Study On The Microstructure,Mechanical Property And Strengthening Mechanism Of High Pressure Die Casting Mg-4Al-5RE-xGd(wt.%) Alloys

Due to the global energy shortage and greenhouse effect,weight reduction becomes an important topic in automotive industry.In this regard,high pressure die cast（HPDC）Mg alloy has become one of the most promising alloys because of its low density and high specific strength.However,compared with steel and aluminum alloy,HPDC Mg alloy has low strength and poor toughness,which limits its application.Due to excellent yield strength,HPDC Mg-4Al-5RE（wt.%）alloy has been received extensive attention,but excessive acicular Al₁₁RE₃ phase deteriorates its ductility.It was reported that Al₂Gd phase formed by adding Gd to Mg-Al casting alloy can significantly refine the grain and homogenize the second phase,thus improving the strength and toughness.Besides,there is an orientation relationship between Al₂Gd and Al₁₁RE₃,and Al₂Gd has a higher melting point.Therefore,Gd addition is expected to refine and modify the microstructure and improve the tensile and impact properties of HPDC Mg-4Al-5RE alloy.In addition,during HPDC process,the melt contacting the die wall is quenched thus forming a fine casting surface with a coarse casting interior,resulting in the heterostructure.It was reported that heterostructure will produce extra strengthening during deformation.However,it is unclear yet whether HPDC heterostructure will produce such strengthening effect during deformation.This work investigated the effect of 0.2 wt.%and 0.7 wt.%Gd addition on the microstructure of HPDC Mg-4Al-5RE alloy using optical microscopy（OM）,scanning electron microscopy（SEM）,electron backscattered diffraction（EBSD）and transmission electron microscope（TEM）.The mechanisms of grain refinement and second phase modification by Gd addition were discussed.At the same time,different casting methods such as sand casting（SC）and permanent mold casting（PMC）were applied to produce the sample in order to figure out the effect of cooling rate on the efficiency of microstructure refinement and modification by Gd addition.Tensile and Charpy impact tests were carried out to evaluate the mechanical properties,and the strengthening mechanisms were established by correlating the measured mechanical properties to the microstructure.The heterostructure of the HPDC Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloys was investigated from the casting surface to the interior.By using in-situ SEM and slip trace analysis methodology,various deformation modes and slip/twinning activities during ambient tension were quantitatively investigated layer-by-layer.The incompatible mechanical behaviors arising from the heterostructure among the HPDC layers and the resultant extra strengthening were investigated.In the last,a physical model was established to predict the contribution of each layer to the extra strengthening.The main achievements are listed as follows:（1）The microstructures of HPDC,PMC and SC Mg-4Al-5RE-x Gd alloys showed that the Al₂（Gd,RE）particle formed by Gd addition exhibited good orientation relationships（ORs）withα-Mg and Al₁₁RE₃,thus refining grains and modifying the acicular Al₁₁RE₃ phase to short-rod like.Besides,more Al₂（Gd,RE）were formed by adding 0.7 wt.%Gd thus promoting the grain refinement and second phase modification effect.Moreover,from HPDC to PMC to SC processes,with the decreased cooling rate,the constitutional undercooling increased,resulting in more significant grain refinement effect of the Al₂（Gd,RE）particle.（2）The tensile property of HPDC,PMC and SC Mg-4Al-5RE-x Gd alloys showed that the 0.2%proof strength（YS）and ultimate tensile strength（UTS）of HPDC Mg-4Al-5RE alloy decreased after Gd addition,which were ascribed to the lower amounts of second phase and Hall-Petch strengthening due to the low area fraction of second phases and the unrefined grains,respectively.While the acicular Al₁₁RE₃ phase was modified to short-rod like,which was less prone to cracking,so they can retard the crack initiation and propagation,resulting in an improved elongation（EL）.For the PMC and SC Mg-4Al-5RE alloys,Gd addition could refine the grains thus increase YS and UTS related to Hall-Petch strengthening.Also,EL was improved due to the modified second phase,（3）The Charpy impact property of HPDC,PMC and SC Mg-4Al-5RE-x Gd alloys showed that the impact displacement of HPDC Mg-4Al-5RE alloy increased monotonously with the increase of Gd content,resulting in the enhanced impact energy.For the PMC and SC Mg-4Al-5RE alloys,Gd addition also improved the impact energy.However,compared to the PMC and SC alloys,the HPDC alloy with fine microstructure showed the worse impact displacement.This abnormal phenomenon was due to that profuse parallel{10（?）2}twins with two V-shaped variants were observed in the PMC and SC Mg-4Al-5RE alloys,providing a considerable deformation;while a few{10（?）2}twins and many cracks at grain boundary were observed in the HPDC Mg-4Al-5RE alloy,exhibiting a limit capacity of deformation.（4）The impact load-displacement curves of HPDC,PMC and SC Mg-4Al-5RE-x Gd alloys were derived to impact stress-strain curves.The impact stress was greater than the tensile stress,exhibiting the strain rate sensitivity（m’）.For the HPDC alloys,twins exhibiting low m’mainly participated in plastic deformation before yielding then the slips with high m’took over with the increase of deformation,resulting in the first decreased,and then increased m’.However,twins may still participate in the plastic deformation when the PMC and SC alloys were close to fracture,which led to the monotonically decreased m’.（5）The microstructure in thickness direction of HPDC Mg-4Al-5RE-x Gd alloys showed that from the casting surface to the interior,the grain and second phase of the HPDC Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloys gradually got coarse,and the area fraction of second phase gradually decreased.Therefore,HPDC Mg-4Al-5RE-x Gd alloys had an intrinsic heterostructure.Besides,it was found that Gd addition mitigated the heterogeneous degree of the heterostructure in the thickness direction.（6）The results of in-situ tensile analysis on the deformation behavior of the HPDC Mg-4Al-5RE alloy in each layer（surface layer,middle layer,and central layer）showed that basal<a>slip and{10（?）2}tensile twinning were mainly activated in all layers.However,the surface layer with fine grains was prone to activate slip deformation.Besides,due to the inhibited twinning activity,more basal<a>slip systems with low Schmid factor m（<0.3）and prismatic<a>and pyramid<c+a>slip systems with high m（>0.3）were instead activated.Conversely,the central layer with coarse grains was liable to activate twinning.The frequency and area fraction of grains activating slip and twinning for the central layer were greater than those in the surface layer,suggesting a readily plastic deformation.Since Gd addition mitigated the heterogeneous degree of the heterostructure,the difference of deformation behavior（such as slip and twinning behaviors）among the layers was decreased.After Gd addition,twinning was liable to be activated in each layer.Meanwhile,the percentage of non-basal and low m basal<a>slips was decreased.Since profuse twins participating in the plastic deformation,the deformation requirements can still be satisfied even though basal<a>slip with high m was simply activated.Moreover,Gd addition increased the frequency and area fraction of grains activating slip and twinning,which can be attributed to the reduction of the second phase and its hindrance to slip and twin boundary migration.（7）The mechanical property in each layer of HPDC Mg-4Al-5RE-x Gd alloys showed that the YS,UTS and EL of HPDC Mg-4Al-5RE and Mg-4Al-5RE-0.7Gd alloy decreased monotonously from the surface layer to the central layer.The flow strength of the HPDC bulk was predicted by averaging the flow strength of all layers using the rule of mixture.Interestingly,both two HPDC alloys exhibited the extra strengthening△σ,in which the experimental flow strength was higher than the predicted one.The mechanism of△σwas investigated by digital image correlation and EBSD.The results showed that when the heterostructure underwent deformation,incompatible mechanical behaviors such as different stress and strain behaviors were produced among the layers.In order to maintain the continuity of deformation,a strain gradient was produced,thus promoting the generation and accumulation of geometrically necessary dislocations（GNDs）and resulting in the△σ.Therefore,Gd addition resulted in the less heterogeneous structure and mitigated mechanical incompatibility among the layers,thus leading to the reduction of GNDs and the decreased△σ.（8）A model was developed at the elastoplastic stage for each layer based on the stress state analysis,which provided a quantitative insight focused on theΔσ.The numerical results showed that the interaction between the surface layer and middle layer led to a definiteΔσclose to the bulk sample,while the middle layer and central layer had a marginalΔσfor both alloys.These indicated that the interaction between casting surface and middle layer dominated theΔσof the bulk sample,due to the significant heterogeneous microstructure between them,which led to the significant mechanical incompatibility.