Study On The Plastic Instability Behavior And Deformation Mechanisms Of Mg-Gd(-Mn-Sc) Alloys During Tensile Deformation

Mg-Gd system is one of the most promising candidates for developing precipitation hardenable magnesium alloys and becomes more and more attractive materials for aerospace and automotive applications due to its high specific strength and excellent mechanical properties at elevated temperatures. In recent years, Mordike et al. developed a new group of creep-resistant Mg-Gd materials microalloyed with Mn and Sc. These newly developed Mg-Gd-Mn-Sc alloys have superior creep resistance at 300℃that are better than those of commercial WE54 or WE43 alloys. The previous investigations on Mg-Gd alloy focused on the age-hardening response, precipitation during ageing, tensile properties at elevated temperatures and creep resistance. The reports on Mg-Gd-Mn-Sc alloy were mainly about the creep resistance and the corresponding microstructures. An improved understanding of the correlation of microstructures and deformation behaviors, the plastic deformability and the plastic instabilities of the Mg-Gd(-Mn-Sc) alloys is of critical importance for any further enhancement in mechanical properties at ambient and elevated temperatures and exploiting application prospect.In this study a Mg-5wt.%Gd alloy and a Mg-5wt.%Gd-1wt.%Mn-0.7wt.%Sc alloy have been selected as model alloys. The effects of Mn and Sc elements on the microstructure and age-hardening response of Mg-Gd alloy, different kinds of plastic instabilities occurred in the Mg-Gd(-Mn-Sc) alloys at temperatures ranging from room temperature (RT) to 400℃and the microstructure evolution and plastic deformation mechanisms during tensile tests under different temperatures and strain rates have been systematically investigated. Optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD) equipment, transmission electron microscope (TEM) and three-dimensional atom probe (3DAP) were used to examine the microstructures. The results show that:(1) With the additions of Mn and Sc elements, a large number of Mn-Sc intermetallic dispersoids precipitate in the quaternary alloy after heat treated at 460～580℃. However, when the quaternary alloy is solution treated at 610℃for 24h, these Mn-Sc dispersoids almost fully dissolve intoα-Mg matrix, which leads to the significant increase of the amount of Mn and Sc atoms in the matrix. The binary alloy exhibite almost no age-hardening response above 200℃, no matter the alloy is solution treated at 520℃or 610℃for 24h before ageing. For the quaternary alloy, no obvious age-hardening above 200℃occurs in the sample suffered from 520℃/24h heat treatment before ageing. However, when the quaternary alloy is solution treated at 610℃for 24h then aged at 200～300℃, a substantial increase in hardness is observed. The peak hardnesses obtained during ageing at 200,250 and 300℃are increased by 63%,44% and 33% respectively. The dense precipitation of very thin Mn-Gd plates and tiny Mn2Sc discs is considered to be mainly responsible for the greatly improved age-hardening response.(2) Initial yield point and strain ageing yield point are observed during the tensile tests at RT. The initial yield point is associated with the significant decrease in the dislocation density during the proper heat treatment before tensile tests. The strain ageing yield point is ascribed to the interaction between solute atoms and the dislocations. When a proper heat treatment is applied to the unloaded sample, the solute atoms can diffuse to and pin the dislocations. After reloading the sample, the strain ageing yield drop happens when the dislocations unpin the solute atoms atmosphere to move. These two yield points are caused by different phenomena, however, they can be ascribed to the same physical principle. In either case, the result of ageing is to reduce the mobile dislocation density below that value needed to sustain the imposed strain rate. When the tensile test is resumed, an increase in stress initially manifests the mobile dislocation deficiency, but when the dislocations multiply quickly and the plastic strain rate exceeds the imposed strain rate, the yield drop is observed.(3) Serrated flow is observed when the as-extruded samples of Mg-5Gd(-1Mn-0.7Sc) alloys are tensile tested at temperatures ranging from 150 to 300℃and at strain rates of 1.67×10-4s-1 to 1.67×10-2s-1. The serrated flow phenomenon is significantly influenced by the heat treatment conditions.β' phase precipitates densely and the content of solute atoms decreases greatly during the ageing treatment at low temperatures, which weakens the PLC effect or even leads to the disappearance of the PLC effect. The serrated flow is attributed to dynamic interactions between solute atoms and gliding dislocations, i.e. dynamic strain ageing.(4) The Mg-Gd-Mn-Sc alloy has a smaller critical strain for serrated flow and larger stress drops for both strain ageing yield point at RT and serrated flow at elevated temperatures than those in the Mg-Gd alloy. This phenomenon may be ascribed to the contribution of Mn atoms. The Mn atoms have a smaller atomic radius than Mg atoms, and when combined with larger atoms of Gd, can provide more effective pinning of dislocations than a single type of solute atoms in the Mg-Gd alloy.(5) When the Mg-Gd extruded samples tensile tested at 350～400℃under different strain rates, serrated flow is observed at the strain rates lower than 4.4×10-4s-1, and absent at strain rates higher than 8.8×10-4s-1.This serrated flow is not ascribed to the PLC effect.(6) Dynamic recrystallization (DRX) occurs in the specimen tested at 400℃at higher strain rate of 8.8×10-3s-1, and leads to the grain refinement. DRX is the main softening mechanism for the samples deformed at higher strain rates, and the grain refinement caused by DRX further leads to the softening effect. During the tensile at lower strain rate of 8.8×10-5s-1, no sign for the operation of DRX can be detected but normal grain growth is observed. TEM microstructure observation shows that the dislocation density is large in the samples tested at high strain rate, but the dislocation density in the samples tested at low strain rate is too small to stimulate DRX. Dynamic recovery is the main softening mechanism for the samples deformed at low strain rates. Meanwhile, the grain growth brings the hardening effect. Because of the competition between continuous dynamic recovery (softening factor) and continuous grain growth (hardening factor), the serrated flow is observed.(7) When the specimens are loaded at 350～400℃, at low strain rates to 1-2% total strain, unloaded slowly, and then reloaded again, an increased yield strength is observed. The magnitude of the increase in the yield strength is larger when a slower unloading speed is applied. The phenomenon is absent at the higher strain rates. This is due to the normal grain growth under stress during slow unloading process, which leads to a strengthen effect when tensile temperature is as high as 350～400℃.(8) At room temperature basal slip and mechanical twinning are predominant in Mg-5Gd extruded samples to accommodate the plastic strain; Non-basal a slip and a+c pyramidal slip can only operate in the minority of the grains. At 200℃basal slip and mechanical twinning are also mainly deformation mechanisms; Non-basal a slip and a+c pyramidal slip are initiated in most of the grains. The grains are stretched gradually along the tensile direction with increasing strain at RT and 200℃. Basal slip, non-basal a slip, cross slip, a+c pyramidal slip, dislocation climbing are all very active at 400℃, but basal slip is still the most active slip system and plays an important role in the texture evolution. Grain boundary slip and grain rotation also operate at 400℃, which leads to the grains remaining equiaxial shape during the whole tensile process.