Elevated-temperature Deformation, Strengthening And Fracture Mechanisms In Cast Creep-resistant Mg-11Y-5Gd-2Zn-0.5Zr (wt.%) Magnesium Alloy

Due to the strong demand for weight-reduction in automotive industry, research and development of creep-resistant Mg alloys, which can be long-time well served for powertrain applications at elevated temperatures, has progressed considerably in the last decade. Particularly, research and development of the high-performance creep-resistant Mg alloys for the key components of powertrain applications (≥300℃), such as engine piston, are at the forefront. However, research on those Mg alloys, especially for systematic research on the elevated-temperature deformation, strengthening and fracture mechanisms in Mg alloys containing rare-earth elements is limited.Mg-11Y-5Gd-2Zn-0.5Zr (WGZ1152, wt.%) is a gravity-casting high-performance creep-resistant Mg alloy developed by our group recently. Previous work showed that this alloy exhibited potential for elevated-temperature applications (≥300℃). Thus, the present work focused on this alloy. The tensile behavior at25～400℃(0.33-0.75Tm, Tm is the melting point) and strain rate ranges of1E-4-1E-2s-1, as well as the tensile-creep behavior at250～340℃(0.58～0.68Tm) and stress ranges of30～140MPa (0.1-0.6Ro2,R0.2is the yield stress at300℃) were investigated. The important characterization methods and techniques included scanning electron microscope (SEM), transmission electron microscope (TEM), electron backscatter diffraction (EBSD), image analysis, slip trace analysis, and in-situ SEM. Based on above results, the elevated-temperature deformation, strengthening and fracture mechanisms were discussed. What’s more, the industry trials of Mg alloy piston by gravity casting as well as the engine bench test were performed successfully.The room-and elevated-temperature properties of the peak-aged WGZ1152-T6alloy were investigated, and the results showed:1) the tensile strength and yield strength of the WGZ1152-T6alloy were considerably superior to those of WE54-T6(the most successful commercial heat-resistant Mg alloy) and AC8A-T6(the most widely used commercial Al alloy for engine piston) at25～400℃. At300℃(0.64Tm), the tensile strength and yield strength of the T6alloy were above250MPa and225MPa, respectively, which maintained86%and95%of those for room temperature.2) At300℃and the same stress, the minimum creep rate of the WGZ1152-T6alloy was almost two orders of magnitude lower than that for WE54-T6, and was more than one order of magnitude lower than that for AC8A-T6, and was comparable to that of HZ32-T5(the structure Mg alloy has the highest service temperature, but it being phased out because of radioactivity).The results of elevated-temperature tensile deformation and fracture behavior of WGZ1152-T6alloy are as follows:1) The flow behavior of WGZ1152-T6alloy was investigated at250～400℃and at strain rate ranges of1E-4～1E-2s-1, and the results showed:the constitution equation could be described by ε=A[sinh(ασ)]nexp(-Q/RT). The stress exponent n=7.7±0.7and the activation energy of deformation Q=274±10kJ/mol. The values of n and Q indicated that dislocation cross-slip was the rate-controlling mechanism. The observed wavy slip traces, which suggested cross-slip was active, supported the above viewpoint.2) The activities of slip modes during tensile deformation for the T6alloy were investigated quantitatively by in-situ SEM, slip trace analysis, and EBSD. The results showed:the dominate slip modes transited from basal<a> slip (100%) to basal<a> slip (73%) combined with prismatic<a> slip (16%) from25℃to250℃. As the temperature further increased up to350℃, the combination of basal<a> slip (67%) and pyramidal <c+a> slip (25%) became the dominate slip modes; the prismatic<a> slip was more active at higher strains for moderate temperatures (200～250℃), while the pyramidal<c+a> slip was more active at higher strains and temperatures; the above results were consistent with temperature dependence of the critical resolved shear stress (CRSS) of Mg single crystal at200～250℃, but when the temperature was above250℃, they were consistent with the simulation results for AZ31alloy used the full-constraint Taylor model by Barnett to a certain extent.3) The fracture mechanisms during tensile deformation for the T6alloy were investigated by in-situ SEM, and the results showed:the specimen fractured by both transgranular cracking (40%) and intergranular cracking (60%) at25℃; the coarsened slip band was important for the transgranular cracking nucleation; at200～350℃, the dominant fracture mode became intergranular cracking. The onset of obvious cracks was from the middle-to late-deformation stage. The intergranular cracking nucleation site tended to be located at grain boundary which was perpendicular to the load direction and the interface between the a-Mg matrix and the large intermetallic grain boundary phase.The results of tensile-creep deformation behavior of the as-cast, solution treated T4, and peak-aged T6alloys are as follows (T=250-325℃,σ=50～140MPa):1) The alloy exhibited an extended tertiary creep stage, which was similar to Ni-Cr-base superalloys. Such creep characteristic was believed to be associated with the β’and β precipitate coarsening. 2) For300℃condition, at lower stresses (σ<50MPa), there was not a significant difference in the minimum creep rates among the T6, T4and as-cast alloys. At higher stresses (σ≥50MPa), the T6alloy exhibited lower minimum creep rates than the as-cast alloy, while the T4alloy exhibited the highest creep rates.3) The creep stress exponent values were4.4～6.0implying that dislocation process creep was the creep mechanism. The measured average activation energy (221～266kJ/mol) was significantly greater than that for lattice self-diffusion of Mg (135kJ/mol). This was considered to be a result of the activation of non-basal slip and cross-slip, and probably cross-slip was the rate-controlling mechanism. This was consistent with the slip traces analysis which confirmed that12～25%non-basal slip was active and the deformation observations which suggested that cross-slip became more active at higher temperatures.4) The activities of slip modes during tensile-creep for the T6alloy were investigated quantitatively by in-situ SEM, slip trace analysis, and EBSD. The results showed:at low temperature and high stress (T=250℃,σ=120MPa), the dominate slip modes were basal<a> slip (88%), and non-basal slip was active including prismatic<a> slip (9%) and pyramidal<c+a> slip (3%). The basal<a> slip was observed before non-basal slip during creep deformation, and the relative contribution of basal<a> slip decreased with increasing creep time; at high temperature and low stress (T=340℃, σ=75MPa), the dominate slip modes became basal<a> slip (75%) combined with pyramidal<c+a> slip (16%), and high amount of non-basal slip were found in the early stage of creep.The important grain-interior strengthening mechanisms were:the prismatic orientated, plate-shaped, and densely-distributed β’ and β precipitates were most effective obstacles for basal slip, while the long period stacking ordered (LPSO) phases can suppress the non-basal slip, dislocation climb and cross-slip. The grain-boundary X phase and eutectic Mg24(GdYZn)5with high hardness (92～112%higher than matrix), high volume fraction (16～24%), and high thermal stability can pin the grain boundaries and strengthen the boundaries effectively.The fracture mechanisms during tensile-creep for the T6alloy were investigated by in-situ SEM, and the results showed:1) At all the conditions tested (T=250～340℃,σ=50～120MPa), intergranular fracture was the dominant creep fracture mechanism for all the tested conditions. The onset of obvious cracks and creep cavities were from the middle-to late-creep stage (0.4～0.65tr), and they tended to nucleate at grain boundary which was perpendicular to the load direction and the interface between the α-Mg matrix and the large intermetallic grain boundary phase. 2) At low temperature and high stress (T=250℃,σ=120MPa), the crack nucleated and propagated by grain-boundary sliding, and edge of the crack was smooth.3) At high temperature and low stress (T=280～340℃,σ=50～75MPa), the coalescence and growth of isolated cavities and their linkage formed the microcrack, and then grain-boundary sliding played an important role in the propagation of the microcrack. Edge of this kind of crack was serrate.4) The mean cavity diameter D and creep rate followed the empirical relationship D=k·εa, and the growth rate of creep cavity was proportional to creep rate;5) these findings indicated that the growth of creep cavity might be consistent with the constrained diffusional cavity growth mechanism.At T=250～325℃and a=50-140MPa, the creep damage tolerance parameter λ ranged between1.2and2.5. The minimum creep rate and fracture followed the original Monkman-Grant relationship. The λ values and Monkman-Grant relationship indicated that creep cavity and intergranular crack played important role in creep fracture, and these were consistent with the in-situ observations.The present work can contribute to a better understanding for the elevated-temperature deformation, strengthening and fracture mechanisms in complex Mg alloy, and provide both theoretical and practical fundamentals for further research and development of high-performance creep-resistant Mg alloys.