Cryogenic Temperature Dependence of the Yield Strength of High-Strength Alloys

The effect of cryogenic temperatures (from 78 F to −423 F) on the yield strength of twenty alloys was studied. Experimental results prove that they do not conform to any of the following theories: Hollomon and Zener’s, Cottrell and Bilby’s, or Fisher’s. However, all the plottings in loge-loge scale of yield strength versus absolute cryogenic temperatures of these alloys fall on straight lines which are governed by one single general equation, σy = bT−m. From the Cottrell’s dislocation theory on yielding and Fisher’s equation of activation energy in forming a dislocation loop, the same type of equation of yield strength versus temperature as expressed by the empirical ones can be derived theoretically. The empirical equations are very useful in predicting yield strengths at any cryogenic temperature within or slightly out of the test range for which data were available. Some limited yield strength data at elevated temperatures for a few alloys were studied for comparison. It was observed the general equation for yield strength versus cryogenic temperatures holds valid for stainless steels but not so well for titanium, nickel, and aluminum alloys at elevated temperatures. However, no conclusion can be drawn until further detailed studies at elevated temperatures are made.