Anomalous high-temperature strengthening in alpha Ti-Al solid-solution alloys

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Tensile tests were performed in Ti- (7-13) at.%Al alloys. Temperature dependence and strain-rate dependence on deformation behavior were investigated. The possibility of the strength anomaly athigh temperatures was examined and its reason was discussed, based on the obtained results and the reported results for alloys with low Al concentrations.In the case of low-aluminum alloys containing less than 5mol%Al, the stress-strain rate relationship showed transition of the stress exponent from 3.5 to 4.6. The concentration dependence of flow stress was found to agree with theroreticaly predicted behavior by an ordinarly solid-solution herdening theory. In the case of concentrated alloys containing more than 5%Al, no transition was observed and the stress exponent was found to be a constant value of 3.5. The concentration dependence of flow stress was more than twice as large as the theoretically predicted calues, indicating the anomalous high-temperature strengthening in concentrated alpha Ti-Al solid-solution alloys.Structural analysis by X-day diffraction and transmission electron microscopy suggested the presence of a short-range ordered (SRO) structure. The possibility of the SRO formation was investigated by Monte Carlo calculation. It was found that the SRO structure became noticeable for a concetration of more than 5 mol%. The SRO tedency was also found to increase with increasing the aluminum concentration. Since the presence of the ordered structure increases flow stress, the anomalous strengthening in the Ti-Al alloy was found to be caused by the formation of the SRO structure.