| Due to the strong demand on energy-saving and emission-reduction,weight-reduction becomes an urgent topic in automotive industry.As a result,magnesium（Mg）alloys show great advantages as they are with high specific strength and low density.However,conventional Mg alloys exhibit low strength and creep resistance at elevated temperatures（>120oC）.High-performance Mg-RE alloys have shown potential for applications in powertrain components such as pistons,where the materials need to work at higher temperatures（250300oC）.Considering the fact that these components are always under complicated loads in service,including tension,compression and localized load,to study the elevated-temperature tension,compression and impression creep behavior and deformation mechanisms will benefit Mg-RE alloys development and application.This work is focusing on the high-performance Mg-10Gd-3Y-0.5Zr（wt.%,GW103）alloy.Tension and compression creep behavior at 200300oC（0.510.62Tm,Tm is the melting point in absolute temperature）and 30150MPa were investigated for the both the cast and the extruded GW103 alloy.Impression creep behavior at 200325oC（0.510.65Tm）and 80505 MPa for the cast and the extruded GW103 alloy was investigated.The following points:creep curve features,effects of extrusion and T5treatment on the creep behavior,creep exponent and activation energy measurements,creep mechanisms,microstructural evolution during creep process,cracking and fracture modes were focused.At the same time,using in-situ scanning electron microscopy（SEM）and slip trace analysis methodology,the deformation modes and slip activity during tension and tensile creep for the cast and extruded GW103 alloy were quantitatively investigated.Slip interactions both within grains and at grain boundaries were found and discussed.For the extruded alloy,samples with various orientationas were used to investigate the anisotropic hehavior.The main achivements are listed as follows:（1）The initial microstructures of the peak-aged cast（cast-T6）,as extruded（as-ex）and peak-aged extruded（ex-T5）GW103 alloy consisted of equiaxed?-Mg matrix and some secondary phase particles,which were distributed both at the boundaries and inside the grains.The cast-T6 microstructure was not textured.The ex-T5 microstructure exhibited a typical basal fiber texture.（2）The results of tension creep behavior of the cast-T6,as-ex and ex-T5 GW103alloy at 200300oC/30150MPa showed that the tension-creep curves exhibited three stages:primary creep stage,steady-state creep stage and tertiary creep stage.The tertiary creep stage dominated the creep process.With increasing temperature and stress,the creep resistance decreased.The cast-T6 alloy exhibited excellent tension creep resistance,i.e.,the minimum creep rate varied from 2.30×10-8 s-1 to 2.30×10-8 s-1 at250oC/30150MPa.The creep resistance of the as-ex and ex-T5 alloy was inferior,and the minimum creep rates were 12 times magnitude greater than that for the cast-T6 alloy at the same T and stress.T5 heat treatment slightly strengthened the creep resistance of the as-ex alloy,but the differences were not significant.For the cast-T6 alloy,the stress exponents（n）at 200,250 and 300oC were 5.8,5.2 and 4.8,respectively,with an activation energy（Q）of 259.4kJ/mol.Dislocation climb was the main secondary-stage creep mechanism.For the ex-T5 alloy,the n values varied from 2.4 to 3.4,with Q values of 182.5±1.3kJ/mol.Dislocation viscous glide was the main creep mechanism.（3）During tension creep of the cast-T6,as-ex and ex-T5 alloy,micro-cracks mainly formed at the grain boundaries roughly perpendicular to the loading direction.At the same time,the cracks mainly initiated at the boundaries where the neighboring grain did not exhibit slip traces.With increasing T and stress,the fracture mode varied from intergranularly brittle fracture to intergranularly ductile fracture.For the as-ex and ex-T5alloy,the grain size increased slightly during creep.Precipitate free zone（PFZ）formed at the grain boundaries roughly perpendicular to the loading direction.Solute-poor theory could be appropriate to interpret the PFZ formation.（4）The results of compression creep behavior of the cast-T6,as-ex and ex-T5GW103 alloy at 200300oC/50120MPa showed that the compression-creep curves mainly exhibited two stages:primary creep stage and steady-state creep stage.The steady-state creep stage dominated the creep process.The minimum creep rates of the as-ex and ex-T5 alloy were one time magnitude greater than the cast-T6 alloy at the same T and stress.For the cast-T6 alloy,dislocation sliding was the main creep mechanism.For the ex-T5 alloy,dislocation sliding was the main creep mechanism in the high-temperature regime（250300oC）,while grain boundary sliding（GBS）may have contributed more in the low-temperature regime（200250oC）.During compression creep of the ex-T5 alloy,PFZ formed at the grain boundaries roughly parallel to the loading direction.Solute-poor theory could be appropriate to interpret the PFZ formation.（5）The results of impression creep behavior of the cast-T6 and ex-T5 GW103 alloy at 200325oC/80505MPa showed that the creep curves mainly exhibited two stages:primary creep stage and steady-state creep stage.The steady-state creep stage dominated the impression creep process.For the cast-T6 alloy,the n values at low stress（<270MPa）varied from 1.42.2,with Q value of86.8kJ/mol.GBS was the main creep mechanism.The n values at high stress（>270MPa）varied from 3.45.1,with Q values of175.5190.7kJ/mol.Dislocation-climb-controlled creep dominated.For the ex-T5 alloy,n and Q values were lower in the low-T and low-stress regime,GBS might have contributed a lot.At 250300oC,the n value was3.4 with Q values of 140.4165.0kJ/mol.Dislocation sliding was the main creep mechanism.By using conversion factors,the punching stress and impression velocity for the impression creep tests could be correlated to the uniaxial tension or compression creep tests.The result showed that the reference stress and creep rate after conversion,were in good agreement with the data in the tension and compression creep.The used stress conversion factor varied from0.260.35.The strain conversion factor was 0.755.（6）The microstructure evolution of the cast-T6 and ex-T5 alloy after impression creep was investigated.The results showed that the microstructure differed a lot for different areas,which could be divided into three parts:one just beneath（<2mm）the indenter and one far away（>5mm）from the indenter,where the microstructure remained almost unchanged,and one between the other two areas,where severe deformation occurred.Intergranular cracking was exhibited in the area at the edges of the indenter for both the cast-T6 and ex-T5 alloy.For the cast-T6 alloy,extension twinning occurred in the severely-deformed area.No twinning was observed for the ex-T5 alloy.（7）Deformation modes and slip activity in tension and tension creep of the cast-T6and ex-T5 alloy at 200325oC were investigated using in-situ observations.Dislocation sliding was the main deformation mode.For the tests at T≥300oC,besides dislocation slip,GBS contributed to the deformation significantly.No twinning occurred.（8）For the tension and tension-creep tests of the cast-T6 alloy,the results showed that basal slip dominated the slip observations,while the contribution of non-basal slip depended on the temperature.For the tension tests,non-basal slip accounted for 3435%of the total active slip systems at lower T（200 and 250oC）.However,at 300 and 325oC,non-basal slip contribution decreased,accounting for 12%and 7%,respectively.For the tension-creep tests,non-basal slip accounted for31%at lower T,while its contribution decreased to 1016%at higher T.The relative activity of pyramidal slip was enhanced compared to prismatic slip at higher T.（9）For the tension and tension-creep test of the ex-T5 alloy,the results showed that non-basal slip contribution decreased with increasing temperature.For example for the ED samples,non-basal slip accounted for 21%,12%and 5%in tension tests at 200,250and 300oC,respectively.And the non-basal slip contribution for the tension-creep tests was 20%,11%and 9%at 200,250 and 300oC,respectively.The ex-T5 GW103 alloy exhibited anisotropic behavior.The ED samples exhibited the greatest strength and creep resistance,while the TD samples had the lowest.The non-basal slip contribution for the ED samples was greater than that for the TD samples when deformed at the same T.The differences decreased with increasing T.The measured r-value varied from 0.821.07for different temperatures,and the average r-values for the three kinds ex-T5 samples were very close to 1.This suggests that the anisotropic behavior was not significant,due to initial weak texture of the ex-T5 GW103 alloy.（10）The critical resolved shear stress（CRSS）ratios of prismatic/basal slip for the cast-T6 and ex-T5 alloys were measured.The results showed that at lower T,the CRSS values were low（35）.However,the CRSS value abnormally increased when T increased,which might be attributed to the significant GBS contribution at higher T.For the tests at the same T,the CRSS values of the ex-T5 alloy were greater than that for the cast-T6 alloy,which suggested that the relative activity of non-basal slip in the ex-T5alloys was lower.（11）Multiple-slip and slip transfer frequently occurred in the tension and tension-creep tests for both the cast-T6 and ex-T5 alloy.There were more multiple-slip and slip transfer observations at lower temperatures（200 and 250oC）.Basal-prismatic types dominated the multiple-slip observations and the involved slip systems were associated with different<a>directions.Multiple-slip with different<a>directions was regarded as an indirect strengthening mechanism for the studied alloy.Basal to basal slip transfer type dominated,while basal to prismatic and prismatic to prismatic slip transfer types were also observed.The involved slip systems of the observed slip transfer pairs were always associated with the same<a>direction.Slip transfer occurred more easily at low angle grain boundaries（LABs）and grain boundaries with angles higher than 75o.And cracks were less likely to initiate at grain boundaries where slip transfer occurred. |
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