R.W.Broomfield
Rolls-Royce plc.
PO Box31, Derby DE24 8BJ
U.K.
The development of new high temperature materials for gas turbines has always been driven by competitive pressures, and by the need to improve fuel economy in times of high oil prices. On the other hand, such developments are restrained by the paramount requirement for safe operation, and by the need to control the cost of the engine.
The direction of technical development has also been towards unchanging goals : performance improvement, cost reduction, weight reduction and (more recently) environmental impact. The relative importance of these factors varies with time and with engine application. This paper sets out to describe Rolls-Royce's current response to these pressures, in the field of high-temperature materials. The term "high temperature" is taken in an absolute rather than a relative sense, so the materials to be described include cast single crystal superalloys, ceramics, intermetallics and materials for use in the combustor. It does not include "high temperature" polymers, titanium alloys and so on. For each class of material, the current options are reviewed before outlining Rolls-Royce's approach.
As far as ceramics are concerned, composite materials based on SiC or oxides are chosen because of their relatively good defect tolerance. Several trial components have been made. However the cost in component form is several times that of the metal version, to achieve a 25-30% weight saving in one example. Our current emphasis is towards lower cost ceramic materials.
On intermetallics, RR made a small foray into nickel aluminide, using a different alloy family from G.E.'s (-NiAl, but this was not successful and the work has been terminated.
Single crystal aerofoil materials are of course well established, but they are still being developed in two ways; new alloys for higher temperature use, and extension into static components such as vanes and seal segments. The emphases for the two types of development are very different. For blades one uses expensive alloying additions and processing for maximum performance, coupled with compatibility with thermal barrier coatings. Rolls-Royce has two alloy development programmes, aimed at cooled HP and uncooled IP blades respectively; the second of these is in collaboration with NRIM in Japan. For static parts the aim is to minimise the cost increase which is associated with the transition from equiaxed to single crystal components.
For combustor materials the position is disappointing from a technical viewpoint, if not from a commercial one. There are many options in materials and design - sheet metal, tiled construction, centrifugal casting, spray forming - but the tried and tested options of sheet metal and conventionally cast tiles are the ones which are being pursued.