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The fabrication of the bulk powder a’-phase nitrogen martensite in (Fe0.99Cr0.01)Nx system using ball milling at cryogenic temperature was investigated in this thesis. The nitrogen austenite powder particles were sealed in a grinding chamber in a nitrogen atmosphere and ball milled at liquid nitrogen temperature using a cryomill. The amount of martensitic phase transformation was greatly enhanced and the yield of the a’-phase was increased from 28 vol.%, obtained from quench, to 93 vol.% due to the deformation. The X-ray diffractometry and vibrating sample magnetometry measurements were used to determine the martensite phase fraction and the saturation magnetization of the samples. The effect of (1) milling frequency, f, (2) grinding ball size, D, (3) number of grinding balls, (4) total milling time, tT, (5) Cr alloying content, and (6) particle size of cryomilled powder on the amount of deformation-induced martensitic phase transformation was studied from a kinetic energy perspective. A model was developed and used to calculate the kinetic energy applied to powder particles during the ball milling process:Ek=4mb·(L-D)2·f3·tT, where mb is the mass of the grinding ball and L the length of the grinding chamber. This was correlated to the martensite start temperature with deformation Md, Md (oC)=(68±8)×10-3·Ek (J). The martensite start temperature can then be expressed as MST'=MST [N]+Md [Ek]. The relation between the deformation-induced martensite start temperature MST', which is a function of nitrogen content, and the kinetic energy applied by ball milling was determined to be MST' (oC)=(68±8)×10-3·Ek (J)-(169±12) for nitrogen martensite with (10.0±0.1) at.%. nitrogen content. This equation was determined to apply until Ek=3000J, at which time no further transformation occurred. At 3000J< Ekk>10000J. The lattice parameters of the gamma-phase as a function of nitrogen content were determined in (Fe0.99Cr0.01)Nx system to estimate the nitrogen content in the nitrogen austenite in the samples: a(nm)=0.3604+[(4.6±0.5)×10-4]·CN (at.%). The saturation magnetization of the a’-phase with (10.0±0.1) at.% nitrogen content was determined to be (233±9) emu/g. |
