Nonferrous Metal Alloys

Superalloys:

The name superalloy has been given to materials that exhibit far greater strength at high temperatures (1500 to 2000oF) than do conventional alloys. These materials are classified according to the predominant metal in the alloy, which may be cobalt, nickel, or iron. Other alloying elements include chromium, titanium, and some refractory materials such as Nb, Mo, W) These alloys have been developed largely for use in gas turbine rotors, because the efficiency of a gas turbine increases with the temperature at which the rotor can be operated. However, the strength at elevated temperatures is necessary not merely in the rotor itself but also in the vanes, the combustion chamber, and even the compressor section. In addition to turbine applications, these alloys are utilized in nuclear reactors and petrochemical equipment.

There are two principal qualities we look for when developing materials for parts operating at high temperatures:

Oxidation resistance

Strength

Oxidation resistance relates essentially to the problem of gas corrosion. When a material is loaded below its yield point at elevated temperatures it will continue to elongate even if the load is kept constant. This phenomenon is called creep. The material ruptures after a certain time, say 20 hours. Even though we never reached the yield point, failure of the material occurs due to the creep behavior. These are two important factors in design. Therefore, if we design a rotor for a gas turbine merely on the basis of yield strength during a tensile test, it will fail when it is in service, because the blades will heat up and either stretch excessively and contact the housing or break after a period of time.

Nickel:

Nickel is one of the metals that has high resistance to corrosion. When used with chromium , it is resistant to high temperature oxidation and corrosion. When combined with manganese it is resistant to oxidation and reduction at elevated temperatures. Nickel with cobalt provides high heat resistance which makes it useful in chemical industry.

Cobalt:

Cobalt is a high performance material. It achieves its strength through solid solution strengthening and precipitation hardening. Cobalt based alloys are combined with chromium, molybdenum, or tungsten all of which have melting points between 2200 and 2500 F range. Cobalt based materials have low machinability because of rapid work hardening that takes place at elevated temperatures and the abrasive particles that exist in the lattice and grain boundaries.

Titanium:

Titanium and its alloys are relatively new engineering materials that possess an extraordinary combination of properties. Pure titanium has a relatively low density, a high melting point (3035F) and an elastic modulus of 15.5 x 106 psi. Titanium alloys are extremely strong; room temperature tensile strengths can be as high as 200,000 psi. The alloys are highly ductile and easily machines. The major limitation of titanium at elevated temperatures is its chemical reactivity with other materials at elevated temperatures. The corrosion resistance of titanium alloys at room temperature is unusually high. They are virtually immune to air, marine and a variety of industrial environments. They are commonly used in airplane structures, space vehicles, and in the petroleum and chemical industries.

Aluminum:

Aluminum and its alloys are characterized by a relatively low density, high electrical and thermal conductivities, and a resistance to corrosion. Many of these alloys are formed by virtue of high ductility. Its ductility is maintained even at very low temperatures. The primary limitation of aluminum is its low melting temperature (1220F). Mechanical strength of aluminum can be enhanced by cold work and by alloying. However, both processes tend to diminish the resistance to corrosion. Principal alloying elements are copper, magnesium, silicon, manganese and zinc.Composition of aluminum is designated by a four-digit number that indicates the principal impurities.

The numbering system designates the series as shown below:

Material

Designation

99% Aluminum

1xxx

Copper

2xxx

Manganese

3xxx

Silicon

4xxx

Magnesium

5xxx

Magnesium and silicon

6xxx

Zinc

7xxx

 

Aluminum falls into two general categories: those that are heat-treatable and those that are not heat-treatable. In general, the 2000, 6000 and 7000 series may be strengthened by heat treatment. The 1000, 3000, 4000 and 5000 series cannot be heat treated. The latter series depend upon the alloys of manganese and magnesium for their strength. They may be further strengthened by strain hardening procedures, or by cold working.

A system of letters and numbers to indicate temper have been approved by the American Standards Association. In this system the basic temper is designated by a letter, whereas modification of the temper is indicated with a number.

Symbol

Meaning

O

Annealed

F

As-fabricated

H

Strain hardened

T

Heat treated

T indicates the heat treatment and the number that follows T indicates the modification on the heat treatment process. Examples: T2 = Annealed, T3=Solution heat treated and cold worked, T4=Solution heat treated and naturally aged, T6=Solution heat treated and artificially aged.

 

Copper:

Copper is an excellent conductor; it is malleable and easily cast and shaped. Pure copper is alloyed with many other elements to produce minor changes in properties. Pure copper is a single phase alloy.

Copper and its alloys may be classified in many different ways. One classification is:

Copper (99% Copper,small amounts of phosphorus, lead, nickel)

Brasses (copper, zinc, lead)

Bronze (copper, tin, silicon, aluminum)

Nickel Alloy (Copper, nickel, tin)

 

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Last Update: November 19, 1999

By: Serdar Z. Elgun