分享:镁合金LPSO/SFs结构间{ }孪晶交汇机制的原子尺度研究
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以含长周期堆垛有序(LPSO)结构的Mg-Zn-Y(-Zr)合金为研究对象,运用透射电子显微方法,从原子尺度解析LPSO结构/富含溶质元素堆垛层错(SFs)对{
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随着可持续发展和绿色环保对轻质材料需求的不断增加,高强度镁合金的研究受到广泛关注,是21世纪最具发展潜力的绿色工程材料。然而,其强塑性低和耐蚀性差严重限制了镁合金的工业应用。沉淀强化是提高镁合金力学性能的最有效方法之一[1]。长周期有序堆垛(LPSO)结构具有良好的热稳定性[2,3],可以有效提升Mg-TM-RE (TM = Zn、Cu、Ni、Co,过渡族元素;RE = Y、Dy、Ho、Gd、Tb、Er、Tm,稀土元素)系合金的室温和高温性能[4]。LPSO结构由AB'C'A堆垛单元和不同层数的Mg原子层有序堆垛组成[5~7],不仅结构有序而且化学有序,其中B'和C'富含溶质元素TM和RE。随过渡元素、稀土元素及制备工艺的不同,LPSO结构有较多类型[8~12],其中以18R和14H最为主要。在变形过程中由于异号基面滑移的协同运动,LPSO结构形成扭折,是其强韧化镁合金的主要原因之一[13,14]。扭折形成伴随溶质元素的再分布,可有效稳定扭折界面,从而提升其对性能的贡献[15~17]。而且,晶粒内局部区域存在扭折和孪晶共生,协同提升其塑形[18]。
由于六方结构Mg及镁合金独立滑移系的数量有限,孪生在其塑性变形中起到重要作用。{
1 实验方法
以高纯Mg (99.99%)、Zn (99.99%)、Mg-25Y (质量分数,%)和Mg-30Zr (质量分数,%)中间合金为原料,通过高频感应炉在Ar气保护下制备Mg97ZnlY2 和Mg96.6Zn1Y2.2Zr0.2 (原子分数,%)镁合金。铸锭经过500℃、12 h的均匀化处理后,线切割制备4 mm × 4 mm × 8 mm的块状试样。压缩变形实验在Gleeble 1500热/力模拟试验机上进行,压缩方向平行于长轴方向,温度为室温,变形速率1 × 10-3 s-1,压缩应变分别为24.1%和36.2%。实验后立即将试样放入水中冷却至室温以保存变形微观结构。透射电镜(TEM)样品主要采用离子减薄制备。用低速锯取下特征位置样品,机械研磨至40 μm,利用Gatan微凹仪挖坑,最后用Gatan 691离子薄化器减薄。为了减少氩离子轰击时对样品的损伤,减薄初始采用4.5 kV、6°入射角,然后逐步降低电压并减小入射角,最后调整至3 kV、3°入射,在结束前降低电压至1.2 kV、3°低能量清扫表面。高倍高角环形暗场像(HAADF)在配备了双球差矫正器的Titan3TM G2 60-300型TEM上获取,操作电压300 kV。拍摄原子尺度扫描透射电子显微镜(STEM)图像时,电子束汇聚角约为25 mrad,束斑直径在0.1 nm以下,所对应分辨率优于0.08 nm。其中TEM像拍摄时电子束平行<
2 实验结果与讨论
2.1 镁合金中{ }孪晶交汇
hcp结构Mg及镁合金含有6个{
图1

图1 hcp结构Mg及镁合金中{
Fig.1 Three crystallographically types of twin-twin interactions formed from six {
(a) co-zone twins: T1-T4 twin-twin pair with the intersection line along <
(b) non co-zone twins: T1-T2 twin-twin interaction with the intersection along <
(c) non co-zone twins: T1-T3 twin-twin interaction with the intersection line along <
(d) configuration of {
(e) {
2.2 LPSO/SFs结构间{10 2}孪晶交汇
图2

图2 含LPSO结构Mg-Zn-Y镁合金室温压缩前后的微观结构
Fig.2 Microstructures of Mg-Zn-Y alloy with LPSO structures before (a, b) and after (c) compression at room temperature
(a) low-magnification HAADF-STEM image showing the basal planes of LPSO structures and SFs enriched with Zn and Y atoms are parallel to the basal plane of the matrix in the Mg-Zn-Y alloy
(b) atomic-scale HAADF-STEM image showing AB'C'A building blocks (red lines) in the LPSO
(c) the multiple twins were triggered during compression, and they intersected with the SFs
图3a为SFs间2组{
图3

图3 LPSO/SFs间{
Fig.3 BB and PP interfaecs introduced by the co-zone {
(a) low-magnification HAADF-STEM image of the wavy {
(b) atomic-resolution HAADF-STEM image showing the TB deflected from the basal plane of ~4° (Inserted inverse fast Fourier transform (IFFT) image shows a periodic array of dislocations in the twin boundary, which was processed by masking (0002) reflection of the matrix and {
(c) TB framed by cyan rectangle in Fig.3b deflecting from the {
(d) the corresponding IFFT image of Fig.3c processed by masking (0002) of the matrix and {
(e) high-magnification HAADF-STEM image showing the impingement of multiple twins, leaving a triangular matrix, denoted by M in Fig.3a
(f) twin boundaries and BB boundary delineated by dashed lines, and the corresponding FFT images of TB1, TB2, and BB boundary
图3e为宽间距SFs内部的高倍HAADF-STEM像,左右两侧分别为2个钝角三角形区域。图3f为图3e不同界面TB1、TB2和孪晶I和II界面对应的FFT图,可见其中包含3个孪晶以及BB界面。表明孪晶I和II与基体为{
图4为LPSO/SFs间{
图4

图4 LPSO/TSFs间{
Fig.4 TEM images of BB interface and {
(a) high-magnification HAADF-STEM image showing the twin-induced BB boundary within the LPSO structures, where the TB is delineated by a white dashed line
(b) enlarged image of cyan rectangle framed area showing that the BB coexists with {
(c) enlarged image of yellow rectangle framed area indicating that the BB (~7°) is connected with TB, and the FFT image is inserted
(d) a set of dislocations are displayed at the BB boundary generated by masking (0002) reflections (shown by the cyan dashed circles) of the two crystals
(e) geometric phase analysis (GPA) further confirming the position of dislocation cores (The colour bar indicates change in strain intensity from -0.25 (compressive) to 0.25 (tensile))
图5进一步显示了间距约为20 nm的LPSO结构与TSFs间形成的BB界面(约11°)。图5b和c分别为5a中b和c区域对应的原子分辨率HAADF-STEM像,表明其中BB界面是由间距为1.5 nm的周期性位错组成。图5b表明BB左右两侧区域的基面与基体基面呈{
图5

图5 LPSO与TSFs间Mg片层内{
Fig.5 Microstructure of BB interfaces formed in the Mg layers sandwiched between LPSO and TSFs with a spacing of ~20? nm
(a) high-magnification HAADF-STEM image of BB boundary between LPSO and TSFs, shown by the yellow arrows (b, c) atomic-resolution HAADF-STEM image of the BB boundary of ~11° denoted by b and c in Fig.5a. The array of dislocations processed by masking (0002) reflections of the left and right twin (cyan-color dashed-line circles in the inset FFT pattern) are imposed on the BB boundaries. The original triangular LPSO remains at the intersection between the intersection of left and right twins, where the AB'C'A building blocks are denoted by the red lines in Fig.5b, and very small local SFs with matrix orientation, which was framed by the dashed rectangle in Fig.5c (Inset shows the FFT pattern of the left and right twin. The shearing of LPSO caused by the twin intersection was indicated by the red arrows in Figs.5a and b)
当受到间距为70 nm的LPSO结构限制时,{
图6

图6 LPSO结构间{
Fig.6 High-magnification HAADF-STEM image of a region containing {
在LPSO/SFs间距为20~100 nm时(图3~6),{
图7

图7 LPSO间{
Fig.7 Schematics of the nucleation (a), propagation of {
2.3 LPSO结构扭折与{ }孪晶交汇的相互作用
图8

图8 LPSO/SFs内扭折界面(KB)与{
Fig.8 Low-magnification HAADF-STEM images of a region containing twin and kink boundary (KB) between the high-density SFs or LPSO structure
(a) the matrix with KB was surrounded by the twins
(b) KB connected with TB, which was also connected with a triangle matrix (denoted by red arrows)
(c) zoom-in image of the area “M” shows the triangle matrix 3 relates to a BB boundary
(d) atomic-resolution HAADF-STEM image of the area d in Fig.8c indicating that the triple intersection consists of BP, KB, and PB
(e) magnification image of area e in Fig.8b suggesting the formation of dislocations with c Burgers vectors in the LPSO structure (red ⊥) due to the twin shear
图8b~d表明LPSO相产生小角度扭折与Mg片层内形成{
当LPSO结构间距减小至30 nm时,其KB与Mg片层内TB相连,孪晶在单侧扭折带内形核长大,如图9所示。图9a显示LPSO结构内35° KB与其间约20 nm Mg片层中TB相连。微小不规则孪晶由BP、PB、左侧孪晶界面(LTB)和右侧孪晶界面(RTB)组成,其与KB左右两侧区域对应的FFT (图9a内插图LTB和RTB)表明它们分别为{
图9

图9 LPSO结构扭折促进{
Fig.9 Atomic-resolution HAADF-STEM image of TB connecting with KB between LPSO structures, where the twinning just nucleated in the left side of the kink (a), and nearly occupied the left side of the kink (b) (LTB—left twin boundary, RTB—right twin boundary, CTB—coherent twin boundary)
3 结论
(1) LPSO/SFs抑制{
(2) LPSO/SFs/TSFs层间{
(3) LPSO层间孪晶交汇与变形扭折可共存。扭折界面处位错促进{