A new, fourth generation, single crystal superalloy has been jointly developed by GE Aircraft Engines, Pratt & Whitney and NASA. The focus of the effort was to develop a turbine airfoil alloy with long-term durability for use in the High Speed Civil Transport. In order to achieve adequate long-time strength improvements at moderate temperatures and retain good microstructural stability, it was necessary to make significant composition changes from 2 nd and 3 rd generation single crystal superalloys. These included lower chromium levels, higher cobalt and rhenium levels and the inclusion of a new alloying element, ruthenium. It was found that higher Co levels were beneficial to reducing both TCP precipitation and SRZ formation. Ruthenium caused the refractory elements to partition more strongly to the ' phase, which resulted in better overall alloy stability. The final alloy, EPM-102, had significant creep rupture and fatigue improvements over the baseline production alloys and had acceptable microstructural stability. The alloy is currently being engine tested and evaluated for advanced engine applications.

Superalloys have contributed markedly to societal benefit. It is difficult to imagine the modern world without superalloys. These materials provide the backbone for many applications within key industries that include chemical and metallurgical processing, oil and gas extraction and refining, energy generation, and aerospace propulsion. Within this broad application space, arguably the highest visibility challenges tackled by these materials have arisen from the demand for large, efficient land-based power turbines and lightweight, highly durable aeronautical jet engines. So impressive has been the success of these materials that the last half of the 20 th century has been known as the Superalloy Age. While superalloys have met many technical challenges, the overarching consideration is that no use of these materials occurs unless value to the customer is demonstrated. This paper discusses the emerging paradigm within the aviation industry that applies customer requirements to drive materials development and implementation on an accelerated timeline. This new paradigm is first of all spurring on competition to materials from other technologies, and secondly opening the door to other material classes to compete with superalloys for key applications. The superalloy community has the opportunity to respond with innovative alloys and processing improvements. In the ideal case, this competition will result in the development of the best ideas, such that the end customer, whether a civilian or military aeronautical system operator, receives optimal value. This new development paradigm is leading to overall faster application of advanced materials.

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A comprehensive characterization of γ′ in inertia friction welded Alloy 720Li has been undertaken to quantify the dramatic variation of the γ′ microstructure in the heat affected zone (HAZ) and the effect of the post weld heat treatment (PWHT). Experiments were performed on samples in the as-welded and two different PWHT conditions. High energy synchrotron diffraction was used to investigate the overall variation of γ′ volume fraction across the weld line with a high spatial resolution. During this experiment it was also possible to determine the misfit between γ and γ′ as a function of position in the HAZ and the base material. In addition high and ultra high resolution field emission gun scanning electron microscopy (FEGSEM) studies were carried out in order to individually characterize primary, secondary, tertiary γ′ and γ′ that had reprecipitated upon cooling at the end of the friction welding process. To study the relation between γ′ variation and any changes of strength in the welds, microhardness tests were performed on the three samples. The high energy synchrotron diffraction measurements revealed that in the as-welded condition the HAZ was depleted in γ′ with a trough observed at about 1.5mm from the weld line. No variation in γ′ volume fraction was observed in the two PWHT conditions. FEGSEM studies revealed that at the weld line primary, secondary and tertiary γ′ had been dissolved during the welding process and that reprecipitated γ′ could be observed, even in the as-welded condition. The reprecipitated γ′ appeared to be smallest not at the weld line but at around 1mm axially away from it. Between 1 and 4mm from the weld line, significant coarsening of tertiary was observed even though the welding process had taken only a few seconds to complete. The recorded microhardness of the as-welded sample can be understood in terms of γ′ depletion and the non-optimal particle size of the fine tertiary and reprecipitated γ′. Both PWHT conditions displayed a pronounced microhardness increase towards the weld line that can be explained by the large volume fraction of reprecipitated and coarsened γ′ in this region. Measurements of the lattice misfit between γ and γ′ showed a variation in the HAZ of the welds. A γ/γ′ misfit between 0.11% to 0.15% was observed in the base material of the three samples studied and a minimum of about 0.05% misfit was measured at about 1mm from the weld line. In all cases the lattice spacing of the matrix (γ) was smaller than the lattice spacing of the coherent γ′ precipitates, i.e. representing a positive γ/γ′ misfit. At the moment the nature of the γ/γ′ misfit variation is not clear but it can be assumed that it is related to the temperature history that the material experienced in this region.

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Conservation and the safeguard of precious anthropological information: this somewhat unusual slunt on the theme of this issue is provided by an ethnographer and a conservation scientist in Australia, whose ‘dialogue’ has been sent to Museum in the form of a jointly signed text.

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