Microcreep of molybdenum crystals

Abstract

Molybdenum single crystals 1/8 inch in diameter were prepared by a zone-melting technique. Crystals were tested in tension in a special creep machine using a very sensitive capacitance extensometer. Flow stress was measured on an Instron machine. Microcreep of the crystals was studied at stresses below half the macroscopic flow stress in the temperature range 200° to 300°K. at resolved shear strains of below 5%. The creep following addition of a stress increment was found to have two parts. In the initial portion the creep rate decreased rapidly, but approached a constant rate in from one to ten minutes. The stresses used produced constant creep rates in the range 10-9 to 10-6 sec.-1. The effect of temperature, strain, and effective stress on that rate was studied. The effective stress is the externally applied stress minus the internal back-stress. The internal stress was measured by finding the stress at which forward creep balanced backward strain recovery. The constant creep rate, following the initial creep, obeyed an equation of the form A sin h(v times T times * over K times T) as predicted by Alefeld and others for creep at low effective stresses. The activation volume v was found to be in the range 15 to 40 b3, decreasing with strain and decreasing at lower temperatures. The parameter A contains the number of activable dislocation segments. The increase of A with strain was proportional to the increase of dislocation density as reported by Lawley and Gaigher. From the temperature dependence of the parameter A, the activation energy for the constant rate microcreep was found to be on the order of 0.1 ev. Results indicate that higher values obtained by other investigators for deformation of b.c.c. metals arises from a neglect of the change of the internal stress. The internal stress was found to rise strongly at low temperature with the same temperature dependence as that of the macroscopic flow stress. Extrapolation showed the internal stress should approach zero at 360°K. which is the point at which the flow stress of molybdenum has been found to become temperature independent. The internal stress did not appear to be an innate property of the b.c.c. lattice, but developed during small amounts of strain. Indications were found that the internal stress then decreases after the material is unloaded or during a return of the material to higher temperature. No detailed model for the creep process or for the recovery of the internal stress is presented. However, it is thought that cross-slip of screw dislocations plays an important role. It was concluded that the strong temperature dependence of the flow stress reflects a very high rate of work-hardening which strongly increases the internal stress during strains of 10-3 or less.