KovXcs and FErTtIAM [1] and Kov 93 and NAGY [2] derived an equation for the total equivalent mean shear strain ~ of a wire plastically twisted about its axis at constant tensile load. In this equation ~ consists of a torsional shear strain ND/L and the shear strain component equivalent to the associated tensile strain dL/L o, as follows:where N is the number of turns of twist, D is the diameter of the wire, L o and L ate the initial and instantaneous lengths of the wire, ~ and/~ are constants. KovAcs and FELTHAM [1] took the numerical values of ~ and fl to be 1/3 and 2.24, respectively, but they indicated that these values were underestimated because the mean torsional strain was evaluated on the assumption that the wire was purely elastic: they pointed out that GAYDON'S [3] assumption of ideal plasticity would be more appropriate. Recent work [4,5] has indicated that the values of ~ and fl are 2~/3 and 3, respectively. Eq. (1) can be written in the form:,JLILo = --(~I~)mblL + Ylt~ of C = --(~/~)e + y/~,where C = AL~Lo and O = ND/L.
Technically pure aluminium-4.5 wtyo magnesium alloy samples are subjected to bidirectional twisting under various applied loads and a t different temperatures. The associated tensile strain is measured up to fracture of the test samples. It is realized that the investigated alloy exhibits superplastic behaviour under appropriate conditions of deformation. This behaviour is attributed to the relieve of internal stresses and recovery of the damage caused by the initial strain. A loss in superplasticity is found a t temperatures higher than 200 "C. Intergranular cracks are believed to be responsible for the observed decrease in ductility. Furthermore, a remarkable decrease in ductility values occurs when the investigated alloy is deformed in the temperature range 400 to 600 "C. This may result from the rapid onset of grain growth under these conditions.Unter verschiedener LuBerer Belastung und bei verschiedenen Temperaturen werden technisch reine Aluminium4,5 Crew% Magnesium-Legierungsproben einer doppeltgerichteten Verdrehung unterzogen. Die damit verknupften Dehnungen werden bis zum Bruch der Testproben gemessen. Es wird gefunden, dalj die untersuchte Legierung unter geeigneten Deformationsbedingungen superplastisches Verhalten zeigt. Dieses Verhalten wird der Entlastung innerer Spannungen und der Ausheilung von Storungen durch innere Verzerrungen zugeschrieben. Bei Temperaturen hoher als 200 "C wird ein Verlust der Superplastizitiit gefunden. E s wird angenommen, dalj intergranulare Spalten fur den beobachteten Abfall der Duktibilitat verantwortlich sind. Dariiberhinaus tritt ein merklicher Duktilitatsabfall auf, wenn die nntersuchte Legierung im Temperaturbereich von 400 bis 600 "C deformiert wird. Dies kann von dem plotzlichen Einsatz von Kornwachstum bei diesen Bedingungen herruhren.
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