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Zirconium and its alloys are used extensively in the nuclear industry. In the Canadian Deuterium Uranium reactor, the primary containment in the primary coolant system is composed of Zr-2.5wt%Nb in the form of a pressure tube. The permissible chemical composition of Zr-2.5wt%Nb for use in the pressure tube in nuclear reactors is dictated by ASTMB353. Oxygen and iron are the highest content controlled elements in the standard alloy, after zirconium and niobium. Oxygen is an alpha-stabilizer, and diffuses preferentially to the alpha phase, leading to a well established increase in the yield strength of the alpha phase. Iron is a beta-stabilizer, and is concentrated in the beta phase, as well as near alpha-beta grain boundaries. While the mechanical properties of standard Zr-2.5wt%Nb alloy are well understood, there is a dearth of knowledge on the individual effect of these alloying additions, especially at non-standard concentrations. Additionally, the experimental evidence that does exist does not directly take into account the two-phase nature of the alloy, or the effect of impurities on specific deformation modes. Notably absent is experimental evidence on the effect of interstitial impurities on twinning in the hexagonal close-packed alpha phase. This work seeks to complement the present understanding of these phenomena. Mechanical tests have been performed on three specially prepared Zr-2.5wt%Nb alloys to clarify the contributions of oxygen and iron to Zr-2.5wt%Nb deformation properties. Traditional mechanical measurements were complemented by in situ and ex situ diffraction measurements. Tests were performed at a range of temperatures (77K - 673K) and strain rates (quasi-static to 10^-2/s). Increasing oxygen content from 1176wppm to 3300wppm increases the macroscopic yield stress at room temperature, and results in a transition in work hardening behaviour at low strain rates. Increasing iron content from 547wppm to 1080wppm has no effect on the macroscopic yield stress, but increases the work hardening rate at room temperature. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-10-11 20:06:57.655

Neutron diffraction is a widely used technique for measuring internal stresses inside polycrystalline materials. By examining the diffraction patterns collected during in situ uniaxial deformation, the lattice strains along various crystallographic directions can be calculated. These lattice strains give insight into the active deformation mechanisms active within the material during plastic deformation. This is most commonly done by fitting model results to the experimentally measured lattice strains through an iterative process of refining the model parameters. A numerical optimization technique was successfully applied to the problem of refining the input parameters of an elastoplastic self-consistent (EPSC) model. The results were found to be comparable to those obtained by a past researcher manually refining the model parameters and subjectively judging the fit to the experimental data. The numerical optimization method was able to reach an acceptable result much faster than is possible by a human being (days as opposed to weeks or months), meaning that it has the potential to reduce the turn-around time from data collection to interpretation/publication significantly. At the same time, common experimental techniques for conducting diffraction experiments during uniaxial deformation tests were examined. It is common to use an interrupted loading scheme where the sample is brought to a certain loading condition and then held steady while the neutron data is collected, a processes that often takes several minutes. This interrupted loading may be done such that the sample is held at constant stress, strain, or simply by having the load frame stay in a constant position. Each of these different loading modes results in a particular type of relaxation within the sample as it is being held, so a series of experiments were conducted to investigate any impact these different relaxation types may have on the measured values of the lattice strains. Overall it was found that both qualitative and quantitative differences in the recorded data can arise as a result of the different loading modes, and that such differences tend to manifest themselves at or near the point at which the material begins to yield macroscopically. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-03-05 19:37:19.995

Hafnium and Zirconium based gate dielectrics are considered potential candidates to replace SiO2 or SiON as the gate dielectric in CMOS processing. Furthermore, the addition of nitrogen into this pseudo-binary alloy has been shown to improve their thermal stability, electrical properties, and reduce dopant penetration. Because CMOS processing requires high temperature anneals (up to 1050 °C), it is important to understand the diffusion properties of any metal associated with the gate dielectric in silicon at these temperatures. In addition, dopant penetration from the doped polysilicon gate into the Si channel at these temperatures must also be studied. Impurity outdiffusion (Hf, Zr) from the dielectric, or dopant (B, As, P) penetration through the dielectric into the channel region would likely result in deleterious effects upon the carrier mobility. In this dissertation extensive thermal stability studies of alternate gate dielectric candidates ZrSixOy and HfSixOy are presented. Dopant penetration studies from doped-polysilicon through HfSixOy and HfSixOyNz are also presented. Rutherford backscattering spectroscopy (RBS), heavy ion RBS (HI-RBS), x-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), and time of flight and dynamic secondary ion mass spectroscopy (ToF-SIMS, D-SIMS) methods were used to characterize these materials. The dopant diffusivity is calculated by modeling of the dopant profiles in the Si substrate. In this disseration is reported that Hf silicate films are more stable than Zr silicate films, from the metal interdiffusion point of view. On the other hand, dopant (B, As, and P) penetration is observed for HfSixOy films. However, the addition of nitrogen to the Hf - Si - O systems improves the dopant penetration properties of the resulting HfSixOyNz films.

The aim of this project has been to develop the field of group 4 indenyl metallocene complexes based upon highly methylated ligands. Previous studies have shown that these compounds can be extremely active ethylene polymerisation catalysts, and, as such, are of both significant academic and commercial interest <strong>Chapter One</strong> introduces metallocene chemistry, discussing developments within the field and the effects of permethylation on indenyl rings. A synopsis of the rise of the ansa-bridge is provided, in addition to highlights from recent zirconocene chemistry. A feature on olefin polymerisation is included, spanning heterogeneous catalysts, homogeneous metallocenes and post metallocenes, as well immobilised complexes and their supports. <strong>Chapter Two</strong> charts updates to syntheses of bridged and unbridged permethylindenyl ligands. The developments have allowed for their use as viable industrial procedures. <strong>Chapter Three</strong> is an account of the group 4 organometallic chemistry of the indenyl ligands from Chapter Two. Four bridged metallocenes, including rac-SBI*ZrCl₂ and meso-EBI*Zr(CH₂Ph)₂, are reported. In addition, six unbridged analogues comprising rac/meso-Ind^#<sub style='position: relative; left: -.8em;'>2</sub>MCl₂ (M = Zr, Hf) and rac/meso-Ind^#<sub style='position: relative; left: -.8em;'>2</sub>(CH₂Ph)₂ are described as well as a half-metallocene. The complexes are characterised by single crystal X-ray diffraction and variable temperature NMR spectroscopy. DFT calculations have been performed, with representations of their optimised geometries and frontier MOs given. <strong>Chapter Four</strong> describes a reliable, reproducible procedure for immobilising group 4 complexes on the surface of solid supports; in total 19 catalysts are prepared. In addition to SSMAO, two new inorganic supports (LDHMAO and Solid MAO) are utilised. The latter has never previously been described in the academic literature. These catalysts have been characterised by IR, UV/visible and solid-state NMR spectroscopy in addition to SEM imaging. Zr K-edge EXAFS experiments were conducted and exceptionally clear data are reported. <strong>Chapter Five</strong> investigates the aforementioned complexes as both solution- and slurry-phase ethylene polymerisation catalysts. Numerous parameters are tested including temperature and time dependence and all of the catalysts produce high molecular weight polymer in the range 150-300,000 daltons. The activity of rac SBI*ZrCl₂ in solution exceeds 22,500 kg_PE/mol_Zr/h/bar, and 7,500 kg_PE/molZr/h/bar immobilised on Solid MAO. meso-EBI*Zr(CH₂Ph)₂ displays double the activity of its dichloride analogue. 1-hexene co polymerisation is carried out as part of a high throughput screening study and activities in excess of 30,000 kg_PE/molZr/h/bar are reported. Scale-up polymerisation runs are also disclosed. The resultant polymer has been characterised by GPC, as well as X-ray diffraction, SEM, ¹³C NMR and IR spectroscopy. <strong>Chapter Six</strong> provides the experimental details and characterising data for the previous chapters. An Appendix consists of crystal structure data while the Electronic Appendix contains the CIFs, DFT output files and the raw polymerisation data.

Polymers incorporating main group elements offer different and interesting properties compared to their all carbon analogues. For example, π-conjugated polymers incorporating phosphorus in the main chain of the polymer have generated interest due to their unique thermal and electronic properties, which primarily result from delocalization of the phosphorous lone pair within aromatic units. Similarly, interest in polysilanes stems from conductivity resulting from σ electron delocalization, though current methods of preparation for both of these types of materials are lacking. In this dissertation, both early and late transition-metal compounds were used to dehydrocouple phosphine and silane substrates. The use of dehydrocoupling catalysis as a method for the synthesis of main group element-linked polymers was explored utilizing substrates designed to engender solubility in their polymeric products. Progress towards the preparation of silane- and phosphine-based conjugated materials via dehydrocoupling catalysis is reported. Catalytic reactions of bisphosphinite pincer-ligated iridium compounds p-XR(POCOP)IrHCl (POCOP) = 2,6-(R2PO)2C6H3, R = iPr, tBu, X = H, COOMe, H, NMe2with primary and secondary silanes have been performed. Compounds featuring the less sterically demanding iPr-substituted ligands facilitate silane redistribution reactions, but dehydrocoupling catalysis is observed for more encumbered silane substrates or with aggressive removal of H2. The bulkier tBu-substituted compounds are silane dehydrocoupling precatalysts that also undergo competitive redistribution with less hindered substrates. Products generated from reactions utilizing tBu ligated Ir include low molecular weight oligosilanes with varying degrees of redistribution present or disilanes when employing more sterically demanding substrates. The interplay of steric and electronic effects of the POCOP ligand on the silane product distribution will be presented. In previous work by our group, a triamidoamine-supported zirconium catalyst,[κ5-(Me3SiNCH2CH2)2NCH2CH2NSiMe2CH2]Zr, 1 has been shown to be effective in catalyzing the formation of phosphorus–element bonds via dehydrocoupling. Substrates including 2,5-bisphosphinofuran and 1,4-bisphosphinobenzene were dehydrocoupled to yield hyperbranched polyphosphine products. Efforts to characterize these products have been limited due to poor solubility. Rational substrate design incorporating aliphatic sidechains in primary phosphine linker molecules to engender solubility has been accomplished. Treatment of these second generation substrates with 1 or [Cp*2ZrH3]Li, 2 leads to sluggish reactions reaching moderate conversions to diphosphine products. The working hypothesis is that steric congestion during the bond forming step hinders additional bond-formation. Efforts toward the characterization and utilization of these insoluble materials as metal ion scavengers will be presented.

Sharp leading edges required for hypersonic vehicles improve the maneuverability as well as reduce aerodynamic drag. However, due to the sharp design, increased surface temperatures require materials that can withstand these extreme conditions. Ultra-high temperature ceramics are a material group being considered for the leading-edge material, specifically ZrB₂/SiC (ZBS) which has a high thermal shock resistance, melting temperature, and thermal conductivity. Studies done by Tan et. al. has shown that adding samarium (Sm) as a dopant to ZBS has an emittance of 0.9 at 1600^oC and develop oxide scales that have excellent ablation performance. However, it remained unknown how the Sm doped oxide scale formed as well as how the emittance and ablation performance are affected by the microstructure. This study investigates the oxide scale development of 3 mol% doped Sm-ZBS billets as well as how differences in microstructure affect the emittance and ablation performance. Samples were prepared via chemical infiltration of samarium nitrate into spray-dried powders of 80 vol.% ZrB₂/20 vol.% SiC; powders were then pressed into billets and pressureless sintered. Samples cut and polished from these billets were then oxidized for 10, 60, or 300 s, respectively, using an oxyacetylene torch. X-ray diffraction was used to determine the sequence of oxidation of Sm-ZBS, beginning with the formation of ZrO₂ and Sm₂O₃. The final oxide scale was determined to be c₁-Sm_0.2Zr_0.8O_1.9, with a melting temperature exceeding 2500^oC. SEM and EDS were also used to investigate the microstructural formation that occurs from the bursting of convection cells. Samples with different microstructures revealed similar topographical microstructures post-ablation due to the sequence of the oxide formation. However, samples with rougher surfaces and higher porosities had a higher concentration of trapped glass in the cross-sectional oxide scale. It was also found that due to differences in heating the sample during emittance testing compared to ablation testing, the oxide developed was identical for all the samples. It was also found that variances in microstructure had no effect on the spectral emittance of Sm-ZBS at ultra-high temperatures. The fabrication of c₁-Sm_0.2Zr_0.8O_1.9 (SZO) as a bulk billet was also investigated to use as a thermal barrier coating (TBC) in replacement of Sm-ZBS.

Os reatores nucleares de potência refrigerados à água pressurizada (Pressurized Water Reactor - PWR) comumente utilizam pastilhas de dióxido de urânio como o seu combustível nuclear, estas por sua vez são montadas e empilhadas em tubos e tampões de ligas de zircônio (M5, Zirlo, Zircaloy, etc), Na usinagem desses componentes são geradas grandes quantidades de cavacos que estão contaminados com fluido de corte, o armazenamento destes cavacos apresenta riscos de segurança e ambientais por ser um material pirofórico e apresentarem alta área específica. Estas são ligas importadas e por isso é mostrado interesse pela indústria de reciclagem na sua reutilização. Este trabalho apresenta o estudo de um processo de reciclagem e os resultados para buscar, eficiência no processo de limpeza; controle de qualidade; obtenção de eletrodos prensados e a fusão em um forno a arco elétrico sob vácuo (Vacuum Arc Remelting - VAR). O processo de reciclagem se inicia com uma separação magnética de possíveis materiais ferromagnéticos presentes, lavagem do fluido de corte, que é solúvel em água, lavagem com um desengraxante industrial, seguido por um enxágue com fluxo contínuo de água em alta pressão e uma secagem por fluxo de ar quente. A primeira avaliação do processo foi feita por uma análise de fluorescência de raios X por dispersão de energia mostrando impurezas provenientes da mistura com cavacos da usinagem de aços inoxidáveis. Os cavacos foram prensados para a obtenção de eletrodos, para fusão em forno VAR, e para isso foi desenvolvida uma matriz para obtenção de eletrodos. Os eletrodos foram fundidos em um forno VAR de laboratório no Laboratório de Fusão do CECTM IPEN. A fusão dos cavacos é possível e viável em um forno VAR o que reduz em até 40 vezes o volume de armazenamento desse material. / The Pressurized Water Reactor commonly uses uranium dioxide pellets as the nuclear fuel, which are then assembled and stacked in zirconium alloy tubes (M5, Zirlo, Zircaloy) and end capped, in the machining of these components large amounts of chips are generated which are contaminated with cutting fluid. Its storage poses security and environmental risks due to a pyrophoric material. These are imported alloys and so it is shown interest by the recycling industry in its reuse. This paper presents the study of a recycling process and its results to seek efficiency in the cleaning process; quality control; obtaining of the electrodes and melting in a Vacuum Arc Remelting (VAR) furnace. The recycling process begins with a magnetic separation of possible ferromagnetic alloys present, washing out the water-soluble cutting fluid, washing with an industrial degreaser, followed by a rinse with continuous high pressure water flow and a flow drying of hot air. The first evaluation of the process was done by an energy dispersive X-ray fluorescence analysis showing the presence of impurities from the mixture with stainless steel machining chips. The chips were pressed to obtain electrodes to be melted in VAR furnace and for this a matrix was developed to obtain electrodes. The electrodes were melted in a laboratory VAR furnace in the Melt Laboratory at CECTM - IPEN. The melting of the chips is possible and feasible in a VAR furnace that reduces the storage volume of this material by up to 40 times, however, it is necessary to correct the composition of the alloy for the melting of these ingots.

Titanium dioxide nanotubes (TiO2) have been applied to enhance the mechanical properties of dental materials. Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) has been increasingly used in dentistry. Aside from its optimal clinical results, Y-TZP is prone to failures related to insufficient thickness of the fixed prostheses connector and debonding due to its difficult adhesion. The purpose of this study is to evaluate the effect of the addition of different concentrations of TiO2 to YTZP in its mechanical properties and microstructure, and also to evaluate the influence of these nanotubes on the bond strength when added to the ceramic or to the resin cement. To evaluate that, the described purposes were divided in two different papers. Paper 1 describes the tests of biaxial flexural strength, fractography qualitative analysis in scanning electron microscopy (SEM), microstructure evaluation in field emission-SEM and X-ray diffraction. Groups evaluated were commercial YTZP (Ivoclar Vivadent) (ZC) and an experimental Y-TZP with different blends of nanotubes [0 (Z0), 1 (Z1), 2 (Z2), and 5% (Z5), in volume]. In paper 2 shear bond strength test is described. It was carried out with the following groups: commercial YTZP (Ivoclar Vivadent) (ZC) and an experimental Y-TZP with different blends of nanotubes [0 (Z0), 1 (Z1), 2 (Z2), and 5% (Z5), in volume] bonded to the resin cement Panavia F2.0; and commercial Y-TZP bonded to resin cement RelyX U200 added with different blends of nanotubes in two curing methods dual-cured [0 (DC), 0.3 (D03), 0.6 (D06) and 0.9% (D09) of nanotubes in weigth] or self-cured [0 (SC), 0.3 (S03), 0.6 (S06) and 0.9% (S09) of nanotubes in weigth]. Values of flexural strength and shear bond strength were subjected to ANOVA and Tukey (=0.05). Flexural strength values were also subjected to Weibull statistics. Grain sizes values were subjected to Kruskal-Wallis and Dunn tests (=0.05). The flexural strength results were: ZC 896.73±122.70; Z0 577.67±62.26; Z1 477.32±75.65; Z2 492.25±63.19; Z5 437.18±53.55. The Weibull modulus results found were: ZC 7.9; Z0 11.2, Z1 8.7; Z2 8.1; Z5 9.3. Results showed that experimental Y-TZP presented lower flexural strength values than commercial one, but the first presented better Weibull modulus (m). Experimental Y-TZP also presented good microstructure, comparable to commercial Y-TZP, with very similar grain sizes. Nanotubes addition to Y-TZP led to lower flexural strength, although higher m than commercial ceramic. Pores containing Ti were observed in Y-TZP as the nanotubes concentration raised. Shear bond strength results found were, from higher to lower values: Z5 6.46±3.36; DC 6.17±0.87; D03 5.74±1.70; S03 5.73±1.71; Z1 5.16±2.62; D06 4.82±1.06; D09 4.75±1.43; SC 4.73±1.43; S09 4.61±0.85; S06 4.51±1.87; ZC 3.70±1.82; Z0 -3.33±2.05; Z2 2.94±1.38. Shear bond strength was also influenced by nanotubes addition, either in the ceramic or in the cement, although not linearly. Y-TZP added of 5% of TiO2 nanotubes presented the highest bond strength, although with no significant difference from most groups. Group Z1 was probably the group that presented the best combination of flexural strength, m, microstructure and bond strength. More studies of other properties could be carried out with this group. / Nanotubos de dióxido de titânio (TiO2) tem sido utilizados para melhorar as propriedades mecânicas de materiais odontológicos. Zircônica tetragonal policristalina estabilizada por ítria (Y-TZP) tem sido amplamente utilizada na Odontologia. Apesar de seus excelentes resultados clínicos, a Y-TZP é suscetível a falhas relacionadas à espessura insuficiente do conector da prótese fixa e a soltura da restauração devido à adesão deficiente. O objetivo desse trabalho é avaliar o efeito da adição de diferentes concentrações de nanotubos de TiO2 à Y-TZP nas suas propriedades mecânicas e microestrutura, além de avaliar a influência na resistência de união da adição desses nanotubos na cerâmica ou no cimento resinoso. Para isso, os objetivos descritos foram divididos em dois artigos diferentes. O artigo 1 descreve os testes de resistência flexural biaxial, análise fractográfica qualitativa em microscopia eletrônica de varredura (MEV), avaliação de microestrutura em MEV de emissão de campo e difração de raios-X. Os grupos avaliados foram: Y-TZP comercial (Ivoclar Vivadent) (ZC) e Y-TZP experimental com diferentes concentrações de nanotubos [0 (Z0), 1 (Z1), 2 (Z2), e 5% (Z5), em volume]. No artigo 2 está descrito o teste de resistência ao cisalhamento que foi realizado com os seguintes grupos: Y-TZP comercial (ZC) e Y-TZP experimental com diferentes concentrações de nanotubos [0 (Z0), 1 (Z1), 2 (Z2), e 5% (Z5), em volume] aderidas ao cimento Panavia F2.0; e Y-TZP comercial aderida ao cimento resinoso RelyX U200 com adição de diferentes concentrações de nanotubos em dois métodos de polimerização: dual [0 (DC), 0,3 (D03), 0,6 (D06) e 0,9% (D09) de nanotubos em peso] ou auto [0 (SC), 0,3 (S03), 0,6 (S06) e 0,9% (S09) de nanotubos em peso]. Os valores de resistência flexural e resistência ao cisalhamento foram submetidos aos testes de ANOVA e Tukey (=0,05). A resistência flexural também passou por análise de Weibull. Os valores de tamanho de grãos foram submetidos a testes de Kruskal-Wallis e Dunn (=0,05). Os resultados de resistência flexural encontrados foram: ZC 896,73±122,70; Z0 577,67±62,26; Z1 477,32±75,65; Z2 492,25±63,19; Z5 437,18±53,55. Os resultados de módulo de Weibull encontrados foram: ZC - 7,9; Z0 - 11,2; Z1 - 8,7; Z2 - 8,1; Z5 - 9,3. Os resultados mostraram que a Y-TZP experimental apresentou menores valores de resistência flexural do que a cerâmica comercial, mas a primeira apresentou maior módulo de Weibull (m). A Y-TZP experimental apresentou boa microestrutura, comparável à Y-TZP comercial, com tamanhos de grão muito semelhantes. A adição de nanotubos à Y-TZP levou a menor resistência flexural, porém maior m que a cerâmica comercial. Poros contendo Ti foram observados na Y-TZP conforme a concentração de nanotubos aumentou. Os resultados de resistência ao cisalhamento foram, do maior para o menor: Z5 - 6,46±3,36; DC - 6,17±0,87; D03 - 5,74±1,70; S03 - 5,73±1,71; Z1 - 5,16±2,62; D06 - 4,82±1,06; D09 - 4,75±1,43; SC - 4,73±1,43; S09 - 4,61±0,85; S06 - 4,51±1,87; ZC - 3,70±1,82; Z0 - 3,33±2,05; Z2 - 2,94±1,38. A resistência ao cisalhamento também foi influenciada pela adição de nanotubos, tanto na cerâmica quanto no cimento, porém não linearmente. A Y-TZP adicionada de 5% de nanotubos de TiO2 apresentou maior resistência de união, porém sem diferença estatística da maioria dos grupos. O grupo Z1 foi provavelmente o grupo que apresentou a melhor combinação de resistência flexural, m, microestrutura e resistência de união. Mais estudos de outras propriedades podem ser realizados com o mesmo.

One of the degradation processes of zirconium-based nuclear fuel assemblies is irradiation-induced growth, an anisotropic, stress-independent, macroscopic deformation mechanism that elongates fuel cladding tubes axially. Irradiation-induced growth is driven by the irradiation-induced formation of dislocation loops, where the evolution of the loop structure can be complex, with the initial formation of loop generating transient growth, while the later formation of component dislocation loops, or loops, leads to accelerated growth. A full mechanistic understanding of loop nucleation is as yet unforthcoming. This thesis utilizes the diffraction peak broadening analysis software, named extended Convolutional Multiple Whole Profile, to study the dislocation structure evolution of neutron- and proton-irradiated zirconium alloys in order to validate proton-irradiation as a effective tool for the study of irradiation damage in relation to irradiation-induced growth. The diffraction profiles obtained exhibit unexpected features present in the tails of the Bragg peaks, tentatively attributed here to either strained regions of matrix, or diffuse scattering from severely distorted regions around nucleating precipitates, both originating from an increased solute concentration. The diffraction results indicate that the proton-irradiated samples exhibit qualitatively similar behaviours as seen from neutron-irradiation, such as a threshold irradiation dose before the formation of loops, however, a continued increase of loop dislocation density determined from peak broadening analysis is not observed by transmission electron microscopy. It is also shown that the Nb-containing Low-Sn ZIRLO® alloy has a lower dislocation density than the Nb-free Zircaloy-2 after the formation of loops correlating well with the relative irradiation-induced growth behaviours observed in- reactor. A correlation between a reduction in the loop dislocation density and the formation of loops is observed in Low-Sn ZIRLO® and Zr-1.60Sn-0.033Fe, providing support for the hypothesis that vacancy loops transform into loops. Zr- 0.61Sn-0.024Fe and Zr-1.60Sn-0.033Fe alloys show a rapid increase in the loop dislocation density in the initial stages of proton-irradiation, likely due to the low irradiation-resistance of the precipitates present in these alloys.