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Stress relaxation measurements were carried out on a plain carbon and four Ti steels over the temperature range 850 to 1050$ sp circ$C. The results show that the stress relaxation of plain carbon austenite after a 5% prestrain can be described by the relation $ sigma$ = $ sigma sb0$-$ alpha$ln(1 + $ beta$t). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is complete. As a result, this new mechanical method is suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. / PTT diagrams were determined by the present technique for the steels containing 0.05, 0.11, 0.18 and 0.25% Ti. The PTT curves obtained are C shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the current values of the solubility product (Ti) (C). This phenomenon is attributed to the catalytic effect of trace amounts of dissolved N on the nucleation rate of Ti(CN) in austenite. / Changes in the size distribution and morphology of the precipitates during relaxation of the 0.25% Ti steel were followed by means of electron microscopy. The cube shaped Ti(CN) precipitates were heterogeneously distributed in either a chain-like or a cell-like manner. Electron microanalysis was additionally carried out to determine the compositions of the Ti carbosulphide-Mn sulphide inclusions. / Finally, the experimental results are compared with the predictions of classical nucleation theory and of the diffusion controlled particle growth theory. Good agreement was obtained between the predictions of the theories and the experimental results. It is demonstrated by means of the thermodynamic analysis of nucleation that the Ti(CN) precipitate/austenite interface is of a semi-coherent nature. It is shown that the interface between a critical nucleus and the matrix can be characterized by the newly introduced coherency loss parameter C.

Aluminum-zirconium alloys exhibit superior strength at elevated temperature in comparison to traditional aluminum casting alloys. These alloys are heat-treatable and their strength depends to a large extent on the quenching and aging steps of the heat treatment process. However, measurements show that the critical cooling rate necessary to retain 0.6 wt. pct. zirconium(the minimum amount necessary for significant strengthening) in a super-saturated solid solution with aluminum is 90ºC/s, which is un-attainable with traditional casting processes. On the other hand, the critical cooling rate necessary to retain 0.4 wt. pct vanadium and 0.1 wt. pct. zirconium in a super- saturated solidsolution with aluminum is only 40ºC/s; which suggests that substituting vanadium for zirconium significantly decreases the critical cooling rate of the alloy. This is an important finding as it means that, unlike the Al-0.6Zr alloy, the Al-0.4V-0.1Zr alloy may be processed into useful components by traditional high pressure die-casting. Moreover, measurements show that the hardness of the Al-0.4V-0.1Zr alloy increases upon aging at 400ºC and does not degrade even after holding the alloy at 300ºC for 100 hours. Also, measurements of the tensile yield strength of the Al-0.4V-0.1Zr alloy at 300ºC show that it is about 3 times higher than that of pure aluminum. This increase in hardness and strength is attributed to precipitation of Al3(Zr,V) particles. Examination of these particles with high resolution transmission electron microscopy (HRTEM) and conventional TEM show that vanadium co-precipitates with zirconium and aluminum and forms spherical particles that have the L12 crystal structure. It also shows that the crystallographic misfit between the precipitate particles and the aluminum matrix is almost eliminated by introducing vanadium into the Al3Zr precipitate and thatthe mean radius of the Al3(Zr,V) particles is in the range from 1nm to 7nm depending on the alloy composition and aging practice. Finally, it is found that adding small amounts of silicon to the Al-0.4V-0.1Zr alloy effectively accelerates formation of the Al3(Zr,V) precipitate.

Precipitation hardening of eutectic and hypoeutectic Al-Ni alloys by 2-4 wt pct. manganese is investigated with focus on the effect of the alloys’ chemical composition and solidification cooling rate on microstructure and tensile strength. Within the context of the investigation, mathematical equations based on the Orowan Looping strengthening mechanism were used to calculate the strengthening increment contributed by each of the phases present in the aged alloy. The calculations agree well with measured values and suggest that the larger part of the alloy’s yield strength is due to the Al3Ni eutectic phase, this is closely followed by contribution from the Al6Mn particles, which precipitate predominantly at grain boundaries

This thesis aims to fabricate the high performance Magnesium/Carbon-Fiber/PEEK five-layer hybrid nanocomposite laminates. The adopted Mg thin sheets are 0.5mm thick. The Carbon-Fiber/PEEK prepregs were stacked into two lay-ups, such as cross-ply [0/90]s and quasi-isotropic [0/45/90/-45], with the adding of nanoparticles SiO2 spreaded among the laminates. After etching of Mg foils by CrO3-base etchants, a five-layered Mg/Carbon-Fiber/PEEK nanocomposite laminate was made according to the modified diaphragm curing process. Then, the mechanical properties, such as stress-strain curve, strength and stiffness were obtained by tensile test at room temperature (25¢J), 50, 75, 100, 125 and 150¢J and the fatigue properties were also obtained under constant stress amplitude tension-tension cyclic loading elevated at room and elevated temperatures 25, 75, 100, 125 and 150¢J. Finally, the Mg sheets and fractured laminates were observed by the SEM and OM. The results according to the experiments were summarized as follows: 1.The slope of stress-strain curve dropped at strain¡Ü0.0015. It can be inferred that fracture occurred in the laminates at this time. Stiffness approached the theoretical value by curve fitting with the strain range of 0 to 0.0015. 2.The mechanical properties decreased with the environmental temperature rise. 3.The resistance to the temperature effect of the quasi-isotropic Magnesium/Carbon-Fiber/PEEK laminate is superior to that of the cross-ply Magnesium/Carbon-Fiber/PEEK laminate. 4.The cross-ply Magnesium/Carbon-Fiber/PEEK laminate is brittler than that of the quasi-isotropic laminate generally. 5.The irregular bright lines were found in the third etched Mg sheet and that resulted in the delamination of Mg sheet after treatment. The unetched part maybe the defect of Mg sheet. 6. It was found that AZ31 has the precipitation hardening effect at 50¢J and 75¢J.

Thesis (Ph. D.)--West Virginia University, 2002. / Title from document title page. Document formatted into pages; contains xvi, 140 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 137-140).

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Thesis (Ph. D.)--University of North Texas, May, 2007. / Title from title page display. Includes bibliographical references.

Thesis (Ph.D.)--Washington State University, December 2005. / Includes bibliographical references.