Effect Of Precipitate Orientation Regulating On Mechanical Behavior And Strengthening-Toughening Mechanisms Of Mg-Sn/Mg-Al Alloys

Precipitates are exploited for strengthening in many Mg alloys.The different precipitate types usually lead to significantly different strengthening responses.For example,the plate-shaped or lath-shaped precipitates formed on basal plane(basal plates or basal laths)generally provide poor strengthening due to the weak impediment for basal slip.However,the plate-shaped precipitates formed on prismatic plane(prismatic plates)can hinder basal slip effectively and then cause a remarkably increase on the critical resolved shear strength(CRSS)for basal slip,thus improving strength remarkably.Since precipitates with different habit planes(precipitate orientation)and shapes will strengthen basal slip to different extents,it may be possible to enhance precipitation strengthening response by regulating precipitate orientation.Recently,a novel method coupling twinning,aging and detwinning(TAD)process was developed to change the orientation of Mg₁₇Al₁₂ precipitates from basal plates to prismatic plates,which resulted in an appreciable improvement in yield strength(YS)of AZ80 Mg alloy.Nevertheless,there is a lack of evidence that the TAD process is suitable for regulating the precipitate orientations in other series of Mg alloys,e.g.Mg-Sn alloys.In addition,the strengthening mechanisms of precipitate orientation regulating have not been investigated systematically and quantitatively.A better understanding of the relationship between precipitate orientation regulating and strength improvement is beneficial to develop novel high strength Mg alloys.To broaden the application of the TAD process and reveal the potential strengthening mechanisms,the TAD process was applied here to regulate the orientation of lath-shaped Mg₂Sn precipitates formed on basal plane(basal laths)of a Mg-5Sn alloy.As a result,in one case,most of the precipitate orientations were purposely transformed from basal laths to prismatic laths.In another case,a portion of the precipitate orientations were altered to prismatic laths while the other precipitate orientations were still basal laths,which formed a basal-prism combined precipitate distribution.Besides,the orientation of Mg₁₇Al₁₂ precipitates was also purposely transformed from basal plates to prismatic plates by a simplified method coupling aging and twinning(AT)process.Furthermore,the microstructure evolutions during TAD and AT process were investigated by scanning electron microscopy(SEM),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM).The mechanical properties of the various samples were evaluated by uniaxial compression and tension tests.For comparing the precipitation strengthening effect on the main slip systems of Mg alloys,the corresponding Orowan models were constructed based on the morphologies of basal laths and prismatic laths,and the strengthening mechanisms were further discussed.The main results are as follows:1)After TAD treatment,most of precipitates approximately lay on one of the{101(?)0}_?prismatic plane with a small rigid body rotation.These prismatic precipitates can obstruct the twin nucleation and growth simultaneously.The statistical results of activated slip systems indicated that the frequency of activated basal slip was decreased from 83%to 79%,while the frequency of activated prismatic slip was increased from 2%to 7%due to the formation of prismatic laths.It revealed that prismatic laths can effectively block basal slip gliding compared to basal laths.Such prismatic laths,however,were observed to be weak in blocking prismatic slip,which enhanced the activation of prismatic slip.The calculated results of Orowan models were validated by these experimental results,which also revealed that the blocking effect from prismatic laths on non-basal slips is relatively weak compared to that from basal laths.Moreover,the strength and toughness of the Mg-5Sn alloy containing prismatic laths were improved simultaneously.Specifically,comparing with the directly aged sample,the tensile yield strength(TYS)and compressive yield strength(CYS)of the sample treated by TAD process were increased by 80 and 70 MPa,respectively.The strength improvement is mainly attributed to precipitation strengthening(36 MPa),dislocation strengthening(33 MPa)and grain boundary strengthening(3 MPa).Besides,the compression to failure(CF)was increased from 8%to 14%,which resulted from the enhanced activation of prismatic slip.2)The calculated results of Orowan models also revealed that although the blocking effect from basal laths is weak on basal slip,it becomes strong on prismatic slip and 2^ndpyramidal<c+a>slip.To effectively hinder basal slip and non-basal slips and then achieve a higher strength,a basal-prism combined precipitate distribution was obtained in Mg-5Sn alloy by controlling the twinning fraction during TAD process.As a result,there were 3different precipitate orientations co-existed in a Mg-5Sn alloy,one is preciptates lay on the basal plane of parent grains,namely PO(20.4%),the other is precipitates lay on the prismatic plane of detwinned grains,namely PO(68.3%),and the third is precipitates lay on the basal plane of twinned grain,namely PO(11.3%).The basal-prism combined precipitates resulted in a CYS improvement of 129 MPa compared to the directly aged sample.The strengthening mechanisms revealed that precipitation strengthening contributed94 MPa,dislocation strengthening contributed 21 MPa and grain boundary strengthening contributed 25 MPa for the improved strength.Importantly,the precipitation strengthening effect arising from the basal-prism combined precipitates(94 MPa)was enhanced by 160%compared to that arising from the prismatic precipitates(36 MPa).3)The orientations of Mg₁₇Al₁₂ precipitates in AZ31 Mg alloy were transformed from basal planes to prismatic planes.As a consequence,the CYS and CF of the sample treated by AT process were increased by 40 MPa and 5%,respectively,compared to the directly aged sample,which overcomed the strength-toughness trade-off.The strengthening mechanisms revealed that precipitation strengthening contributed 15 MPa and grain boundary strengthening contributed 14 MPa for the improved strength.Besides,the residue dislocation also strengthened matrix to some extent.The formation of prismatic plates also led to a decreased CRSS ratio of prismatic slip to basal slip by preferentially hardening against basal slip compared to prismatic slip,which provided more opportunity to activate non-basal slips and further improved toughness.