UNIT - III

MECHANICAL BEHAVIOUR OF MATERIALS

NITROGEN STEEL All steels contain some nitrogen which is effective in

improving the mechanical and corrosion properties of steels if it remains in solid solution or precipitates as very fine and coherent nitrides

When nitrogen is added to austenitic steels it can simultaneously improve fatigue life, strength, work hardening rate, wear and localized corrosion resistance.

High nitrogen martensitic stainless steels show improved resistance to localized corrosion (pitting, crevice and intergranular corrosion) over their carbon containing counterparts

However, solidification of steel may result in three nitrogen-related phenomena: formation of blowholes; precipitation of one or more nitride compounds; and/or the solidification of nitrogen in interstitial solid solution.

EFFECT OF NITROGEN ON STEEL PROPERTIES The effect of nitrogen on steel properties can be either

detrimental or beneficial, depending on the other alloying elements present, the form and quantity of nitrogen present, and the required behavior of the particular steel product.

In general, however, most steel products require that nitrogen be kept to a minimum. High nitrogen content may result in inconsistent mechanical properties in hot-rolled products, embrittlement of the heat affected zone (HAZ) of welded steels, and poor cold formability. 

Hardness is the resistance of a material to surface indentation

Nitrogen absorbed during steelmaking results in interstitial solid solution strengthening and grain refinement, both of which increase hardness. 

further, the diagram shows that nitrogen absorbed during the steelmaking process has a more significant impact than that absorbed during batch annealing in a nitrogen-rich atmosphere, although both have a measurable effect

INTERMETALLICS When a solution solidifies, alloys of metals which have a

limited mutual solubility may form new phases at certain ratios. These new phases possess crystal structures different from either component and are called intermetallic compound

 It refers to solid-state phases involving metals Intermetallic compounds are generally brittle and have a

high melting point.  They can also display desirable magnetic,

superconducting and chemical properties, due to their strong internal order and mixed (metallic and covalent/ionic) bonding, respectively

Note that many intermetallic compounds are often simply called 'alloys', although this is somewhat of a misnomer. Both are metallic phases containing more than one element, but in alloys the various elements substitute randomly for one another in the crystal structure, forming a solid solution with a range of possible compositions; in intermetallic compounds, different elements are ordered into different sites in the structure, with distinct local environments and often a well-defined, fixed stoichiometry. Complex structures with very large unit cells can be formed.

NICKEL ALUMINIDENickel aluminide (Ni3Al) is an intermetallic alloy

of nickel and aluminumThe alloy is very resistant to heat and

corrosion, and finds use in heat-treating furnaces and other applications where its longer lifespan and reduced corrosion give it an advantage over stainless steel.

 Nickel aluminide is unique in that it has very high thermal conductivity combined with high strength at high temperature.

Used in coating blades of  gas turbines and jet engines.

COMPOSITION AND STRUCTURE Boron additions were critical for achieving the reasonable

level of ductility in the alloys

Boron is thought to increase grain boundary cohesiveness, thereby reducing the tendency for brittle intergranular structure

The unit cell consists of FCC arrangement in which the aluminium atoms occupy the corner position, nickel atom preferably occupy the face centred position

This type of atom packing leads to mechanical properties

As the temperature increases, also the yield strength increases,

CORROSION RESISTANCE

CORROSION RESISTANCEOxidation and carburization resistance of

nickel aluminide alloy are shown in above diagram

Nickel aluminide alloy form a protective Al2O3 scale on the surface have significantly better oxidation resistance than aluminium free alloy

Carburization resistance also high under oxidation

APPLICATIONSPistons and valves, automotive turbo charges,

aircraft fasteners,gas water and steam turbines

TITANIUM ALUMINIDE. It is lightweight and resistant to oxidation

[1]and heat, however it suffers from low ductility.

Titanium aluminide has three major intermetallic compounds: gamma TiAl, alpha 2-Ti3Al and TiAl3

TiAl based alloys have a strong potential to increase the thrust-to-weight ratio in the aircraft engine.