Interphase Precipitation Research Articles

Homogeneous precipitation of niobium nitride in alpha-iron during isothermal treatment

Abstract Evidence for homogeneous precipitation of niobium nitride in α-iron has been found in a Fe-0·2 wt.-% Nb-0·018 wt.-% N alloy isothermally treated in the range 600°-800°C after rapid cooling from the γ-field. The observed stages of this type of precipitation seem to agree with the model of nucleation and evolution of mixed Guinier-Preston zones in iron proposed by Jack (1972), but the results of this work suggest that homogeneous nucleation may not be restricted to the case of constant high-nitrogen supersaturation, and could be of general occurrence in a defect-free ferrite once interphase precipitation is avoided.

Alloy carbide precipitation and aging during high-temperature isothermal decomposition of an Fe-4Mo-0·2C alloy steel

An electron metallographic study, coupled with X-ray diffraction, has been carried out on the decomposition and aging of an Fe-4Mo-0·2C low-alloy steel at 750%C in an attempt to characterize the nature of the alloy carbide dispersion and its stability. Intragranular precipitation of both the intermediate carbide Mo2C and the equilibrium carbide M6C was dispersed in sheetform at short aging times denoting interphase precipitation at the austenite/ferrite transformation interface. The orientation relationship of this interphase precipitate of M6C with the ferrite matrix was identified as either Kurdjumov-Sachs or Nishiyama-Wasserman. Mo2C also occurred with a fibrous morphology. At longer aging times further nucleation and growth of M6C occurred and the Mo2C dissolved, and the observation of new aligned dispersions of M6C interleaved with rows of Mo2C creates strong circumstantial evidencefor nucleation of M6C on the ferrite/Mo2C interface, although no unambiguous electron microscopy evidence of this phenomenon could be obtained.

Interphase precipitation of cementite in a continuously-cooled plain-carbon steel

Interphase precipitation of cementite in a continuously-cooled plain-carbon steel

Precipitation of carbides atγ─αboundaries in alloy steels

Austenite containing 1 to 2 wt % vanadium and 0.2 wt % carbon, held in the range 600 to 850°C transforms to an extremely fine dispersion of V4C3in ferrite. The carbide particles are nucleated on theγ─αphase boundary at successive positions during the transformation, forming sheets of precipitate which are 5 to 30 nm apart, while the individual particles are 3 to 10 nm diameter (interphase precipitation). High resolution electron microscopy has provided details of the morphology and crystallography of the carbide particles and the transformation front. A similar reaction has been studied in an iron +6.3 wt % tungsten + 0.23 wt % carbon alloy in which M6C precipitates on a much coarser scale enabling precise observations to be made on the transformation front. Interphase precipitation has also been examined in an iron + 3.45 wt % molybdenum + 0.22 wt % carbon alloy. A model is proposed for the interphase precipitation which takes into account the observed morphology and crystallography of the carbide phases and of the ferrite.

Interphase Precipitation in Iron Alloys

AbstractDuring the decomposition of austenite to ferrite in certain iron-base alloys a new form of precipitation has been observed at the ferrite/austenite interface. In appropriate alloys, V4C3, Mo2C, and Cr7C3 form very fine particles on the interface, which repeatedly moves on to provide fresh sites for further nucleation. The transformation leads to a very fine banded dispersion of carbide in ferrite having striking morphological differences from the same carbides in tempered martensite.