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UNIT - III

MECHANICAL BEHAVIOUR OF

MATERIALS

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NITROGEN STEEL All steels contain some nitrogen which is effective in improving

the mechanical and corrosion properties of steels if it remainsin solid solution or precipitates as very fine and coherentnitrides

When nitrogen is added to austenitic steels it can

simultaneously improve fatigue life, strength, work hardeningrate, wear and localized corrosion resistance.

High nitrogen martensitic stainless steels show improvedresistance to localized corrosion (pitting, crevice andintergranular corrosion) over their carbon containing

counterparts

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

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EFFECT OF NITROGEN ON STEEL

PROPERTIES The effect of nitrogen on steel properties can be either

detrimental or beneficial, depending on the other alloyingelements 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 nitrogenbe kept to a minimum. High nitrogen content may result ininconsistent mechanical properties in hot-rolled products,embrittlement of the heat affected zone (HAZ) of weldedsteels, and poor cold formability.

Hardness is the resistance of a material to surface indentation

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Nitrogen absorbed during steelmaking results in interstitialsolid solution strengthening and grain refinement, both ofwhich increase hardness.

further, the diagram shows that nitrogen absorbed during thesteelmaking process has a more significant impact than that

absorbed during batch annealing in a nitrogen-richatmosphere, although both have a measurable effect

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Note that many intermetallic compounds areoften simply called 'alloys', although this issomewhat of a misnomer. Both are metallicphases containing more than one element, but

in alloys the various elements substituterandomly for one another in the crystalstructure, forming a solid solution with a rangeof possible compositions; in intermetallic

compounds, different elements are ordered intodifferent sites in the structure, with distinct localenvironments and often a well-defined, fixedstoichiometry. Complex structures with very

large unit cells can be formed.

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NICKEL ALUMINIDE

Nickel aluminide (Ni3Al) is an intermetallic alloy ofnickel and aluminum

The 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.

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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 intergranularstructure

The unit cell consists of FCC arrangement in which thealuminium 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,

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CORROSION RESISTANCE

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CORROSION RESISTANCE

Oxidation and carburization resistance of nickelaluminide 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

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TITANIUM ALUMINIDE

. It is lightweight and resistant to oxidation [1]andheat, 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.