<p>The properties and microstructure of Ti-5Al-5V-5Mo-3Cr were characterized under various stress states after the following heat treatments: 1) annealing above the β transus, followed by cooling at various rates and ageing for different times; 2) solution heat treatment in the α-β range, fan-cooling and ageing for various temperatures and times.</p> <p>Heat treatment above the ptransus temperature causes complete recrystallization of the as-forged microstructure. The as-cooled microstructure consists of equiaxed β grains with an average grain size of 200μm. Water quenching from above the β transus results in precipitation of a dispersion of nano-sized ω phase; while the fan-cooled microstructure contains nano-sized ω and α precipitates. Ageing of the fan-cooled microstructure at 790°C or 600°C precipitates sub micron acicular α throughout the β grains. The tensile properties of this condition could not be determined using standard tensile specimens due to brittle failure at the grips.</p> <p>Controlled cooling from above the β transus to the ageing temperature at slower rates produces a coarser α+β microstructure. Acicular α laths are produced with cooling rates of 1°C/min, while lamellar α develops at cooling rates of 3.4°C/min. The β annealed and fan-cooled condition is characterized by relatively low strength (~850MPa) and low ductility (~6% elong.).The fracture mode is by intensely localized slip and the creation of transgranular cracks. Localization of slip is attributed to shearing of the nano-scale ω precipitates by dislocations. A linear relationship between the grain size, d⁻¹/² , and the yield and fracture stresses was established, as described by the Hall-Petch relation. With controlled cooling, the strength and ductility improve by precipitation of lamellar α within the β matrix. Improvements in ductility and strength are achieved by reducing the slip length.</p> <p>Solution heat treatment below the β transus and fan-cooling results in complete dissolution of the as-forged acicular α phase. Solutionizing at 50°C below the ptransus yields a volume fraction of 16.5% primary α in a matrix of retained β. Low angle grain boundaries and globular primary α, each measuring 2-4μm average in diameter, are uniformly distributed throughout the retained β matrix. The tensile strength in this condition is relatively low, i.e. (900MPa) and the ductility relatively high (~16% elong.). With ageing in the 500°C to 600°C temperature range, precipitation of α within the retained β begins within 5 minutes of the start of ageing. Precipitating is heterogeneously nucleated at dislocations and grain boundaries. The yield and ultimate tensile strengths reach values of roughly 1200 and 1300MPa, respectively, and remain relatively constant for up 48 hours ageing</p> <p> The fracture stresses for the solution treated condition and for material subsequently aged at 500°C and 600°C are quite similar in magnitude. This similarity is due to the fact that the fracture mechanism, which controls the fracture stress, is the same for all these conditions. The fracture mechanism for all the solution treated conditions begins with shear decohesion of the primary α/β interfaces.</p> <p>For each condition, the damage mechanisms and final fracture modes were evaluated and rationalized on the basis of microstructural features. The yield and fracture stresses for the various conditions were calculated and plotted on a two-principal stress axis coordinate system, thus creating the failure envelope for Ti-5553. For the β annealed and fan cooled and for the α-β solution heat treated and aged conditions the yield and fracture envelopes are two concentric ellipses in good agreement with the shear strain energy (van Mises) model for failure.</p> <p>The fracture toughness and stress corrosion cracking behaviour for the STA condition were evaluated and compared against other β titanium alloys.</p> / Thesis / Doctor of Philosophy (PhD)

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Yes / Brittleness in manganese steels can be associated with processing in a "wet¿ [micro]climate resulting in the formation of continuous oxide networks. The formation of these networks can be prevented by sintering in an atmosphere, also ¿local¿ in a semiclosed container, adhering to the Ellingham-Richardson oxide reduction criteria. When this requirement is satisfied, however, further types of ductile ¿ brittle transitions are observed. Rapid cooling, typically above 40°C/min, produces enough martensite to render Fe-(3-4)Mn-(0·6-0·7)C material macroscopically brittle. Quenched and conventionally tempered structures remain brittle. It is tentatively suggested that segregation of minor alloying/tramp element(s), as in cast materials, is responsible for this temper embrittlement. To overcome it, heat treatment at a temperature no higher than 200°C, recovery/stress relief, is recommended.

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No description available.

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The objectives of this project were:<br />1.����To compare the drying cost and drying time for oak and poplar pallets for the following mold mitigation strategies for hardwood pallets: air drying, forced air drying (fan shed), kiln drying to 25% moisture content and chemical treatment, and<br />2.����Develop and evaluate a procedure for preventing and controlling mold growth on heat treated hardwood pallets<br />Twenty red oak pallets and twenty yellow-poplar pallets were tested for each drying method to compare costs and to determine drying times. �Additional pallets were obtained to conduct a more thorough air drying procedure. �Drying data was extrapolated to allow estimates of the drying time from green (83% moisture content for poplar and 64% moisture content for oak) to 25%. �<br />After the pallets reached the desired 25% moisture content, they were placed in a 40�" enclosed trailer, inoculated with mold (Aspergillus, Stachybotrys, and Penicillium) and were left undisturbed for a period of 14 days. �After the 14 day incubation period, the pallets were inspected for mold using the ASTM D-4445 Standard Test Method for Fungicides for Controlling Sapstain and Mold on Unseasoned Lumber. �<br />A comparison of drying costs was then conducted to determine which method was the most cost efficient based on the data obtained in this study. �The cost to treat the pallets with each treatment was calculated including electrical cost, labor, and tax values. �In addition to the cost comparison, a Net Present Value (NPV) was calculated to determine which method produced the best outcome over a longer period of time.<br />Two heat treatment and drying schedules were then developed to meet both IPPC-ISPM #15 requirements and achieve the desired 25% moisture content with minimal degrade. �This was accomplished by testing several HT/drying schedules on green yellow-poplar and white oak pallets until the pallets met the criteria for being heat treated and had minimal degrade. �The schedules developed are a modified oak HT/KD schedule that required 30 hours to complete and a modified poplar HT/KD schedule that required 16 hours to complete.<br />The results demonstrated that that mold would not grow on the pallets stored in an enclosed container when the dew point is not reached. Air-drying pallets, chemical application in conjunction with air-drying pallets, fan shed drying pallets and kiln drying pallets to a 19-24% moisture content was demonstrated to prevent mold growth on oak and yellow poplar pallets. Estimates for the time required to dry yellow-poplar and oak pallets to 19% and 25% moisture content were developed for air-drying, forced air-drying and kiln drying for the conditions experienced in Blacksburg, VA between 7/30/2008 and 11/10/2008. Air-drying pallets was found to have the lowest daily operational cost but not the lowest total drying cost. �Fan shed drying had the lowest drying cost to achieve 25% moisture content. �Kiln drying was the most expensive daily and total cost, but yielded the fastest method of drying pallets to 25% moisture content. A NPV cost comparison showed that over a 3 year (36 month) time period, fan shed drying is the most cost effective method of drying pallets based on the values used in this study. Given the environmental conditions experienced between 7/30/2008 and 11/10/2008, no mold grew on the air-dried, fan shed, and kiln dried pallets during the drying process. <br /> / Master of Science

This study investigates use of the organosilane 3-(trimethoxysilyl) propyldimethyl octadecyl ammonium chloride (Si-Quat) as a wood treatment to impart residual moisture and organism control on wood substrates. Study 1, which utilized experimental testing procedures to evaluate mold growth after standardized heat treatment, indicated less surface mold on treated samples. Study 2, which utilized standardized testing procedures to evaluate Si-Quat treated wood’s resistance to subterranean termite attack, indicated greater termite mortality and less feeding on treated wood, as well as increased termite feeding preference for untreated wood. Study 3, which utilized standardized testing procedures to evaluate water repellency, indicated significantly reduced moisture gain at higher silane-based treatment levels in comparison to untreated wood. It is concluded that a silane based treatment utilized in this study can be effective for organism control and the possible supplementation to current phytosanitation of wood packaging materials.

Concentrically braced frames (CBFs) have been widely used in seismic areas as efficient structural systems to provide both lateral stiffness and strength. They dissipate earthquake energy through the inelastic deformation of the braces in both tension and compression. While these frames are efficient in providing lateral stiffness and strength, their inelastic mechanism is not ductile when compared to other systems such as moment resisting frames (MRFs). This student proposes a new approach to enhance the ductile behavior of CBFs by locally heat treating gusset plate connections or braces. In this method, the steel is heated locally to austenitizing temperature and then cooled with the appropriate rate to achieve the desired material properties. In gusset plate connections, to permit the rotation imposed from brace buckling, the conventional approach is to use linear fold lines, which can result in overly large plates. A more compact design uses elliptical fold lines, but both designs can lead to damage to welds with surrounding components. To enhance the performance of the gusset plate connection, a yield path is defined with a locally weakened zone within a high strength steel gusset plate. The weakened zone, created through heat treatment concentrated the inelastic deformation, resulting in an efficiently sized connection in which the failure mechanism is tightly controlled. A design methodology for the heat treated gusset plate is proposed, and finite element analysis is used to analyze the behavior of the heat treated gusset plates. In conventional braces, repeated buckling leads to deterioration and low-cycle fatigue which limits the ductility capacity of the CBF, compared to MRFs. As a novel approach, heat treatment is used to increase the local yield strength in the brace. Through this method, the buckling is permitted to occur, but an enhancement in the buckling behavior is intended. Various heat treated configurations are investigated, and finite element analysis is used to compare the behavior of heat treated braces. / Thesis / Master of Applied Science (MASc)

A coupled electromagnetic/heat transfer model capable of simulating microwave processing of composite materials in a cylindrical resonant cavity was developed. The two-dimensional model simulates processing of axisymmetric material loads in cylindrical resonant cavities operating in the TM₀₁₀ mode. The model consists of an electromagnetic model and a heat transfer model which are coupled by the heat generation term in the heat transfer equation. Heat generation in the process material is due to dielectric loss in the material and is related to the dielectric loss factor ofthe material, the processing frequency, and the magnitude of the electric field. The finite-element method was used to develop both the electromagnetic and heat transfer models. The electromagnetic model, based on Maxwell's equations, allows anisotropic conductivity and permittivity and accounts for resonance. A novel technique for determining resonance was developed for use in the electromagnetic model. The technique can be used to design microwave applicator/material systems. The heat transfer model allows anisotropic thermal conductivity and can be used to simulate heating by microwaves only, by convection only, or by a combination of microwaves and convection. The coupled model can account for the temperature dependence of dielectric properties. The electromagnetic and heat transfer models were verified by comparison to cases for which analytical solutions were available. The coupled model was then used to simulate microwave processing of nylon 66 and composite specimens of S-glass/polycarbonate. Microwave and convective heating were used alone and in combination to heat a thick cylinder of material. Comparisons are made between microwave, convective, and combined processes and the advantages and disadvantages of microwave processing are discussed. / M.S.