Dependence of m-value on the Strain Rate in Superplastic Al-Cu Alloys

The strain rate sensitivity index, m, of Al-33Cu and Al-27Cu-5Si alloys was measured by the two methods, (a) the differential closs-head speed method and (b) the strain rate change method. The m-value by the method (a) was dependent strongly on strain and decreased with increasing strain. The m-value by the method (b) was dependent slightly on strain, but strongly on the width and the sense of the change from a given strain rate, \dotε1, to another one, \dotε2. The dependence of m on the width, J∗=∂m⁄∂\sqrt(lnN)2 where N=\dotε2⁄\dotε1, was positive or negative depending on whether the strain rate was lower or higher than the strain rate having the maximum m-value on the m-ln\dotε curve when N=2 (the reference curve of m). At the strain rate having the maximum m-value, the value of |J∗| showed the minimum value (|J∗|∼0). Since the m-value is defined by the strain rate and strain rate change, it is desirable to measure m by the method which excludes the effect of strain on flow stress, and it is suggested that the strain rate change method is the one in accordance with the definition. When specimens were deformed under the condition of the same m-value on the reference curve of m, the fracture strain and the uniformity of deformation in the strain rate region where J∗>0 were higher than those in the region where J∗<0.