Stainless steels are very important engineering materials in a variety of applications such as in the food industry and nuclear power plants due to their combination of good mechanical properties and high corrosion resistance. However, ferrite-containing stainless steels are sensitive to the so-called ‘475°C embrittlement’, which is induced by phase separation of the ferrite phase, where it decomposes into Fe-rich ferrite (α) and Cr-rich ferrite (α'). The phase separation is accompanied with a severe loss of toughness. Therefore, the upper service temperature of ferrite-containing stainless steels in industrial applications has been limited to around 250°. In the present work, Fe-Cr based steels were mainly investigated by atom probe tomography. A new method based on the radial distribution function (RDF) was proposed to quantitatively evaluate both the wavelength and amplitude of phase separation in Fe-Cr alloys from the atom probe tomography data. Moreover, a simplified equation was derived to calculate the amplitude of phase separation. The wavelength and amplitude was compared with evaluations using the auto-correlation function (ACF) and Langer-Bar-on-Miller (LBM) method, respectively. The results show that the commonly used LBM method underestimates the amplitude of phase separation and the wavelengths obtained by RDF shows a good exponential relation with aging time which is expected from the theory. The RDF is also an effective method in detecting the phenomena of clustering and elemental partitioning. Furthermore, atom probe tomography and the developed quantitative analysis method have been applied to investigate the influence of different factors on the phase separation in Fe-Cr based alloys by the help of mainly mechanical property tests and atom probe tomography analysis. The study shows that: (1) the external tensile stress during aging enhances the phase separation in ferrite. (2) Phase separation in weld bead metals decomposes more rapidly than both the heat-affected-zone metals and the base metals mainly due to the high density of dislocations in the welding bead metals which could facilitate the diffusion. (3) The results show that Ni and Mn can enhance the phase separation comparing to the binary Fe-Cr alloy whereas Cu forms clusters during aging. (4) Initial clustering of Cr atoms was found after homogenization. Two factors, namely, clustering of Cr above the miscibility gap and clustering during quenching was suggested as the two responsible mechanisms. (5) The homogenization temperatures significantly influence the evolution of phase separation in Fe-46.5at.%Cr. / <p>QC 20140910</p> / Spinodal Project

In this work, new reconstruction and analysis methods were developed for 3D field ion microscopy (FIM) data, motivated by the goal of atomic scale characterisation of radiation damage for fusion applications. A comparative FIM/ atom probe tomography (APT) study of radiation damage in self-implanted tungsten revealed FIM advantages in atomistic crystallographic characterisation, able to identify dislocations, large vacancy clusters, and single vacancies. While the latter is beyond the detection capabilities of APT, larger damage features were observed indirectly in APT data via trajectory aberrations and solute segregation. An automated 3DFIM reconstruction approach was developed to maintain reliable, atomistic, 3D insights into the atomic arrangements and vacancies distribution in ion-implanted tungsten. The new method was utilized for the automated ‘atom-by-atom' reconstruction of thousands of tungsten atoms yielding highly accurate reconstructions of atomically resolved poles but also applied to larger microstructural features such as carbides and a grain boundary, extending across larger portions of the sample. Additional tools were developed to overcome reconstruction challenges arising from the presence of crystal defects and the intrinsic distortion of FIM data. Those were employed for the automated 3D mapping of vacancies in ion-implanted tungsten, analysing their distribution in a volume extending across 50nm into the depth of the sample. The new FIM reconstruction also opened the door for more advanced analyses on FIM data. It was applied to the preliminary studies of the distortion of the reconstructed planes, found to depend on crystallographic orientation, with an increased variance in atomic positions measured in a radial direction to the centre of the poles. Additional analyses followed the subtle displacements in atomic coordinates on consecutive FIM images, to find them affected by the evaporation of atoms from the same plane. The displacements were found to increase with size as the distance to the evaporated atom decreased, and are likely to be the result of a convolution between image gas effects, surface atoms relaxation, and charge re-distribution. These measurements show potential to probe the dynamic nature of the FIM experiment and possibly resolve contributions from the different processes effecting the final image. Finally, APT characterisation was performed on bulk and pre-sharpened needles to determine the effect of sample's geometry on the resulting implantation profiles, and the extent to which pre-sharpened needles could be employed in radiation damage studies. While the ions depth profiles in needles were not found within a good match to SRIM simulations, the damage profiles exhibited closer agreement. Further, the concentration of implanted ions in bulk samples was found significantly higher than in the respective needle implanted samples, with excessive loss found for the light ion implantation.

High entropy alloys (HEAs) is a concept wherein alloys are constructed with five or more elements mixed in equal proportions; these are also known as multi-principle elements (MPEs) or complex concentrated alloys (CCAs). This PhD thesis dissertation presents research conducted to develop precipitation-hardenable high entropy alloys using a much-studied fcc-based equi-atomic quaternary alloy (CoCrFeNi). Minor additions of aluminium make the alloy amenable for precipitating ordered intermetallic phases in an fcc matrix. Aluminum also affects grain growth kinetics and Hall-Petch hardenability. The use of a combinatorial approach for assessing composition-microstructure-property relationships in high entropy alloys, or more broadly in complex concentrated alloys; using laser deposited compositionally graded AlxCrCuFeNi2 (0 < x < 1.5) complex concentrated alloys as a candidate system. The composition gradient has been achieved from CrCuFeNi2 to Al1.5CrCuFeNi2 over a length of ~25 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. With increasing Al content, there was a gradual change from an fcc-based microstructure (including the ordered L12 phase) to a bcc-based microstructure (including the ordered B2 phase), accompanied with a progressive increase in microhardness. Based on this combinatorial assessment, two promising fcc-based precipitation strengthened systems have been identified; Al0.3CuCrFeNi2 and Al0.3CoCrFeNi, and both compositions were subsequently thermo-mechanically processed via conventional techniques. The phase stability and mechanical properties of these alloys have been investigated and will be presented. Additionally, the activation energy for grain growth as a function of Al content in these complex alloys has also been investigated. Change in fcc grain growth kinetic was studied as a function of aluminum; the apparent activation energy for grain growth increases by about three times going from Al0.1CoCrFeNi (3% Al (at%)) to Al0.3CoCrFeNi. (7% Al (at%)). Furthermore, Al addition leads to the precipitation of highly refined ordered L12 (γ′) and B2 precipitates in Al0.3CoCrFeNi. A detailed investigation of precipitation of the ordered phases in Al0.3CoCrFeNi and their thermal stability is done using atom probe tomography (APT), transmission electron microscopy (TEM) and Synchrotron X-ray in situ and ex situ analyses. The alloy strengthened via grain boundary strengthening following the Hall-Petch relationship offers a large increment of strength with small variation in grain size. Tensile strength of the Al0.3CoFeNi is increased by 50% on precipitation fine-scale γ′ precipitates. Furthermore, precipitation of bcc based ordered phase B2 in Al0.3CoCrFeNi can further strengthen the alloy. Fine-tuning the microstructure by thermo-mechanical treatments achieved a wide range of mechanical properties in the same alloy. The Al0.3CoCrFeNi HEA exhibited ultimate tensile strength (UTS) of ~250 MPa and ductility of ~65%; a UTS of ~1100 MPa and ductility of ~30%; and a UTS of 1850 MPa and a ductility of 5% after various thermo-mechanical treatments. Grain sizes, precipitates type and size scales manipulated in the alloy result in different strength ductility combinations. Henceforth, the alloy presents a fertile ground for development by grain boundary strengthening and precipitation strengthening, and offers very high activation energy of grain growth aptly suitable for high-temperature applications.

The relatively low oxidation resistance and subsequent surface embrittlement have often limited the use of titanium alloys in elevated temperature structural applications. Although extensive effort is spent to investigate the high temperature oxidation performance of titanium alloys, the studies are often constrained to complex technical titanium alloys and neither the mechanisms associated with evolution of the oxide scale nor the effect of oxygen ingress on the microstructure of the base metal are well-understood. In addition lack of systematic oxidation studies across a wider domain of the alloy composition has complicated the determination of composition-mechanism-property relationships. Clearly, it would be ideal to assess the influence of composition and exposure time on the oxidation resistance, independent of experimental variabilities regarding time, temperature and atmosphere as the potential source of error. Such studies might also provide a series of metrics (e.g., hardness, scale, etc) that could be interpreted together and related to the alloy composition. In this thesis a novel combinatorial approach was adopted whereby a series of compositionally graded specimens, (Ti-xMo, Ti-xCr, Ti-xAl and Ti-xW) were prepared using Laser Engineered Net Shaping (LENS™) technology and exposed to still-air at 650 °C. A suite of the state-of-the-art characterization techniques were employed to assess several aspects of the oxidation reaction as a function of local average composition including: the operating oxidation mechanisms; the structure and composition of the oxides; the oxide adherence and porosity; the thickness of the oxide layers; the depth of oxygen ingress; and microstructural evolution of the base material just below the surface but within the oxygen-enriched region. The results showed that for the Ti-Mo, Ti-Al and Ti-W systems a parabolic oxidation rate law is obeyed in the studied composition-time domain while Ti-Cr system experiences a rapid breakaway oxidation regime at low solute concentrations. The only titanium oxide phase present in the scale for all the binary systems was identified as rutile (TiO2) and formation of multiphase oxide scales TiO2+Al2O3 in Ti-Al system and TiO2+TiCr2 in Ti-Cr system was observed. A thermodynamic framework has been used to rationalize the oxygen-induced subsurface microstructural transformations including: homogeneous precipitation of nano-scaled β particles and discontinuous precipitation of +β phases in Ti-Mo and Ti-W system, evolution of TiCr2 intermetallic phase in Ti-Cr system and ordering phase transformation in Ti-Al system.

A new materials system of a single layered Co(W) barrier/liner coupled with a Cu(Mn) alloy seed was investigated. Atom probe tomography visualized the sub-nanoscale structure of Cu(Mn)/Co(W) system, and thereby revealed Cu diffusion behavior of Co(W). Grain boundaries of Co were found to be the diffusion path, and successfully stuffed by W. Mn in Cu(Mn) also segregated to stuff the grain boundaries of Co. Combination of these two additives enabled high barrier property against Cu diffusion of Cu(Mn)/Co(W). Foreseeing tiny and high-aspect-ratio Cu interconnect features, Cu(Mn)/Co(W) was fabricated by ALD/CVD processes. To maximize the performance, minor impurities of the film incorporated from the ligand of the precursors were controlled by precursor selection. Thin, conformal, and smooth films were finally demonstrated onto a trench substrate.

This thesis explores nanostructuring in TiSiCN, ZrAlN, and TiAlN thin films deposited by cathodic arc evaporation onto cemented carbide substrates, with intended applications for cutting tools. The three systems were found to exhibit age hardening upon annealing, by different mechanisms, into the superhard regime (≥30 GPa), as determined by a combination of electron microscopy, X-ray diffraction, atom probe tomography, erda, and nanoindentation tech- niques. TiSiCN forms nanocomposite films during growth by virtue of Si segregation to the surface of TiCN nanocrystallites while simultaneously pro- moting renucleation. Thus, the common columnar microstructure of TiCN and low-Si-content (≤5 at. %) TiSiN-films is replaced by a “feather-like” nanos- tructure in high-Si-content (≥10 at. %) TiSiCN films. The presence of C promotes the formation of this structure, and results in an accelerated age hardening beginning at temperatures as low as 700 °C. The thermal stability of the TiSiCN films is, however, decreased compared to the TiSiN system by the loss of Si and interdiffusion of substrate species; C was found to ex- acerbate these processes, which became active at 900 °C. The ZrAlN system forms a two-phase nanostructure during growth consisting of cubic ZrAlN and wurtzite ZrAlN. Upon annealing to 1100 °C, the c-Zr(Al)N portion of the films recovers and semicoherent brick-like w-(Zr)AlN structures are formed. Age hardening by 36 % was obtained before overageing sets in at 1200 °C. As-deposited and annealed solid solution Ti0.33Al0.67N thin films were characterized for the first time by atom probe tomography. The as-deposited film was found to be at the very initial stage of spinodal decomposition, which continued during annealing of the film at 900 °C for 2 h. N preferentially segregates to Al-rich domains in the annealed sample, causing a compositional variation between Ti-rich and Al-rich domains, to maintain the stoichiometry for the developing AlN phase. That effect also compensates for some of the coherency strain formed between cubic domains of TiN and AlN. Finally, a possible Kirkendall effect caused by an imbalance in the metal interdiffusion during the spinodal decomposition was discovered.

High entropy alloys (HEAs), also referred to as complex concentrated alloys (CCAs), are a relatively new class of alloys that have gained significant attention since 2010 due to their unique balance of properties that include high strength, ductility and excellent corrosion resistance. HEAs are usually based on five or more elements alloyed in near equimolar concentrations, and exhibit simple microstructures by the formation of solid solution phases instead of complex compounds. HEAs have great potential in the design of new materials; for instance, for lightweight structural applications and elevated temperature applications. The relation between grain size and yield strength has been a topic of significant interest not only to researchers but also for industrial applications. Though some research papers have been published in this area, consensus among them is lacking, as the studies yielded different results. Al atom being a large atom causes significant lattice distortion. This work attempts to study the Hall-Petch relationship for Al0.3CoFeNi and Al0.3CoCrFeNi and to compare the data of friction stress σ0 and Hall-Petch coefficient K with published data. The base alloys for both these alloys are CoFeNi and CoCrFeNi respectively. It was observed by atom probe tomography (APT) that clustering of Al-Ni atoms in these two base CCAs was responsible for imparting such high values of K. Additionally the high value of K in the CoCrFeNi HEA can also be attributed to the presence of Co-Cr clusters.

The self-assembly of magnesium aluminate spinel as a result of dewetting an overlaid thin film of (chiefly) gold was investigated. Thin films of gold were deposited on single-crystalline spinel substrates and were heat-treated to dewet gold film which led to self-assembly of intricate structures consisting of faceted spherical particles atop of frustums. The current work was conducted in continuation of previous studies which reported formation of such intricate structures. The most recent studies had evidently overruled a pure gold self-assembly scenario as was pointed out in preliminary investigations. It was in fact proven that these structures consist of three distinct parts: (i) a single or polycrystalline gold faceted sphere, (ii) quasi-phase interfacial bilayer, and (iii) a crystalline MgAl2O4 necking structure spontaneously risen from spinel substrate. In the current work, samples were produced through different film deposition methods of sputter, thermal evaporation, and e-beam evaporation coating which underwent thermal annealing to induce dewetting of gold film and subsequent self-assembly of intricate structures. Several characterization methods such as electron microscopy, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy, and atom probe tomography were utilized to survey the different features of the intricate structures with focus on chemical analysis of the gold-spinel interface. The results rejected the previous findings about formation of interface complexion at the boundary of gold-spinel. It was found out that gold-spinel interface is in fact an ordinary metal-oxide boundary with sharp atomic distinction and no inter-diffusion or formation of interfacial complexion. It was further discovered that dewetting pure gold thin films does not result in formation of spinel self-assembly and existence of elemental impurities of copper (Cu) in the initial film is vital in development of such structures. Finally, it was concluded that chemical composition of metallic overlayer and the heat treatment parameters most fundamentally influence formation and physical characteristics of those self-assembled structures. / Thesis / Master of Applied Science (MASc)

No description available.

Among the commercial aluminum alloys, aluminum 319 (Al-7wt%Si-4wt%Cu) type alloys are popularly used in automobile engine parts. These alloys have good casting characteristics and excellent mechanical properties resulting from a suitable heat treatment. To get a high strength in the 319 type alloys, grain refining, reducing the porosity, solid solution hardening, and precipitation hardening are preferred. All experimental variables such as solidification condition, composition, and heat treatment are influence on the precipitation behavior; however, precipitation hardening is the most significant because excess alloying elements from supersaturated solid solution form fine particles which act as obstacles to dislocation movement. The challenges of the 319 type alloys arise due to small size of precipitate and complex aging response caused by multi components. It is important to determine the chemical composition, crystal structure, and orientation relationship as well as precipitate morphology in order to understand the precipitation behavior and strengthening mechanism. In this study, the mechanical properties and microstructure were investigated using transmission electron microscopy and three dimensional atom probe tomography. The Mn and Mg effects on the microstructure and mechanical properties are discussed with crystallographic study on the iron intermetallic phases. The microstructural evolution and nucleation study on the precipitates in the low-Si 319 type aluminum alloys are also presented with sample preparation and analysis condition of TEM and 3DAP tomography.