Paul Alfred Siemers

Characterization of the high-temperature phase fields near stoichiometric γ-TiAl

The phase fields above ∼1250°C in the Ti−Al system containing 46 to 55 at. pct Al were characterized using consolidated rapid solidification-processed materials. The characterization techniques used included metallography after heat treatments and dilatometry. Ingot solidification microstructures were also evaluated. The results indicate the existence of a hexogonal phase with transus temperatures ranging from ∼1325°C at 46 at. pct Al to ∼1440°C at the solubility limit of ∼51 at. pct Al. These results agree well with the Margolin Ti−Al phase diagram. Other results relating to the melting and solidification of the hexagonal phase are also consistent. The present measurements, however, show a much wider γ+hexagonal phase field, since the determined γ-TiAl phase boundary runs from the peritectic composition of ∼52.5 at. pct Al at ∼ 1440°C to ∼51 at. pct Al at 1250°C. It is possible that the large two-phase field is partially due to the 600 to 900 ppm oxygen present in the studied materials.

A comparison of the mechanical properties and microstructures of intermetallic matrix composites fabricated by two different methods

The tensile properties of SCS-6 SiC fiber-reinforced Ti-24Al-11Nb (at. pct) have been measured over the past several years by a number of investigators. These composites have been fabricated by different techniques and tend to exhibit a large amount of scatter in the longitudinal tensile properties. To date, it is not known if one optimized fabrication method provides composites with improved mechanical properties over those produced by other optimized methods, since carefully controlled experiments have not been performed to determine this. Thus, the purpose of the present study was to compare the longitudinal tensile strengths of SCS-6 SiC/ Ti-24Al-11Nb composites that had been fabricated by the powder-cloth method and the lowpressure plasma spray (LPPS) method. In this study, the same lots of matrix powder and reinforcing fiber were used for fabricating the composites. It was determined that the powder-cloth and plasma spray methods produced composites having very similar tensile properties. Both fabrication methods induced damage in a small percentage of fibers, which manifested itself in the form of bimodal Weibull distributions of extracted fiber strengths. It appeared that the particular lot of SCS-6 fiber used in fabricating both types of composites was more susceptible to fabrication damage than those used in previous studies. This article also shows the dramatic effect that different handling and testing techniques can have on measured fiber strengths.

Bearing Wear Due to Mechanical Stresses and Electrical Currents

Passage of current through moving conductive contacts results in electrical discharge and then melting of the material, which leads to wear. Such kind of bearing wear is common in electrical machines. There are however certain patterns which are unique to this kind wear. This wear pattern is called ‘fluting’, which are repetitive in nature. Electrical discharge can create higher surface roughness. Also the thermal and rheological properties of the lubricant play a big role in the film thickness formation. The passage of current through the lubricant also changes this and is determined by the electrical properties of the lubricant. In this work effect of bearing currents on a 7204 angular contact ball bearing is studied. This is tested with and without different cage materials with an axial load and no radial load, rotating at 2700 rpm. Four experiments were done at different-level of voltage, lube and cage material. Type of lubricant was seen to play a significant role in fluting.Copyright