Schmid's law deviation in molybdenum

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The observation that the critical resolved shear stress is dependent upon the text axis in body-centered cubic metals is important in developing an adequate theory of strength in these materials. Therefore, it is important that effects due to substructure or impurity variation on the yield stress be carefully separated from the true orientation dependence. This was accomplished by growing single crystals of molybdenum along the 〈110〉 axis so that specimens having either a 〈100〉 or 〈110〉 tensile axis could be prepared from the same parent single crystal. The yield strength of the crystals was then measured as a function of temperature from 78°K to 298°K. It was found that the CRSS in the 〈110〉 tensile axis was 30% greater than for the 〈100〉 tensile axis when tested at 78°K. This result differs greatly from previous work on crystals that had been grown along a specific axis and then mechanically tested along the same axis. The present results strongly indicate that much of the orientation effect of previous work is actually due to a variation in the impurity content of test specimens. © 1974.

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Materials Science and Engineering