Creep Behaviour Of An Mg-4Y-3Nd-0.5Zr Alloy

The poor high temperature properties, especially creep property, ofmagnesium alloys has significantly impacted the industrial application ofmagnesium. The development of high temperature creep resistant magnesiumalloy has been a hot research. Mg-4Y-3Nd-0.5Zr alloy was employed in thisresearch. By using XRD, optical microscope, scanning electron microscopy,transmission electron microscopy and other methods, the structures of as-castand peak-aged state of this alloy were analyzed. The tensile and creep propertieswere tested by using Instron5500R electronic universal material testing machineand SWT2504high temperature creep and sustained testing machine,respectively. Based on the tensile and creep properties of Mg-4Y-3Nd-0.5Zralloy and the analysis of structure after creep, fracture, stress exponent andactivation energy, Mg-4Y-3Nd-0.5Zr alloy creep mechanism in differenttemperature and stress could be finally determined. The results show that:The grain of as-cast Mg-4Y-3Nd-0.5Zr alloy are like a chrysanthemum withthe size of79microns. The eutectic organizations are Mg5RE, while some tinylumps are isolated and dispersed distribution. The solution treatment caneliminate the eutectic phases between the grains and precipitate the ellipticspherical beta’ phases along <11-20> direction in the interior of the grains.When the temperature is below250℃, the tensile strength and yieldstrength has no obvious change. When the temperature is higher than250℃, theintensity decreases with the rise of temperature. And when the temperature isbelow150℃, the elongation has no obvious change, too. While, when thetemperature is higher than150℃, the elongation increases with the increase oftemperature. Under low temperature, tensile fracture mechanism belongs to themixed quasi cleavage fracture; along with the temperature increases, fracturemechanism turns into ductile fracture mechanism gradually. With thetemperature above300℃, the cleavage planes almost disappear, and the fracturemechanism turns into microporous gathered ductile fracture.In the process of high temperature creep, decelerating creep stagesometimes does not appear; secondary creep stage is the longest stage with the minimum creep rate among the whole creep process. Along with the increase ofstress or temperature, secondary creep rate accelerate. The beta’ phase in thepeak aged state which continues to grow up in high temperature turns into betaphase. The higher the temperature rise, the faster the beta phase grows. Thestress provides conditions for precipitated phase to nucleate and grow up byprecipitating dislocation lines and dislocation network, so that the precipitatedphase separate from the original direction.The creep fracture of Mg-4Y-3Nd-0.5Zr alloy, under high temperature andlow stress condition, is ductile intergranular fracture. With the increase of thestress, the fracture mechanism turns into brittle intergranular fracture. While inthe low temperature high stress condition, the fracture mechanism is brittleintergranular fracture.The stress exponent at200℃is5.96, and dislocationclimbing mechanism is dominant, the dislocation slip creep mechanism andgrain boundary sliding creep mechanism are concomitant. At high temperature,the average activation energy is261kJ/mol, which corresponds creep mechanismfor cross slip creep mechanism. At300℃, the stress exponent is5.46, anddislocation creep mechanism is dominant. When the stress is low, diffusion creepmechanism plays a role in creep mechanism;while the grain boundary slip creepmechanism works in high stress. The stress exponent at250℃and at275℃are3.16and3.46,respectively, which corresponds creep mechanism for grainboundary sliding mechanism.