High temperature thermodynamics of the Cr-Cr 2 N-N 2 system
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(KJ 9 mol -I Cr2N)
AG ~ = -104. + 0.0498T -+ 3.8
I.
INTRODUCTION
CHROMIUM is an important element in the field of hightemperature materials. As an alloying element, chromium has a great effect on the properties of steels. For example, chromium can improve the oxidation and corrosion resistance and increase hardness and toughness of steels. EL2'3j In addition, varieties of chromium-base alloys have been used as engineering materials because of their attractive high temperature strength, and creep and oxidation resistance. [4,51The effects of chromium are influenced substantially by nitrogen, since the formation of chromium nitride is beneficial to the hardness and toughness of steels. I''31 However, nitrogen is detrimental to chromium-base alloys because it reduces room-temperature ductility of these materials, t6'71 Therefore, it is important to understand the thermodynamics of chromium-nitrogen reactions.
II.
PREVIOUS WORK
For the chromium-nitrogen system, two intermediate phases exist: (1) Cr:N with a hexagonal close-packed lattice and a range of homogeneity from 11.2 to 15.1 wt pct nitrogen according to Blix, I81or 9.3 to 11.9 wt pct nitrogen according to Eriksson; I91 and (2) CrN which has a cubic lattice. However, Sully and Heal f~~ have reported that CrzN is always present in chromium-rich alloys which are contaminated with nitrogen. Elliott c~1 also pointed out that Cr~N is in equilibrium with the metal phase at least over most of the temperature range. The present investigation was concerned only with equilibria between the nitride, Cr2N, the metal and N: gas. Blix I81 also found that in the ranges of 0 to 11.2 wt pct and 11.2 to 21.2 wt pct nitrogen, the solid phases of the chromium-nitrogen system were Cr and Cr2N, and CrzN and CrN, respectively. Sano ~21 confirmed Blix's results and first investigated pressure-temperature relationships in the CraN-Cr-N2 and Cr2N-CrN-N 2 systems by dissociating chromium nitride which contained 9.7 wt pct and 18.5 wt pct nitrogen, respectively, over the temperature range 1229 to 1395 K. He started with CrN and removed some nitrogen to obtain the desired nitrogen content. Seybolt and Oriani t~3J investigated the pressure and composition of the LIYA WANG, Graduate Student, and ROBERT D. PEHLKE, Professor, are with The University of Michigan, 2300 Hayward Street, Ann Arbor, MI 48109-2136. Manuscript submitted June 3, t987.
METALLURGICAL TRANSACTIONS B
(KJ 9 mol -~ Cr2N)
solid solution of nitrogen in chromium as a function of temperature over the temperature range 1273 to 1673 K by a controlled pressure method in which the equilibrium contents of nitrogen in chromium at different temperatures and pressures were measured. The investigation of Sano was limited to relatively low temperature ranges (below 1395 K), whereas most chemical reactions in metallurgical processes take place at higher temperatures. Also, the results of Sano were substantially different from those of Seybolt and Oriani, so further investigation is necessary.
III.
FUNDAMENTAL CONSIDERATIONS
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