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Date of Award

6-1995

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Supervisor

Professor G.C. Weatherly

Co-Supervisor

Professor J.S. Kirkaldy

Abstract

A two part study was conducted on the nitridation behaviour of dilute Ni-Ti alloys, and on the orientation relationships (OR)s and interfacial structures between TiN and Ni. Beginning with the Fickian relationships first proposed by Kirkaldy (1969) to describe internal sulphidation and assuming a solubility product of the form K=BC, these equations were combined to yield for the metal solute:

(δB/δt)=[Dʙ+(KDc/B²)](δ²B/δx²)

This second order differential equation shows that the concentration profile of the metal solute, B, is dependent not only on the diffusivity of the metal, Dʙ, but also on the solubility product K and on the gas diffusivity Dc. Two limiting cases were identified from the previous equation. The first which depends only on the metal diffusivity, Dʙ, corresponds to Wagner's (1952) error function solution for the superficial oxidation of alloys dilute in B. The second limiting case (Dʙ=0) corresponds to Wagner's (1959) oxygen controlled internal oxidation as proven analytically by Ohriner and Morral (1979). A criterion describing the transition from internal to external nitridation (oxidation) was deduced from the previous equation as:

(KDc/B₁²Dʙ)<0.1

where B₁ is the interfacial composition of the metal solute.

A finite difference algorithm was presented based on these ideas. A comprehensive test of this approach showed that it could generate the well known analytical solutions of Wagner's two limiting conditions. It was also shown that provided the solubility product K is small, Ohriner and Morral's (1979) analytical solution is valid even for an alloy not saturated in the gas phase (i.e. the composition need not be confined to the solvus of the ternary isotherm). As well as reproducing these analytical solutions, the model was successfully tested against the experimental work of Swisher (1968) and Kirkaldy (1969). A detailed investigation was conducted on the concentration profiles of Ti in Ni as a consequence of either internlal nitridation or superficial nitridation using Energy Dispersive X-Ray (EDX) analysis. It was found that the program always generated the correct shapes of the diffusion profiles as well the transition to external nitridation in the Ni-Ti-N system. Based on the measured interfacial composition of the interface, first time determinations of both KDN and DTi values were obtained between 800°C and 1020°C. The diffusivity of Ti in Ni is similar to other substitution solutes in Ni and can be expressed as

Dn=0.07 exp(-320000/RT) m²/s (900°C-1020°C)

where R is the universal gas constant 8.314 J/mole-K

A solubility product calculation of TiN in Ni showed values between 1.4 x 10ˉ⁶ and 5x10ˉ⁸ [w/o]² (between 1020°C and 800°C respectively). Using this data a diffusivity of N in Ni was obtained which compares favourably to other interstitial diffusivity data in Ni and ϒFe. The upper bound value for the diffusivity of N in dilute Ni-Ti alloys can be expressed as:

DN= 0.0003 exp(-17000/RT) m²/s (900°C-1020°C)

Stoichiometric TiN was the only precipitated phase found between 800°C-1020°C in this work. A first time investigation of the interfacial structure between TiN and Ni was conducted using transmission electron microscopy (TEM). It was determined that at least five different orientation relationships between TiN and Ni exist. All these ORs had relatively small near coincidence site lattice (CSL) relationships. The planar Σ value of Ni (δɴᵢ) with respect to the Ti sublattice were all found to be under 50. No correlation was found between the size of the CSL unit cell and the frequency of observation of a particular OR. In only two cases were misfit dislocations observed. Both O-Iattice and plane matching theory made correct predictions as to the misfit dislocation configurations. In both cases one predominate set of dislocations was calculated. It was not possible, however, to unambiguously identify the Burgers vector. The habit planes were dominated by {010}TiN indicating that this was an important factor in selecting the morphology and orientation relationships of these particles. This plane coincided both with a primary O-lattice plane and, as already noted, with a planar CSL relationship. These factors probably all play a role in the selection of these orientation relationships and habit planes in the Ni/TiN system.

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