated through microsample tensile testing techniques. The mechanical behavior and deformation mechanisms observed were found to depend strongly on colony orientation. A near-Burger’s orientation relationship (OR) has been observed for the a (hcp) and b (bcc) phases in these colonies, grown utilizing a vertical float zone technique. The Burger’s OR is believed to be responsible for the observed anisotropy in the critical resolved shear stress for the three a:3e11-20e{10-10} prism slip systems. A more pronounced anisotropy was observed in the critical resolved shear stress (CRSS) for the three a:3e11-20e{0001} hcp basal slip systems. Scanning Electron Microscopy (SEM) investigations of the slip line morphology of the deformed samples indicates an inhibition of b-lath shearing for colonies oriented for basal slip, while frequent shear offsets of the b phase by slip lines were observed in the samples oriented for prism slip. The effects of the Burger’s OR, slip line spacing and elastic interactions stresses arising from the a ‐b interface are considered in explaining the observed anisotropy in CRSS.

The plastic collapse and energy absorption capacity of egg-box panels

The plastic collapse response of aluminium egg-box panels subjected to out-of-plane compression has been measured and modelled. It is observed that the collapse strength and energy absorption are sensitive to the level of in-plane constraint, with collapse dictated either by plastic buckling or by a travelling plastic knuckle mechanism. Drop weight tests have been performed at speeds of up to 6 m s-1, and an elevation in strength with impact velocity is noted. A 3D finite element shell model is needed in order to reproduce the observed behaviours. Additional calculations using an axisymmetric finite element model give the correct collapse modes but are less accurate than the more sophisticated 3D model. The finite element simulations suggest that the observed velocity dependence of strength is primarily due to strain-rate sensitivity of the aluminium sheet, with material inertia playing a negligible role. Finally, it is shown that the energy absorption capacity of the egg-box material is comparable to that of metallic foams.

Development of High-temperature Microsample Testing

Microsample tensile testing has been established as a means of evaluating the room temperature mechanical properties of specimens with gage sections that are tens to hundreds of microns thick and several hundred microns wide. The desire to characterize the mechanical response of materials at elevated temperatures has motivated the development of high-temperature microsample testing that is reported here. The design of specially insulated grips allows the microsamples to be resistively heated using approximately 2 V DC and currents ranging between 2 to 6 A. An optical pyrometer with nominal spot size of 290 μm and 12 μm diameter type K thermocouples was employed to measure and verify the temperature of the microsamples. The ability of the pyrometer to accurately measure temperature on microsamples of different thicknesses and with slightly different emissivities was established over a temperature range from 400°C to 1100°C. The temperature gradient along the length and thickness of the microsample was measured, and the temperature difference measured in the gage section used for strain measurements was found to be less than 6.5°C. Examples of elevated temperature tensile and creep tests are presented.

Yielding behavior of aluminum-rich single crystalline γ-TiAl

Abstract The yielding behavior of Al-rich single crystalline γ-Ti 55.5 at% Al has been measured along near [0 0 1], [0 1 0] and [ 1 1 0 ] orientations in both tension and compression and as function of temperature. All three orientations displayed anomalous yielding and a pronounced tension/compression asymmetry. The relative strength of the material, location of the anomalous yielding peak, and tension/compression asymmetry are all orientation dependent. A micromechanical superdislocation model addressing crystal geometry, sense of the applied load, Escaig forces and the Yoo torque is presented to explain the observed experimental results.

Application of Fourier Analysis to the Laser Based Interferometric Strain/Displacement Gage

The laser based interferometric Strain/Displacement Gage (ISDG) measures the in-plane surface deformation between two small reflecting surface markers. When illuminated with a coherent beam of light, the reflected beams from the two markers form an interference pattern, and monitoring the shift of the fringe pattern allows strain in the gage section of a specimen to be directly measured. A minimum on the fringe pattern can be isolated and tracked as the test proceeds, but this technique utilizes only a small part of the optical signal and often requires a complex programming scheme. This paper presents the application of Fourier transform and phase shifting techniques to the use of the ISDG during microsample tensile testing. The Fourier transform samples the entire fringe pattern and greatly improves the optical signal to noise ratio, and the phase shifting fringe pattern analysis has proven to be more robust and less affected by speckle or optical noise than fringe pattern minimum tracking. This results in the ability to measure larger deformations with a system resolution of ∼5 microstrain and an uncertainty of ±15.5 microstrain. An example involving the microsample tensile testing of a MEMS related LIGA Ni specimen is included to demonstrate the utility of these new techniques.

Mechanical Performance of Pumice-reinforced Epoxy Composites

Currently, most cellular solid sandwich panel cores require multiple advanced manufacturing procedures that drive increasing costs for applications. Pumice is a naturally occurring cellular solid that exhibits a microstructure very similar to advanced ceramic foams. This work presents a manufacturing process for producing composite panels of pumice stones and epoxy binder. The cost and weight efficient structure is evaluated through mechanical testing and investigated for possible energy absorption applications. Mechanical performance as a function of relative density is presented and compared to currently available core materials, and scaling laws developed.

Tensile and Compression Testing of Single-Crystal Gamma Ti-55.5 Al

Gamma based titanium aluminides are considered to be promising high temperature application alloys because of their exceptional high temperature mechanical properties and good oxidation resistance. Moreover, with a density less than half of current nickel based super alloys, the increased power to weight ratio that can be realized by using titanium aluminides is very attractive to the automotive and aircraft industries. Here the orientation and temperature dependence of the flow strength of {gamma}-TiAl is being measured to promote a fundamental understanding of the deformation mechanisms that are active in this alloy. High quality single crystals of {gamma}-Ti-55.5 Al have been grown using an optical float zone furnace, which allows for crystal seeding and provides a containerless growth environment. These crystals have been oriented using back reflection Laue and TEM and cut into microsample tensile specimens by electric discharge machining. The microsample testing technique developed at Johns Hopkins is being utilized to measure the orientation, temperature and tension/compression dependence of the flow strength of TiAl. An outline of the microsample testing techniques that have been developed for this study and preliminary results follow in this paper.

Microsample Testing of Single Crystalline Ti-52 at%Al and Ti-55.5 at%Al

Dislocation activity in two-phase commercial TiAl alloys occurs most readily in the {gamma}-TiAl phase, and measurements of the CRSS of single crystalline {gamma}-TiAl provides a solid foundation for understanding the mechanical performance of these alloys. Single crystals of {gamma}-TiAl with greater than 54.5 at% Al have been grown using the optical float zone crystal growing technique, but single crystals with lower Al content, closer to that of commercial alloys, have not. In the present study, polycrystalline ingots of Ti-52 at% Al have been heat treated to form very large grains, and microsample tensile specimens, which have a nominal gage cross-section of 250{micro}m x 300{micro}m, a gage length of 250{micro}m and an overall length of 3 mm, have been machined from within single grains. Microsample high-temperature stress-strain curves for Ti-55.5 at% Al single crystals oriented along the [001] and [010] are presented. Tensile test results for Ti-52 at% Al tested at 1,073K along the[{bar 2}37], [{bar 1}73] and [{bar 3}44] orientations will also be discussed.