多道次压缩变形对AZ80镁合金微观组织演化的影响

图1 恒温三次变形工艺及阶梯降温三次变形工艺流程图

Fig.1 Process maps of three-step deforming process with constant-temperature (a) and three-step deforming process with step-cooling (b) (P—pass, ε—strain)

2 实验结果

2.1 晶粒形貌与尺寸演化

图2给出了恒温、阶梯降温2类变形工艺晶粒形貌和尺寸变化规律。由图2a~c可知，微观组织演化与塑性变形过程密切相关。由图中可以看出，恒温一次变形(ε = 0.2)后的微观组织中存在很多不连续晶界，经过恒温二次变形(ε = 0.4)、恒温三次变形(ε = 0.6)后，发现不连续晶界已急剧减少，且晶粒尺寸随着变形而逐渐细化。图2d为阶梯降温三次变形(ε = 0.6)微观组织，其晶粒细化程度明显高于恒温三次变形。

图2

图2 恒温、阶梯降温2类变形工艺的微观组织演变

Fig.2 Microstructure evolutions of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d)

图3为恒温、阶梯降温2类变形工艺下平均晶粒尺寸和平均晶粒纵横比的定量描述(重点分析变形量ε = 0.2和0.6变形工艺)。从图3a可知，恒温多次变形平均晶粒尺寸从18.315 μm (ε = 0.2)下降到14.152 μm (ε = 0.6)，而阶梯降温多次变形平均晶粒尺寸则从18.315 μm (ε = 0.2)下降到10.197 μm (ε = 0.6)；从图3b可知，恒温多次变形平均晶粒纵横比从2.077 (ε = 0.2)下降到1.708 (ε = 0.6)，阶梯降温多次变形平均晶粒纵横比从2.077 (ε = 0.2)下降到1.781 (ε = 0.6)。即恒温多次变形时，随变形程度增加，晶粒尺寸更加细小、晶粒形状更趋于等轴状(这与图2a~c吻合)。而阶梯降温三次变形(ε = 0.6)则比恒温三次变形(ε = 0.6)晶粒尺寸更加细小，因此本工作中300℃是AZ80镁合金晶粒细化的适宜变形温度。

图3

图3 恒温、阶梯降温2类变形工艺的平均晶粒尺寸与平均晶粒纵横比变化

Fig.3 Average grain sizes (a) and average grain shape aspect ratios (b) of constant-temperature and step-cooling deformation processes

图4为恒温、阶梯降温2类变形工艺下的晶界分布图。如图4a所示，恒温变形时存在大量“弓弯”晶界(如红色箭头所示)，由于压缩变形导致材料内部产生应变，进而导致晶界滑移，“弓弯”晶界成为动态再结晶的潜在形核中心^[20,21]。恒温一次变形(ε = 0.2) (图4a)时，晶界和亚晶界分布不均匀，随着变形程度(ε = 0.2、0.4、0.6)增加，原始晶粒中亚晶界从晶粒内部向晶界迁移并逐渐减少，这是因为亚晶界的可动性，随着压缩变形进行亚晶界逐渐被大晶界所吞噬^[22]。相比恒温三次变形(ε = 0.6) (图4c)，阶梯降温三次变形(ε = 0.6) (图4d)中亚晶界数量较多并且分布不均匀，这是由于外加应力作用的结果^[22]。

图4

图4 恒温、阶梯降温2类变形工艺的晶界分布

Fig.4 Grain boundaries distributions of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d) (The green lines represent subgrain boundaries, and the grain boundaries indicated by the red arrows are the bow-bend grain boundaries)

图5为恒温、阶梯降温2类变形工艺的晶粒尺寸图。可以观察到典型的“项链”结构，这通常被认为是动态再结晶的重要特征，表明恒温、阶梯降温2类变形工艺均发生了动态再结晶。

图5

图5 恒温、阶梯降温2类变形工艺的晶粒尺寸图

Fig.5 Grain size diagrams of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d) (The colored small grains are dynamically recrystallized grains, the red grains are basal orientation, and the other colored grains are non-basal orientation)

图6为恒温、阶梯降温2类变形工艺的大角度晶界(θ > 10°)、小角度晶界(θ ≤ 10°)占比图与动态再结晶占比图。如图6a所示，5°~15°范围内晶界占比最大只有6%，其占比较少，因此，本工作将重点讨论晶界取向差< 5°、> 15° 2类，不再讨论5°~15°部分。由图6a可知，恒温一次变形时小角度晶界和大角度晶界占比分别为57%和39%；恒温二次变形时2者占比分别为39%和55%；恒温三次变形时2者占比分别为35%和59%。即恒温多次变形时，随着变形程度增加，小角度晶界逐渐减少而大角度晶界逐渐增加。与恒温三次变形相比，阶梯降温三次变形时，小角度晶界和大角度晶界占比分别为73%和25%，即小角度晶界增加而大角度晶界减小。

图6

图6 恒温、阶梯降温变形2类工艺晶界占比图与动态再结晶占比图

Fig.6 Grain boundary ratios (a) and dynamic recrystallization ratios (b) of constant-temperature and step-cooling deformation processes

如图6b所示，恒温多次变形随着变形程度(ε = 0.2、0.4、0.6)增加，动态再结晶占比分别为4.3%、26.1%和42.5%，而阶梯降温三次变形(ε = 0.6)动态再结晶占比为22.9%。即恒温多次变形动态再结晶程度随变形程度增加而增加，这是由于随着热压缩变形量增加，连续动态再结晶(CDRX)起始于小角度晶界且晶格位错连续积累，随着小角度晶界逐渐转变为大角度晶界，动态再结晶逐渐增多^[23]，这与图6a结果相吻合。

由图5和6结果可知，恒温三次变形更有利于动态再结晶发生。

图7为恒温、阶梯降温2类变形工艺的取向差图。在图中“Correlated”表示相邻晶粒取向差分布，“Uncorrelated”表示不相邻晶粒取向差分布，“Rondom”表示随机或理论取向差分布。由图7a可知，恒温一次变形后，相邻晶粒取向差呈“双峰”分布，分别在2°和86°左右出现峰值。如图7b~d可知，分别在2°、30°和86°左右出现了峰值，但取向差在2°左右的峰值仍然比较高，这是由于压缩变形所致，同时取向差在30°左右出现峰值，这是由于六方结构的六次对称轴限制，2个理想(0001)晶粒间最大取向差是30°，转角为<0001>^[24]。由图7a~d知，2种工艺均在86°左右出现峰值，表明恒温、阶梯降温2类变形工艺均存在{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶，从图7c和d可知，恒温三次变形的{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶含量为0.010，而阶梯降温三次变形的{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶含量为0.015，即阶梯降温三次变形(ε = 0.6)比恒温三次变形(ε = 0.6)具有更多{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶。“不相邻晶粒取向差”分布随着“相邻晶粒取向差”峰值进行变换，在图7a~d中“不相邻晶粒取向差”分布与“理论取向差”分布在30°处差异较大，故在恒温多次变形、阶梯降温多次变形中均有明显织构产生。

图7

图7 恒温、阶梯降温变形工艺下的取向差统计图

Fig.7 Misorientation distributions of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d)

2.2 位错密度演化

为了定量研究不同变形工艺下位错密度演化，利用EBSD取向数据的局部取向差计算几何必需位错密度(ρ^GND)演化规律，采用核平均取向差(KAM)方法^[17]：

ρ^{G N D} = \frac{2 θ}{u b}

(1)

式中，θ表示局部取向差平均值，b表示Burger矢量模，u表示单位长度。图8为恒温、阶梯降温2类变形工艺GND密度分布图。从图8a~c可以看出，恒温多次变形随着变形程度增加，GND密度总体水平下降且分布不均匀。相比恒温三次变形(图8c)，阶梯降温三次变形(图8d) GND密度较高且分布不均匀。

图8

图8 恒温、阶梯降温2类变形工艺的几何必需位错密度分布图

Fig.8 Geometrically necessary dislocation (GND) density distribution maps of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d) (The green is the dislocation concentration area, and the blue is the dislocation-free area)

图9为恒温、阶梯降温2类变形工艺的GND密度统计图。从图9a~c可知，恒温多次变形的GND密度分别为1.33 × 10¹³、0.93 × 10¹³和1.10 × 10¹³ m^-2。即恒温多次变形时，随变形程度(ε = 0.2、0.4、0.6)增加，GND密度出现先降低、后升高的变化趋势。由图9d知，阶梯降温三次变形(ε = 0.6)的GND密度为1.91 × 10¹³ m^-2。相比恒温三次变形(ε = 0.6)，阶梯降温三次变形(ε = 0.6)的GND密度更高。

图9

图9 恒温、阶梯降温变形工艺的几何必需位错密度(ρ^GND)统计图

Fig.9 Geometrical necessary dislocation density (ρ^GND) statistical maps of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d)

2.3 Schmid因子变化

图10为恒温、阶梯降温2类变形工艺的晶粒Schmid因子分布图。不同的颜色代表不同晶格变形能力，晶体由“硬取向”向“软取向”转变时，颜色相应从蓝色到红色。为了更直观地分析恒温多道次变形和阶梯降温多道次变形，图11给出了不同变形条件下AZ80镁合金的晶粒Schmid因子分布直方图。

图10

图10 恒温、阶梯降温变形工艺的Schmid因子分布图

Fig.10 Schmid factor distribution maps of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d) (The blue is hard orientation, the red is soft orientation, and other colors are between hard orientation and soft orientation)

图11

图11 恒温、阶梯降温变形工艺的Schmid因子分布直方图

Fig.11 Schmid factor statistical maps of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d)

如图11所示，当Schmid因子从0到0.5变化时，晶体由硬取向向软取向转变。恒温一次变形(ε = 0.2)、恒温二次变形(ε = 0.4)的Schmid因子分布并不均匀，主要集中在0.2~0.5之间，以软取向为主导，而恒温三次变形(ε = 0.6)的Schmid因子变得更均匀，大部分集中在0.1左右，以硬取向为主导，即经过恒温多次变形后材料中出现明显“加工硬化”现象(图11c)。由图11d知，阶梯降温三次变形(ε = 0.6)Schmid因子主要集中在“中间部分”(0.2~0.3之间) (与图10情况一致)，表明阶梯降温三次变形并未出现很强烈的“加工硬化”现象，而是介于软化与加工硬化之间，故阶梯降温三次变形(ε = 0.6)比恒温三次变形(ε = 0.6)更有利于塑性变形。

2.4 变形过程中织构演化

图12给出了恒温、阶梯降温2类变形工艺的织构演化过程。恒温一次变形(ε = 0.2)极密度最大为17.72 (图12a)，恒温二次变形(ε = 0.4)极密度最大为15.03 (图12b)，恒温三次变形(ε = 0.6)极密度最大为11.44 (图12c)。即恒温三次变形时，随变形程度(ε = 0.2、0.4、0.6)增加极密度逐渐减小，这是由于随着变形程度增加，再结晶程度不断增加而导致基面织构弱化。而阶梯降温三次变形(ε = 0.6)时，基面(0001)织构极密度最大为9.77 (图12d)，相比于恒温三次变形，阶梯降温三次变形(ε = 0.6)极密度变得更小，这是因为拉伸孪晶增加(图7)，织构得到弱化使塑性提升，这与文献[11]吻合。

图12

图12 恒温、阶梯降温2类变形工艺的极图

Fig.12 Pole figures of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d) (ED—extrusion direction, TD—transverse direction)

如图13所示，通过与六方晶系镁合金中常见的几种理想织构取向分布函数(ODF)图^[25]进行对比判断，恒温一次变形(ε = 0.2)主要存在{0001}<10 $\bar{1}$ 0>基面织构；恒温二次变形(ε = 0.4)主要存在{11 $\bar{2}$ 0}< $\bar{1}$ 010>和{10 $\bar{1}$ 0}< $\bar{2}$ 110>柱面织构，恒温三次变形(ε = 0.6)则主要存在{0001}<11 $\bar{2}$ 0>基面织构，阶梯降温三次变形(ε = 0.6)主要存在{11 $\bar{2}$ 0}< $\bar{1}$ 010>和{10 $\bar{1}$ 0}< $\bar{2}$ 110>柱面织构。

图13

图13 恒温、阶梯降温2类变形工艺的取向分布函数(ODF)图

Fig.13 Orientation distribution function (ODF) maps of constant-temperature deformation process at ε = 0.2 (a), ε = 0.4 (b), and ε = 0.6 (c); and step-cooling deformation process at ε = 0.6 (d) (The red area is strong texture, the blue area is weak texture, and the texture intensity of the green area is between them; φ2—Euler angle)

3 分析讨论

3.1 多道次压缩变形镁合金织构演变

由图12和13可知，恒温三次变形时，随变形程度(ε = 0.2、0.4、0.6)增加，不仅极密度逐渐减小，而且织构类型也发生转变：基面织构→柱面织构→基面织构(见2.4节)，但阶梯降温三次变形(ε = 0.6)主要是柱面织构，其主要原因是：(1) 动态再结晶的影响(图6)。经过不同路径压缩变形均伴随着动态再结晶的发生，而这会弱化基面织构。(2) 晶粒细化的影响。晶粒细化导致晶粒转动和晶界迁移更加容易，随着晶粒不断转动，使晶粒的取向发生变化(图2)。(3) GND密度的影响(图9)。阶梯降温三次变形(ε = 0.6)较恒温三次变形(ε = 0.6)的GND密度急剧增大，GND密度的升高阻碍孪晶生长和晶粒间旋转移动^[19,26]，使晶粒取向不易发生变化，因此阶梯降温三次变形主要为柱面织构。(4) 孪晶的影响(图7)。孪晶可以改变晶粒取向，阶梯降温三次变形(ε = 0.6)比恒温三次变形(ε = 0.6)产生更多86°{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶，这导致基面织构弱化进而产生柱面织构。

3.2 晶界演化对孪晶的影响

孪晶能促使织构转变，而孪晶的形成主要受晶粒取向差、小角度晶界数量、GND密度3方面的影响。

(1) 晶粒取向差对孪晶形成的影响。晶粒取向差对孪晶影响主要表现在“是否能够增强晶界迁移能力(晶界迁移与晶界结构密切相关)”进而有利于孪晶形成。本工作中，恒温三次变形(ε = 0.6)、阶梯降温三次变形(ε = 0.6)局部晶粒取向差平均值分别为34.10°和16.36°，根据小角度晶界迁移阻力 $F = θ Z$ (其中，Z为常数)可知，随 $θ$ 减小，F减小，晶界迁移能力增强。阶梯降温三次变形(ε = 0.6)晶粒取向差(16.36°)较小，F减小，晶界迁移能力增强，故有利于孪晶形成，这与图7结果相吻合。由此可知，晶粒取向差越小，就越有利于孪晶的形成，从而改善塑性。

(2) 小角度晶界数量对孪晶形成的影响。小角度晶界(晶界结构的一种重要形式)数量对孪晶形成的影响主要表现在“是否产生足够亚结构区为孪晶形成提供能量”。本工作中，恒温三次变形、阶梯降温三次变形的小角度晶界占比分别为35%和73% (图6)，且阶梯降温三次变形(ε = 0.6)小角度晶界占比(73%)增大不利于再结晶，这样“再结晶区减少”就迫使“亚结构区^[26]增大”，从而为孪晶的形成提供了能量，因此小角度晶界数量增多(73%)有利于孪晶形成使塑性得到改善。

(3) GND密度是调控孪晶的另一个重要因素。文献[19,26]表明，GND密度增加会阻碍孪晶生长和晶粒间旋转。本工作中，恒温三次变形、阶梯降温三次变形的GND密度分别为1.10 × 10¹³和1.91 × 10¹³ m^-2 (图9)，阶梯降温三次变形(ε = 0.6) GND密度急剧增加，导致孪晶生长受到抑制，从而为孪晶形成提供更多空间，即GND密度也是影响孪晶形成的重要因素。

因此，阶梯降温三次变形(ε = 0.6)比恒温三次变形(ε = 0.6)出现更多孪晶主要原因是：(1) 低晶粒取向差使晶界迁移能力增强，从而促进孪晶形成；(2)小角度晶界增多使亚结构区增大，从而为孪晶形成提供能量；(3) GND密度增加使孪晶生长受阻，从而为孪晶形成提供空间。这样，阶梯降温三次变形(ε = 0.6)在3者共同作用下促使孪晶增多，从而改善了镁合金塑性。

3.3 塑性调控机理

根据2.4节变形过程中Schmid因子变化，恒温三次变形(ε = 0.6) Schmid因子表现为“硬化现象”(在0.1左右)，而阶梯降温三次变形(ε = 0.6) Schmid因子介于“软化与硬化之间(0.2~0.3)”，即阶梯降温三次变形比恒温三次变形更有利于塑性变形，其主要原因是：(1) 恒温三次变形、阶梯降温三次变形的平均晶粒尺寸分别为14.152和10.197 μm，阶梯降温三次变形的晶粒更细小，即单位体积内晶粒更多，因此压缩变形时相同变形程度可分散到更多晶粒产生较均匀形变而不会引起裂纹过早形核与扩展。(2) 由于阶梯降温三次变形通过“晶粒取向差减小、小角度晶界增加、GND密度增加”3者共同作用，产生更多86°{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶，进而有利于塑性变形。(3) 恒温三次变形过程中，织构类型发生转变(基面织构→柱面织构→基面织构)，而阶梯降温三次变形促使更多非基面织构启动，柱面织构启动更有利于塑性改善。

综上，“晶粒尺寸、孪晶、织构3者综合作用”对AZ80塑性的调控明显优于“单一动态再结晶”对其塑性调控。

4 结论

(1) 恒温三次变形更有利于动态再结晶。恒温多道次变形随变形程度(ε = 0.2、0.4、0.6)增加，小角度晶界逐渐减少，大角度晶界逐渐增加；而阶梯降温多次变形随变形程度(ε = 0.2、0.4、0.6)增加，小角度晶界逐渐增多，大角度晶界减少，因此恒温三次变形(ε = 0.6)更有利于动态再结晶，而阶梯降温三次变形(ε = 0.6)则不利于动态再结晶。

(2) 阶梯降温三次变形(ε = 0.6)工艺更有利于塑性变形。主要原因是阶梯降温三次变形(ε = 0.6) 比恒温三次变形(ε = 0.6)晶粒尺寸更细小且产生更多孪晶，阶梯降温三次变形(ε = 0.6)主要为非基面织构(柱面织构)，而恒温三次变形(ε = 0.2、0.4、0.6)过程中织构类型发生转变(基面织构→柱面织构→基面织构)。

(3) 晶粒取向差减小、小角度晶界增多、几何必需位错密度增加是导致阶梯降温三次变形(ε = 0.6)出现更多86°{10 $\bar{1}$ 2}<1 $\bar{2}$ 10>拉伸孪晶的主要原因。

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