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Nucleation

The initiation of a phase transformation at discrete sites, the new phase growing on nuclei. See nucleus .

 

Nucleus

The first structurally stable particle capable of initiating recrystallization of a phase or the growth of a new phase, and possessing an interface with the parent matrix. The term is also applied to a foreign particle that initiates such action.

 

Natural Aging

Spontaneous aging of a supersaturated solid solution at room temperature. See aging , and compare with artificial aging .

 

Neutral Flame

A gas flame in which there is no excess of either fuel or oxygen in the inner flame. Oxygen from ambient air is used to complete the combustion of CO2 and H2 produced in the inner flame.

 

Neutralization Number

An ASTM number given to quenching oils sludging. See saponification number .

 

Nitriding

Introducing nitrogen into the surface layer of a solid ferrous alloy by holding at a suitable temperature (below Ac1 for ferritic steels) in contact with a nitrogenous material, usually ammonia or molten cyanide of appropriate composition. Quenching is not required to produce a hard case. See aerated bath nitriding , bright nitriding , and liquid nitriding .

 

Nitrocarburizing

Any of several processes in which both nitrogen and carbon are absorbed into the surface layers of a ferrous material at temperatures below the lower critical temperature and, by diffusion, create a concentration gradient. Nitrocarburizing is done mainly to provide an antiscuffing surface layer and to improve fatigue resistance. Compare with carbonitriding .

 

Narrowband

1. A frequency band in which the difference between upper and lower limits is small compared with bandwidths typical of the service specified. 2. Pertaining to a radio-frequency emission whose bandwidth is limited or restricted. 3. Pertaining to a circuit or device that operates over a comparatively small range of frequencies. 4. In a digital network, a channel having a bandwidth of 64 kbps or less.

 

Natural Disintegration

1. The decay of a radioactive substance as a result of the continuous emission of particles and rays. 2. Also called half life. The time required for half of a quantity of a radioactive substance to decay into a different isotope or element.

 

Natural Resonant Frequency

1. The frequency at which a circuit or device responds with maximum amplitude to applied signals. 2. The frequency at which a circuit or device generates maximum energy. 3. The frequency at which an object vibrates at maximum amplitude.

 

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