Nice introduction and overview of superalloys

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Superalloys are high-performance metallic alloys designed for extreme temperatures, primarily used in aerospace applications, with nickel-based superalloys being the most effective. Key elements like aluminum and titanium create a microstructure that enhances strength and creep resistance, crucial for high-temperature performance. The rising costs of precious metals, particularly rhenium and ruthenium, are significant challenges for superalloy production, as they contribute to improved performance but also increase material expenses. Rhenium enhances creep resistance in superalloys but is extremely rare and costly, while ruthenium is even rarer and more expensive, complicating the development of advanced high-temperature materials. The economic implications of these materials may hinder the advancement of innovative superalloy technologies.
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Here is a nice introduction and overview of superalloys.

http://www.msm.cam.ac.uk/phase-trans/2003/Superalloys/superalloys.html
Nickel Based Superalloys
H. K. D. H. Bhadeshia
A superalloy is a metallic alloy which can be used at high temperatures, often in excess of 0.7 of the absolute melting temperature. Creep and oxidation resistance are the prime design criteria. Superalloys can be based on iron, cobalt or nickel, the latter being best suited for aeroengine applications.

The essential solutes in nickel based superalloys are aluminium and/or titanium, with a total concentration which is typically less than 10 atomic percent. This generates a two-phase equilibrium microstructure, consisting of gamma (γ) and gamma-prime (γ'). It is the γ' which is largely responsible for the elevated-temperature strength of the material and its incredible resistance to creep deformation.
Lots of other pages at the bottom

See also - http://www.msm.cam.ac.uk/phase-trans/2003/nickel.html
 
Engineering news on Phys.org
The cost of raw materials used in producing superalloys has become an increasingly important issue. The precious metal rhenium confers enhanced performance in Cannon-Muskegon's second and third generation single crystal alloys like CMSX-4 and CMSX-10 used extensively in gas turbine engines. Meanwhile, the effects of ruthenium additions are being studied across the world and new alloys containing ruthenium for increased temperature capability are under testing and evaluation. Unfortunately, the costs of these precious metal additions are staggering, representing increasing fractions of the total raw materials costs. . . .

Rhenium (Re) has a density of 21.04 grams per cubic centimeter (surpassed only by platinum, iridium, and osmium) and a melting temperature of 3,180°C (surpassed only by tungsten and carbon). Its atomic number is 75 and its atomic weight is 186.207. It is extremely rare, present in the Earth's crust at only 1 part per billion.1 It is basically a byproduct of a byproduct, being extracted from flue dusts from molybdenum sulphide concentrates, which are derived from purifying copper concentrates. . . .

Superalloys that contain rhenium include CMSX-4, CMSX-10, CM186LC, CMSX486, PW1484, Rene N5, Rene N6, TMS-75, TMS-138, and TMS-162. Levels are typically in the range of 3% to 6%. According to Bhadeshia, rhenium is an expensive addition, but it provides enhanced creep resistance by promoting rafting, making lattice misfit more negative, and by reducing the overall diffusion rate in the nickel-base superalloys. . . .

Ruthenium (Ru) has a melting temperature of 2,334°C, an atomic number of 44, and an atomic weight of 101.07. Ruthenium is ten times more rare than platinum and is difficult to refine and extract, . . . .

As of January, 2007, rhenium occupied the eighth position (of precious metals) at a value of $5,500 per kg. According to the Metal-Pages website, the price per kg of rhenium (in the form of ammonium perrhenate) has increased from about $5,500 in January 2007 to more than $7,000 in May 2007!

Ruthenium occupied the eighth position in 1999 with a value of $1,225 per kg and has risen to the third position with its value of $21,540 per kg.
from http://materialstechnology.tms.org/sup/supHome.asp (8/01/07)

The cost and limited quantities of these materials will necessarily limit the development of exotic high temperature systems.
 

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