1. Summary
Jointly organized by The Institute of Materials and the Institute of Mechanical Engineers, the Charles Parsons Conference offered a rare opportunity for turbine designers and materials scientists to meet, discuss progress and identify remaining challenges. Strong technical papers were presented in two concurrent programs addressing gas and steam turbine technologies. This report focuses on the Gas Turbine sessions where talks on Ni-based superalloys dominated. Emphasis was placed on the reduction of life-cycle costs, the relationships between alloy chemistry and performance and on the mechanisms of damage accumulation during service. Notable advances were reported in the use of computer models to design less-expensive alloys with competitive properties and in the development of cost-effective, innovative processing and fabrication methods. Also summarized here are interesting papers on the development of steels for land-based turbines and on designing with titanium-aluminides.
2. Introduction and general impressions
This three-day meeting of engineers and scientists celebrated the turbine engine: its history, recent developments in design and materials, and its future. In general, the technical content was substantial and of a very high caliber. In the sessions on Gas Turbine technology, many papers described advances in the development, processing, physical metallurgy and damage mechanisms of superalloys. Notably, significant advances have been made in the reliability of computational models for predicting mechanical properties as functions of alloy chemistry. Several papers spoke to innovative designs and fabrication technologies for improved performance and lower component cost. One interesting observation: for the first time, the Best Paper in the Gas Turbine category was given to one on titanium aluminides; an analysis of design issues by Ian Perrin of EGT; it is summarized later.
Throughout the conference there was a clear emphasis on the reduction of material, production and maintenance costs, and the role of computational models to meet these objectives dominated discussions. Several models were introduced during the conference: those that describe and predict the influences of alloy composition and processing parameters on material behavior, those that predict remaining component life and determine maintenance schemes, and even those that track component production and material inventories in global manufacturing scenarios. It is evident that the amount of investment in these technologies is significant and can be effectively leveraged if approached intelligently. A clear priority is the development of common, robust, core models to serve as foundations for more specific applications by engine designers, producers and users. As pointed out by Chris Bullough (GEC Alstrom), the immediate need is for industry-wide standardization of terms and definitions for these models.
3. Overviews and future trendsIn his talk "Future Trends in Design and Technology for Aero-Turbines," A. Bradley of Rolls-Royce, described the advantages of their new swept fan-blades for increased air flow and efficiency. A unique design at the blade tip simultaneously provides an efficient thin leading edge and improves foreign object damage resistance
Phil Ruffles of Rolls-Royce presented "Beyond Whittle Innovation in Gas Turbines," describing two engine concepts under development. Noting that for subsonic flight, thrust-to-weight ratio (T:W) is inversely proportional to the number of airfoils, Rolls-Royce proposes that a fifty-percent enhancement in T:W can be achieved. The current EJ200 engine with 1800 airfoils has a T:W of 10; the ACME demonstrator will have only 1200 airfoils and T:W = 15. Also under development is the Trent 8104, an 104,000-pound thrust upgrade for the Trent 892 which will incorporate wide-chord fan blades in a swept fan motor for reduced weight and enhanced F.O.D. resistance. The enabling technologies here are diffusion bonding and super-plastic forming to create wide-chord, twisted blades in six steps.
4. SuperalloysMike Burke (Westinghouse Electric Corp.) described progress in the processing of single crystal components for an Advanced Turbine System in "Cost-Effective Manufacturing of High Performance Power Generation Combustion Turbine Components Using the Fabricated Component Method." In order to operate at higher temperatures and achieve greater power generation efficiency, it would be advantageous to apply the advanced single crystal superalloy technology developed for aerospace applications to the large-scale blades and vanes required in land-based turbines. Simultaneously, production costs must be kept low. One novel approach involves precision casting of single crystal subcomponents which are subsequently joined by transient liquid phase (TLP) bonding. Careful optimization of subcomponent geometries, bond line location, bond surface characteristics, and TLP parameters has resulted in component yields of 90-percent where the strength of the bonded CMSX-4 displays at least 90-percent of the thermal and mechanical capabilities of pure single crystal castings of CMSX-4.
Gordon McColvin, EGT, offered "Second Generation DS Alloy CM186LC for Industrial Gas Turbines," in which he reported on EGT's application of the new superalloy for first stage turbine blades in small industrial gas turbines. CM186LC was developed as a lost-cost variant of first-generation single crystal alloys. Cost minimization is achieved in two ways. First, processing costs are reduced by the low carbon content (LC) and 3 weight percent rhenium addition which improve castability and eliminate the need for solution heat treatment. Additionally, the alloy's composition is similar enough to the CMSX-2, -3 and -4 family of alloys that it can be produced from CMSX foundry revert with essentially no degradation of properties when compared with virgin material.
"Hf on Fatigue Crack Propagation in Gamma-Gamma-Prime Polycrystalline Ni-Based Superalloys," from A.J. Manning (Cambridge University) provided some interesting SEM and TEM observations from two alloys whose only variation is the presence of hafnium. He has found that hafnium significantly alters the nature of grain boundaries in polycrystalline superalloys. When Hf is present Hf-rich MC carbides are generated at the grain boundaries resulting in a four-fold increase in grain boundary carbides; these provide nucleation sites for gamma-prime. Hafnium also promotes eta-phase formation at grain boundaries by substituting for Ti in gamma-prime; the excess Ti in gamma-prime then facilitates the precipitation of eta-phase. Grain boundary strengthening from these effects may be responsible for the delayed transition from transgranular to intergranular crack growth and the significantly lower fatigue crack growth rate observed in Hf-containing superalloys. The researchers are also investigating the influence of Hf on environmental degradation; it is suggested that Hf plays an additional role as oxygen scavenger and thus reduces oxygen-induced intergranular failure.
5. Titanium AluminidesONR-IFO- Europe's home page contains information about our staff and programs, a list of European conferences, and an archive of newsletters. Please look us up at
http://www.ehis.navy.mil
Please note. The opinions and assessments in this report are solely those of the author and do not necessarily reflect official US Government, US Navy or ONR-IFO-EUR positions.
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