The temperature- and strain rate-dependent mechanical behavior of a polycrystalline galena ore

Experiments on the compressive deformation of a polycrystalline galena ore (G2 galena) at temperatures from 20 degrees to 400 degrees C and at strain rates from about 10 (super -4) sec (super -1) to 10 (super -8) sec (super -1) have been performed in a triaxial test apparatus at a confining pressure of 1.5 kb. About 90 volume percent of the galena ore consists of lead sulfide with about 10 ¹⁸ atoms/cc of sulfur in excess of that expected from the stoichiometric composition (PbS).The differential stress supported by G2 galena and its rate of work hardening decrease rapidly as either the temperature is raised or the strain rate is lowered. Differential stresses (Sigma ) measured at 12 percent strain range from 3,100 bars at 20 degrees C and strain rate (e) of approximately 10 (super -4) sec (super -1) , to 320 bars at 400 degrees C and strain rate of approximately 10 (super -8) sec (super -1) . At normalized differential stresses (given by Sigma /[radic]3mu ; where mu = average shear modulus) greater than about 2.2 X 10 (super -3) dislocation glide is the dominant deformation mechanism and e is related to Sigma by an "exponential equation"; e = A exp(-Q/RT) exp(BSigma ); where A and B are constants and Q is the activation energy for flow. At lower stresses dislocation glide plus polygonization (subgrain formation) occurs. Provided that strains greater than about 8 percent are achieved, a "power equation" holds under these conditions: e = C exp(-Q/RT) Sigma ^N ; where C and N are constants. N is about 7.3 and Q approximately 22.5 kcal mole (super -1) . Deformation involving subgrain formation is believed to proceed at a rate controlled by dislocation climb.Calculations have been made to determine whether Nabarro-Herring creep (i.e., diffusive mass transfer creep; e infinity Sigma ) is an important mechanism in the natural deformation of galena ores with the same composition as G2 galena. These show that at 300 degrees to 400 degrees C flow according to the "power equation" may give way to Nabarro-Herring creep at strain rates that are slower than those attained in the laboratory yet fast enough to be tectonically significant, provided that grain size does not exceed a few mm.The differential stresses that G2 galena (mean grain size = 0.07 mm) would support at the slow strain rates typical of folding and related tectonic deformation are estimated to range from about 154 bars at 300 degrees C and a strain rate of 10 (super -11) sec (super -1) , to about 4.5 X 10 (super -2) bars at 400 degrees C and a strain rate of 3 X 10 (super -14) sec (super -1) . If grain growth occurs so that Nabarro-Herring creep is precluded the stress supported under the latter conditions might be as high as 47 bars.

This record provided courtesy of AGI/GeoRef.