On the physical basis of a Larson-Miller constant of 20

Larson-Miller (LM) and Orr-Sherby-Dom (OSD) parameters are widely used when formulating and predicting creep rupture life t(r). The LM constant C and the activation energy Q in the OSD parameter characterize temperature dependence of t(r). Q = Q(LSD) (Q(LSD): activation energy for lattice self-diffusion) and C = 20 are often assumed in the formulation. Creep rupture datasets of 11 heats of nickel alloy A617, 304H and 304J1 stainless steels, 6 heats of A3004N aluminum alloy and four Mg-Al alloys are formulated with the parameters for determining C and Q of each dataset. Based on the correlation between C and Q among similar materials, it is discussed what is the C value equivalent to Q = Q(LSD). It is also examined how the C value changes with melting temperature of the material. The value of C equivalent to Q = Q(LSD) varies from 14 to 9.5 depending on average values of log(rupture life) and of reciprocal temperature of the dataset. These values of C point out that C = 20 is not equivalent to Q = Q(LSD). The equivalent C values are similar in all the materials independently of their melting temperatures.