The precipitation of nitrogen fron iron-molybdenum-nitrogen alloys has been studied by X-ray, electron microscopic and optical metallographic techniques in the temperature range 100 - 800°C. By experimental control of reaction conditions, nine different precipitated phases are characterized in alloys containing up to 5.0 wt. % molybdenum. In the temperature range 100 - 8oo°c it is shown that all known molybdenum nitrides and one of the two known ternary iron-molybdenum nitrides occur as precipitates. Previous work on the influence of oxygen on these phases is confirmed and shown to be applicable to the phases when precipitated in iron. Below about 400oC the diffusion of molybdenum is too slow to precipitate binary or ternary alloying-element nitrides and para-precipitation or iron nitrides occurs. Quench-ageing observations show that the decrease of the activity coefficient of nitrogen in iron solid solution with increase in molybdenum concentration leads to a marked retardation in the rate of precipitation at these low temperatures. Materials of exceptional hardness are obtained at 450° - 650°C by "constant nitrogen activity ageing" of alloys containing 2.0 - 5.0 wt. %Mo. It is suggested that the unique properties of these materials are due to homogeneous precipitation of molybdenum and nitrogen which goes through each of the stages well-recognised in face-centred cubic alloys, i. e. G. P. zone formation-metastable precipitate->equilibriun precipitate The occurrence of Guinier-Preston zones involving both substitutional and interstitial solute atoms is demonstrated for the first time and is thought to be of wide scientific and technological application.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:550314 |
| Date | January 1969 |
| Creators | Speirs, David L. |
| Publisher | University of Newcastle upon Tyne |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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