Abstract: A method and apparatus for the superplastic forming of a workpiece are disclosed. The invention comprises securing the workpiece within a die assembly located within an autoclave. The die assembly is of a type suitable for use in the superplastic fabrication of workpieces. The die assembly includes a lid, a die and first and second sealing mechanisms. The first sealing mechanism seals a first volume formed between the lid and the workpiece from a main volume of the autoclave. The second sealing mechanism is for sealing a second volume formed between the workpiece and the die from the main volume of the autoclave. The second volume has a backpressure, P.sub.b. The first volume has a pressure P.sub.f +P.sub.b where P.sub.f is the forming pressure for achieving superplastic forming of the workpiece. The main volume of the autoclave has a pressure, P.sub.m. The back pressure, P.sub.b is established so as to minimize grain boundary cavitation. A minimal pressure differential .DELTA.P=P.sub.m -P.sub.