The Effects Of Alloying And Grain Size On Fatigue Life Behavior, Cyclic Stress-Strain Behavior, and Microstructural Evolution of Unalloyed Mg and A Mg-Al Alloy

Physical Metallurgy;Mechanical Metallurgy;Metal Fatigue;Recrystallization;Magnesium;Materials Science and Engineering;Engineering;Science;Materials Science and Engineering

Murphy-Leonard, AerielThornton, Katsuyo S ;

University of Michigan

关键词: Physical Metallurgy;    Mechanical Metallurgy;    Metal Fatigue;    Recrystallization;    Magnesium;    Materials Science and Engineering;    Engineering;    Science;    Materials Science and Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/150010/aerielm_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Mg and Mg alloys are important materials in structural applications where light weighting is important since the material exhibits an excellent strength to weight ratio and has a lower density than alloys such as Al and Ti. The formation of these materials requires thermomechanical processing during which the material undergoes deformation and then subsequent annealing which leads to recrystallization and grain growth. Both of these mechanisms have a significant influence on the cyclic stress-strain response and the fatigue life during cyclic loading. Therefore, understanding the influence of grain size and alloying on the microstructural evolution that occurs during annealing and its effect on low cycle fatigue behavior is important for increased use of these materials in structural applications. The influence of grain size and alloying on the microstructural evolution that occurs during static recrystallization and grain growth and its effect on the mechanical behavior during fatigue is being investigated. It is widely understood that grain characteristics (e.g. size and crystallographic orientation) can have a significant effect on the mechanical behavior of metallic materials. The static recrystallization behavior of unalloyed Mg and Mg-4Al was characterized over a range of annealing temperatures. The electron backscatter diffraction grain orientation spread technique was used to quantify the level of recrystallization at various annealing times. Recrystallization kinetics were characterized using the Johnson-Mehl-Avrami-Kolomogorov (JMAK) relationship and it was found that two sequential annealing stages exist. Stage 1 involves heterogeneous nucleation of recrystallization in regions with a high stored energy, including twins and grain boundaries. During Stage 2, recrystallization occurred predominately in the interior of deformed grains with incomplete recrystallization generally observed even at annealing times in excess of two weeks. Increasing the starting grain size in the unalloyed Mg condition led to a significant delay in recrystallization. The addition of Al had minimal effect on the recrystallization kinetics of Mg. To understand the influence of grain size and alloying on mechanical behavior, low cycle fatigue experiments using in-situ and ex-situ techniques were performed. The twinning and detwinning behavior of extruded fine-grained unalloyed Mg and Mg-4Al was investigated using in-situ high energy x-ray diffraction (HEXD) under displacement controlled, fully-reversed low cycle fatigue conditions.Measurements were conducted at three levels of applied strain. At cycle strains greater than 0.5%, tension-compression loop asymmetry was observed during cyclic loading which can be related to twinning during the compressive portion of the cycles followed by detwinning during the unloading (from peak compression) and tensile loading portions of the cycles. The twinning and detwinning were characterized by monitoring the evolution of x-ray diffractions peaks associated with the basal {0002} planes throughout a cycle. It was determined that the stress needed to initiate twinning during compression was independent of grain size and Al addition and that the stress needed for complete detwinning was closely related to the twin volume fraction. Ex-situ, strain-controlled LCF experiments were performed on both fine and coarse grained unalloyed Mg alloys as well as Mg-4Al. It was also found that for a given total strain amplitude the fatigue life was slightly increased in the fine-grained unalloyed Mg condition.