传统基面织构镁合金板材六方密排的晶体结构，使其在室温下只能激活有限的滑移系，降低了室温强度和成形性能，不利于其在工业领域的应用[1~3]。研究表明，使镁合金板材织构非基面化可提高其成形性能。Tu等[4]进行等径角轧制-连续弯曲-退火(Equal channel angular rolling and continuous bending process with subsequent annealing，ECAR-CB-A)制备的双峰分离非基面织构AZ31镁合金板材具有良好的室温成形性能，其室温杯突值高达7.4 mm。但是，Chen等[5]的研究结果表明，这种双峰分离非基面织构的引入会产生织构软化效应，即AZ31镁合金板材的初始屈服强度仅为73 MPa，比基面织构板材的初始屈服强度162 MPa显著降低约54.94%，严重影响其商业化应用。
目前，作为常规热处理的补充，深冷处理能使镁合金兼具较高的强度和良好的延伸率[6~8]。Mónica等[6]将AZ91镁合金在时效前分别进行T6处理和T6处理+深冷处理，发现深冷处理的样品在时效过程中析出了第二相Mg17Al12。不同样品的拉伸实验结果表明，深冷处理使AZ91镁合金具有相近抗拉强度且使其材料延伸率提高。Qi等[7]将GW83K镁合金分别在室温、-70℃和-196℃进行拉伸实验，结果表明：随着温度的降低强度逐渐提高；而从室温到-70℃延伸率提高，从-70℃到-196℃降低。对其微观组织的观察发现，随着温度的降低{10
1 实验方法
用电火花沿该板材的轧制方向(Rolling direction，RD)切取“狗骨头”状拉伸试样，其标称尺寸为22.5 mm，矩形截面的宽度和厚度分别为6 mm和1.2 mm。
用SANS试验机进行CT样品的室温单轴拉伸实验，拉伸速率为0.001 s-1，重复3次。为了研究单轴拉伸过程中样品微观组织的演化，分别将CT样品单轴拉伸至变形量6%和12%。在6%和12%变形样品标称段的中间位置取样，以观察在相应变形条件下的微观组织。用FEI NOVA 400 Zeiss sigma场发射扫描电子显微镜和FEI Talos F200X场发射透射电子显微镜观察深冷处理后双峰分离非基面织构AZ31镁合金板材单轴拉伸变形前后的微观组织。用文献[14]中的方法制备EBSD样品，变形前后扫描步长分别为0.5 μm和0.3 μm，加速电压为20 keV。TEM样品的制备：先用SiC砂纸将样品磨抛至厚度约为100 μm的薄片，在磨抛过程控制样品的平整度；随后用冲孔仪冲出一个直径为3 mm的圆片，将圆片继续磨抛使其厚度约为30 μm，然后用Gatan 495离子减薄仪将其减薄。
2 实验结果
2.1 样品的初始微观组织和织构
图1给出了CT样品的初始微观组织和织构，图1a给出了该样品的反极图(Inverse pole figure，IPF)。可以看出，CT样品的微观组织由等轴晶粒组成。统计结果表明，CT样品的平均晶粒尺寸为19.10 μm(图1b)，与Chen等[5]用ECAR-CB-A工艺制备得的WQ样品的平均晶粒尺寸(14.91 μm)只有微小差别。本文制备的CT样品在520℃退火5 h后进行深冷处理，而WQ样品在300℃退火1 h后水冷。这表明，退火工艺的不同是其微观组织不同的主要原因。图1c给出了CT样品的(0002)、(11
图1
图1
AZ31镁合金板材的初始微观组织和织构
Fig.1
Initial microstructure and texture of fabricated AZ31 Mg alloy sheet (a) inverse pole figure (IPF) (b) statistical analysis of grain size (c) (0002) (11
2.2 单轴拉伸力学行为
图2给出了CT样品室温单轴拉伸实验得到的真应力-真应变曲线和相应的加工硬化曲线。表1列出了两种样品的初始屈服强度(Yield stress，YS)、最大抗拉强度(Ultimate tensile strength，UTS)、断裂延伸率(Fracture elongation，FE)和屈强比(YS/UTS)。由表1可以看出，与对照组WQ样品相比，CT样品的YS和UTS分别提高了43.8%和5.2%，而FE降低了20.4%。由图2b可以看出，CT样品的加工硬化曲线出现3个明显不同的加工硬化阶段[5]，即在塑性变形初始阶段随着真应变的增大应变硬化率急剧降低，对应弹塑性过渡阶段。继续增大真应变，CT样品的应变硬化率比WQ样品的应变硬化率下降得更快。这表明，CT样品和WQ样品在室温单轴拉伸变形过程中激活的位错滑移和变形孪生机制以及两者间的相互作用不同[15,16]。
图2
图2
CT样品和WQ样品的真应力-应变曲线和相应的加工硬化曲线
Fig.2
True stress-strain curves (a) and working hardening curves (b) of CT and WQ samples
表1 CT样品和WQ样品单轴拉伸时的力学性能
Table 1
| Sample | YS / MPa | UTS / MPa | FE / % | YS / UTS |
|---|---|---|---|---|
| CT-sample | 105 | 284 | 19.1 | 0.37 |
| WQ-sample[5] | 73 | 270 | 24 | 0.27 |
2.3 在单轴拉伸过程中微观组织和织构的演化
图3给出了CT样品在变形量为6%和12%时的EBSD微观组织。在IPF图(图3a，d)和晶界(Grain boundaries，GBs)图(图3b，e)中，分别用红色、绿色和蓝色表示{10
图3
图3
CT样品的变形量为6%和12%时的微观组织
Fig.3
Microstructure analysis of CT sample at deformation degree of 6% and 12% (a, d) Inverse pole figure (IPF) maps; (b, e) Grain boundaries (GBs) maps; (c, f) Kernel average misorientation (KAM) maps
图4
图4
单轴拉伸过程中CT样品取向差角的分布
Fig.4
Misorientation angle distribution of CT sample during tensile deformation
分析GB图可知，变形程度为6%和12%时的孪晶类型大部分为{10
其中μ为单位(100 nm)长度，b为Burgers矢量的大小。变形量为6%和12%样品的KAM值分别为0.40°和0.65°。这表明，变形12%的样品中GND密度更高。同时，晶界和孪晶界表现出更高的KAM值。这表明，上述位置在塑性变形过程中协调塑性应变的需求更为迫切[17]。
为了研究{10
图5
图5
CT样品的变形量为6%和12%时选取{10
Fig.5
IPF maps and (0002) PF maps of selected {10
图6给出了CT样品和WQ样品的TEM照片，分为明场(Bright field，BF)像和选区电子衍射(Selected area electron diffraction pattern，SAEDP)像。图6a与g的对比表明，在CT样品中出现了较多的位错，与文献[19]的结果一致。深冷处理使金属材料中的位错密度提高，主要原因是低温使材料的晶格收缩和微应力场增大，使材料发生塑性变形和晶格畸变，最终使位错数量增多[20]。从图6b、h可见，在CT样品和WQ样品中都存在弥散分布的球形、椭球形和棒状纳米析出相。图6d、f、j、l给出了相应析出相的SAEDP图，表明析出相为Mg17Al12 (图6d、j)和Al8Mn5 (图6f、l)，与文献[8]的结果一致，即棒状析出物为Al8Mn5相。特别在WQ样品中析出相的形貌大多为球形和椭球型，最大尺寸为77.38 nm，最小尺寸为9.67 nm，平均尺寸为23.15 nm，体积分数为1.45%；CT样品中析出相的形貌大多为椭球形和棒状，最大尺寸为124.43 nm，最小尺寸为15.94 nm，平均尺寸为41.38 nm，体积分数为2.40%。深冷处理使材料的晶格收缩，加速了第二相的析出[21,22]。同时，深冷处理后出现了较多的位错，有利于附近区域溶质原子的富集和析出相的成核[23]。
图6
图7给出了CT样品拉伸变形6%的TEM照片，分为明场(BF)像和高分辨(High resolution TEM，HRTEM)像。可以看出，在变形样品中出现了明显的孪晶界面(图7a)；同时，在孪晶和孪晶附近的基体中出现了高密度位错，在基体和孪晶中还可见部分直线位错。Gong等[24]观察到与此类似的结果，并推论这些位错可能是基面位错。图7b给出了基体/孪晶界面处的高分辨照片，可见基体与孪晶的取向差角为86.3°。根据基体/孪晶界面处对应的SAEDP图，可进一步确定基体与孪晶之间的取向关系，即基体与孪晶有公共平面{10
图7
图7
CT样品变形量为6%时的TEM照片
Fig.7
TEM micrographs of 6%-deformed CT sample (a, c, d) BF images of different regions (b) HRTEM image of selected region in Fig.7a
3 讨论
根据上述分析，给出了CT样品和WQ样品室温单轴拉伸变形的变形机制(图8)。在总体上，CT样品和WQ样品的塑性变形机制类似，即在塑性变形的初期，双峰分离非基面织构使AZ31镁合金板材在室温单轴拉伸变形中不仅激活了大量的位错滑移(包含基面<a>位错和非基面<a>位错)，还激活了相当数量的{10
图8
图8
在单轴拉伸过程中WQ样品和CT样品的变形机制示意图
Fig.8
Schematic diagram of involved deformation mechanisms during uniaxial tension deformation of WQ (a) and CT (b) samples
与WQ样品相比，CT样品的YS提高43.8%，UTS提高5.2%，主要原因是深冷处理使晶格收缩而产生了高密度位错(图6g)。同时，CT样品的微观组织中尺寸更大、体积分数更高的析出相，在拉伸变形过程中钉轧位错[30]。在变形过程中位错产生应变强化，析出相产生沉淀强化，两者的共同作用使CT样品的强度显著提高。与WQ样品相比，CT样品的FE降低了20.4%。CT样品的拉伸变形激活了更多的{10
4 结论
(1) AZ31镁合金板材的CT样品和WQ样品具有类似的初始晶粒尺寸，都具有双峰分离非基面织构特征。与水冷处理相比，深冷处理不改变AZ31镁合金板材析出相的类型，但是使板材样品析出相的尺寸增大，析出相体积分数和位错密度提高。
(2) WQ样品的YS为73 MPa，UTS为270 MPa，FE为24%，经过深冷处理的CT样品的YS为105 MPa，UTS为284 MPa，FE为19.1%，分别提高43.8%，5.2%和降低20.4%。
(3) AZ31镁合金板材的CT样品和WQ样品的变形机制类似，但是深冷处理在晶界和晶粒内富集了大量位错和析出相，随着塑性应变的增大激活了大量{10
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[J].为了改善传统热轧AZ31镁合金板材的微观组织,提升其综合力学性能,将AZ31镁合金板材经360℃热轧后,采用不同时间液氮浸泡进行深冷处理,研究不同深冷时间处理对热轧AZ31镁合金显微组织和力学性能的影响。结果表明:经深冷处理后,热轧AZ31镁合金中析出第二相,同时合金内部晶粒尺寸得到细化,导致孪晶产生。AZ31镁合金热轧板材经过20 min深冷处理后,综合力学性能得到显著改善,其硬度、屈服强度和极限抗拉强度分别为75.4 HV、182 MPa和278 MPa,较传统热轧分别提高了9.4%、8.3%和14%,此外,随着深冷时间的延长,AZ31镁合金的伸长率逐渐提高,最大伸长率达到14.4%。
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