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Mechanical properties of dilute zinc - titanium alloys

University of British Columbia

Zinc-titanium alloys (0.07-0.6 wt.%Ti.) in the form of compacted powder and chill castings have been extruded at temperatures between 150°C and 350°C. The mechanical properties of these alloys have been studied as a function of temperature, strain rate, grain size and intermetallic (Zn₁₅Ti) distribution. Due to a high value of "k" in the Hall-Petch relationship, maximum strengthening is obtained by a reduction in grain size. However because of an increasing amount of grain boundary shear, this potential is not realized. The operation of dynamic recovery mechanisms at 20°C and higher also results in limitations upon the development of high strength. The use of powder metallurgical techniques gives rise to the formation of intermetallic distributions which inhibit these processes and results in high strength (>60,000 p.s.i.) and low strain rate sensitivity (m ∼ 0.02). The mechanical properties are not a function of initial powder size. The properties obtained using chill castings do not reach these levels due to the difficulty associated with forming a fine second phase on solidification. Such a distribution is required to obtain a small stable grain size during subsequent extrusion. To satisfy compatibility requirements deformation modes other than the two supplied by basal slip must be invoked. High strengths are observed when grain boundary shear and migration are inhibited by the distribution of the second phase or by orientation effects. Under such conditions, non basal slip and basal slip are the operative deformation mechanisms. Significantly lower strengths result if grain boundary shear and basal slip satisfy the conditions necessary for ductile behaviour. The strain rate sensitivity parameter at 20°C lies in the range 0.02-0.07. Varying amounts of grain boundary shear occur, nevertheless deformation is slip controlled. Increased strain rate sensitivities are observed at high temperatures, but failure by cavitation limits ductility. The strain rate sensitivity is not a function of titanium concentration. Under constant fabrication conditions the strength generally increases with increased Zn₁₅Ti content. The thermal stability of the intermetallic distribution prescribes the fabrication conditions which must be used to develop high strength, and the temperature to which the mechanical properties can be retained. The high strength microstructures appear to be stable up to at least 150°C for short periods of time.

Text

2011-06-02

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http://hdl.handle.net/2429/35049

Metals and Materials Engineering

University of British Columbia

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