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August 2015

Oxidation and Mechanical Behavior of Super alloys

Superalloys are those nickel, iron and cobalt based corrosion and heat resistant alloys that are normally employed above temperature 540oC or1000oF. Iron based nickel superalloys are an extension of stainless steel technology and normally are melted and cast to electrode and ingot shapes for final fabrication to parts. The Nickel-iron based superalloys are wrought formed to shape or shape by hot rolling and forging. On the other hand, subsequent to primary production through melting and ingot casting, the cobalt base and nickel based superalloys are utilized in wrought or cast form depending on application or alloy composition included. The stainless steels, nickel-chromium alloys and cobalt dental alloys that are included in the superalloys consist of chromium to offer high temperature corrosion resistance. A chromium-oxide layer produced on the surface is an excellent oxidation resistant layer. The cast superalloys for the maximum temperatures are always secured from oxidation due to presence of chromium and aluminum. Or we can say, superalloys must have some content of chromium for providing suitable corrosion resistance. Following table gives an overview of certain effects of different alloying elements

Properties	Iron based Superalloys	Cobalt based superalloys	Nickel based Superalloys
Solid solution strengtheners	Chromium, molybdenum	Niobium, chromium, molybdenum, nickel, tungsten, tantalum	Cobalt, chromium, iron, molybdenum, tungsten, tantalum, Rhenium
Fcc matrix stabilizer	Carbon, tungsten, nickel	Nickel	-
Carbide form
MC	Titanium	Titanium	Tungsten, tantalum, Titanium, molybdenum, niobium, hafnium
M7C3	-	Chromium	Chromium
M23C6	Chromium	Chromium	Chromium, molybdenum, tungsten
M6C	Molybdenum	Molybdenum, tungsten	Molybdenum, tungsten, niobium

Mechanical function Usually metals strength at low temperatures is not related with time, at high temperatures operation load period becomes crucial for mechanical characteristics. The presence of oxygen at high temperature increases the rate of transformation of metal atoms to oxides. Oxidation occurs more quickly at the elevated temperatures rather at low temperatures. For short term tensile characteristics such as yield strength and tensile strength, the mechanical performance of metals at high temperature is alike to that at room temperatures however metals become weaker with increase in temperature. Although, when constant loads below the normal yield or ultimate strength found in short time analyses, are applied for longer periods at the elevated temperatures, the result is unlike. If an alloy is subjected to a specific application condition for prolong period, it shows cracks. The decomposition process is known as creep, causing damage, the alloys are chosen on the base of potential to offer resistance to creeping and creeping rupture damage. Superalloys can be kept stress free for a specific period like 100 hours, against temperature. A factor that adds to high temperature damage is that alloy materials are separated at the grain boundaries when analyzed for prolong periods above 0.5 of their melting points.Hence fine grained alloys that are often recommended for low temperature applications may not be the suitable materials for creep based operations at the elevated temperatures. Elimination or alignment of grain boundaries is a prime aspect in extending the high temperature service of any alloy. Static modulus such as Young modulus of an alloy is affected upon increasing temperature. On the base of load application, moduli measured from tensile test gradually decrease lower than dynamic moduli. As moduli are significant in design and influence the life and durability of alloy, each process should be performed to evaluate the dynamic, static, moduli of superalloys for elevated temperature operations. Moduli for cast alloys may be influenced frequently through orientation of grains. A precise method of describing moduli would be to use the suitable single crystal elastic constants however these are rarely available. Cyclically implemented loads resulting in damage at low temperatures also cause degradation in short periods at the elevated temperatures.