(Al11La3+Al2O3)/Al复合材料的高温性能及其强化机制

图1 铝粉、氧化镧颗粒和球磨粉末的形貌

Fig.1 Morphologies of pure Al powders (a), La₂O₃ particles (b), and mixed powders after HEBM (c)

高能球磨后的混合粉末如图1c所示。为了确保完全反应，将混合粉末在630℃热压烧结2 h，烧结时真空度为10^-1 Pa。最后将烧结后的坯锭在450℃以16:1的挤压比制备出复合材料棒材。

为评价Al₁₁La₃和Al₂O₃的强化效果，对纯铝粉进行工艺相同的球磨、烧结和挤压，制备出对比样品。本文所用实验样品，均取自热挤压棒材。

使用X射线衍射仪(X-ray diffraction, XRD)分析复合材料的物相，用扫描电子显微镜(Scanning electron microscopy, SEM, FEI Apreo)和透射电子显微镜(Transmission electron microscopy, TEM, FEI Talos)观察复合材料的微观结构。用于TEM观察的样品需用5000目砂纸打磨后再用凹坑研磨仪研磨，最后用Gatan PIPS Ⅱ 695精密离子减薄仪将其离子减薄。在平行于挤压方向加工标距尺寸为15 mm×4 mm×2 mm的狗骨形拉伸试样，使用MTS E45.105万能试验机分别在室温和350℃进行拉伸实验，应变速率为1×10^-3 s^-1，350℃高温拉伸的保温时间为10 min，每个材料至少测试3次。用SEM(FEI Apreo)观察拉伸断口。

2 实验结果和分析

2.1 复合材料的的相组成

图2给出了高能球磨后的混合粉末和复合材料的X射线衍射谱。可以看出，在混合粉末的谱中出现了Al₁₁La₃衍射峰，表明在高能球磨过程中发生了原位反应。在复合材料的谱中出现了显著的Al₁₁La₃衍射峰而没有La₂O₃的衍射峰，表明原位反应十分完全。计算结果表明，La₂O₃与Al完全反应将得到Al₁₁La₃和Al₂O₃总体积分数为10%的铝基复合材料(Al₁₁La₃体积分数为8.02%，Al₂O₃体积分数为1.98%)。

图2

图2 高能球磨粉末和复合材料的X射线衍射谱

Fig.2 XRD patterns of mixed powders after HEBM (a) and composite (b)

Sakamoto等^[20]用Al-La₂O₃制备复合材料时，在球磨过程中并未发生原位反应，烧结后剩余大量的La₂O₃。本文实验中充分的原位反应，可归因于球磨和烧结两个过程。在Sakamoto等^[20]使用容积为50 mL的球磨罐，球料比为10:1，转速为700 r/min，球磨100 h，可估算出磨球的平均速度为0.83 m/s；而本文实验中的罐体容量为5 L，转速为400 r/min，则磨球的平均速度为3.14 m/s。同时，15:1的球料比也使本文实验中的球磨强度更高，从而球磨6 h便激活了原位反应。这表明，为了激活Al-La₂O₃原位反应，球磨强度比球磨时间更重要，而较高的烧结温度则可使反应充分。

2.2 复合材料的显微组织

图3给出了复合材料的SEM照片和能谱。从图3a可见，材料中细小颗粒的分布十分均匀。在较高的放大倍数下(图3b)可见这些颗粒的边缘呈不规则的锯齿状。能谱面扫图3d，e和能谱分析图3f表明，这些微米级颗粒由Al和La元素组成，可推断其成分为Al₁₁La₃。使用截距法和面积法测量统计多张SEM图片中的Al₁₁La₃颗粒，可得其平均尺寸为3.2 μm，体积分数为7.94%。这个结果，与完全反应后的理论值8.02%十分接近，据此可认为La₂O₃的反应完全。但是，由于本文的烧结温度较高，Al₁₁La₃的尺寸较大^[20]。使用更高倍数的SEM可以发现，材料中弥散分布有大量纳米级的针状相(图3c)，推测其为另一种反应产物Al₂O₃。

图3

图3 复合材料的低倍和高倍SEM照片、元素面扫和能谱分析

Fig.3 SEM images of composite with low (a, b) and high magnifications (c), EDS mapping (d, e) of the same position of (b), and EDS result of position A (f)

图4给出了复合材料的TEM照片和对应位置的能谱分析。对图4a中微米级的A相衍射斑点(图4b)进行标定，可进一步确认A相为Al₁₁La₃。图4c~e分别给出了与图4a相同位置的元素Al、La和O的面扫结果。可以看出，除了微米级的Al₁₁La₃相，材料中还生成了大量纳米级的针状Al₂O₃，其长度约为150 nm，宽度约为10 nm。生成纳米尺寸Al₂O₃的主要原因：首先，原料中的La₂O₃即为纳米尺寸。其次，Al与La₂O₃的反应是通过热压烧结过程中的固相扩散控制的，Al₂O₃在点接触处成核且O在Al中的扩散速率低，使Al₂O₃的尺寸为纳米级。最后，生成的Al₂O₃粗化率极低，不易长大^{[19, 22]}。进一步的高分辨TEM观察(图4f)发现，Al₂O₃与基体界面的结合良好。

图4

图4 复合材料的TEM照片

Fig.4 TEM images of composite (a) macrostructure and Al₁₁La₃ in the matrix, (b) a selected area diffraction pattern of A-phase, (c) (d) (e) EDS mapping of the same position of (a), (f) HRTEM image of Al₂O₃with its FFT in the inset, and (g) grain size

从图4g可见，材料的晶粒尺寸在超细晶范围，截距法的测量统计结果表明，其晶粒尺寸约为350 nm。随着材料的晶粒尺寸变小，晶粒数量和晶界增多，使室温下位错的滑移变得困难，有助于使材料的强度提高。

图5给出了用于对比的纯铝材料的微观组织TEM照片。通过截距法统计可知，材料的晶粒尺寸约为480 nm。这表明，Al₁₁La₃和Al₂O₃的加入进一步促进了基体晶粒的细化。在球磨过程中引入的部分氧，使材料中生成了少量的氧化铝(图5a箭头所示)。

图5

图5 纯铝基体的TEM照片和相同位置的能谱分析

Fig.5 STEM-BF image of Al matrix (a), and EDS mapping (b, c) of the same position of (a)

2.3 室温和350℃的高温拉伸性能

(Al₁₁La₃+Al₂O₃)/Al复合材料和对比材料的室温(Room temperature, RT)和350℃高温拉伸曲线，如图6所示，拉伸性能列于表1。可以看出，复合材料的室温屈服强度(Yield strength, YS)为292 MPa，抗拉强度(Ultimate tensile strength, UTS)为328 MPa，延伸率(Elongation, EL)为10.5%；350℃屈服强度为113 MPa，抗拉强度为119 MPa，延伸率为10.2%。与用相同工艺制备的纯铝相比，复合材料的室温和高温抗拉强度分别提高了约30%和19%。同时，本文制备的(Al₁₁La₃+Al₂O₃)/Al复合材料其性能也优于典型的耐热铝合金Al-5.3Cu-0.8Mg-0.6Ag和Al-9.4Si-1.9Cu-0.5Mg，这两种合金的350℃抗拉强度均低于100 MPa^{[6, 23, 24]}。与同样有Al₂O₃增强体的(B₄C+Al₂O₃)/Al复合材料对比，本文制备的复合材料其高温强度提升超20%^[25]。

图6

图6 复合材料和铝基体的室温和350℃高温抗拉曲线

Fig.6 Tensile stress-strain curves of composite and Al matrix at RT (a) and 350℃ (b)

表1 复合材料和铝基体的拉伸性能

Table 1 Tensile properties of composite and Al matrix

Sample	RT			350℃
Sample	YS/MPa	UTS/MPa	EL/%	YS/MPa	UTS/MPa	EL/%
Composite	292±4	328±3	10.5±0.9	113±5	119±4	10.2±2.2
Al matrix	208±8	252±3	18.5±1.0	99±6	100±4	15.2±2.0

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(Al₁₁La₃+Al₂O₃)/Al复合材料室温强化的主要原因：一是增强相Al₁₁La₃和Al₂O₃的载荷传递强化( $∆ σ_{L - T}$ )和位错强化作用，其中位错强化主要源于Orowan强化( $∆ σ_{O r o}$ )和几何必需位错强化( $∆ σ_{G N D}$ )，其中Orowan强化效果与材料中纳米颗粒的尺寸大小和体积分数有关。随着纳米颗粒尺寸的减小和体积分数的提高，Orowan强化效应增强；另一个是铝基体细晶粒产生的晶界强化，可根据Hall-Petch公式计算^{[19, 26, 27]}。根据对材料的定量分析，可求出材料各强化作用的贡献。复合材料的屈服强度可表示为^[25]

σ_{y c} = σ_{y m} + ∆ σ_{A l_{2} O_{3}} + ∆ σ_{A l_{11} L a_{3}}

(1)

σ_{y m} = σ_{O} + \frac{K_{H P}}{\sqrt[]{D}}

(2)

其中 $σ_{y c}$ 为复合材料的屈服强度， $σ_{y m}$ 为基体的屈服强度， $∆ σ_{A l_{2} O_{3}}$ 为Al₂O₃引起的强度增加， $∆ σ_{A l_{11} L a_{3}}$ 为Al₁₁La₃引起的强度增加。 $σ_{O}$ 为摩擦应力(为20 MPa)， $K_{H P}$ 为Hall-Petch斜率(40 $M P a \sqrt[]{μ m}$ )，D为晶粒尺寸。Al₂O₃的强度提升，包含了载荷传递的直接强化( $∆ σ_{A l_{2} O_{3}}^{L - T}$ )、Orowan强化( $∆ σ_{A l_{2} O_{3}}^{O r o}$ )和几何必需位错强化( $∆ σ_{A l_{2} O_{3}}^{G N D}$ )。其中Al₁₁La₃尺寸达到微米级，而材料中的Orowan强化主要来自于纳米颗粒，因此Al₁₁La₃相的Orowan强化效果可忽略不计。因此，根据公式可计算出本文的复合材料与铝基体屈服强度的差值为

σ_{C} - σ_{A l} = ∆ σ_{y m} + ∆ σ_{A l_{2} O_{3}} + ∆ σ_{A l_{11} L a_{3}}

(3)

∆ σ_{A l_{2} O_{3}} = ∆ σ_{A l_{2} O_{3}}^{L - T} + ∆ σ_{A l_{2} O_{3}}^{O r o} + ∆ σ_{A l_{2} O_{3}}^{G N D}

(4)

∆ σ_{A l_{11} L a_{3}} = ∆ σ_{A l_{11} L a_{3}}^{L - T} + ∆ σ_{A l_{11} L a_{3}}^{G N D}

(5)

其中Al₂O₃的直接载荷传递的直接强化可表示为^[28]

∆ σ_{A l_{2} O_{3}}^{L - T} = \frac{S σ_{y m}}{4} V_{A l_{2} O_{3}}

(6)

其中S为Al₂O₃的长径比， $V_{A l_{2} O_{3}}$ 为Al₂O₃的体积分数。而Al₁₁La₃的强化公式可简化为^[29]

∆ σ_{A l_{11} L a_{3}}^{L - T} = 0.5 V_{A l_{11} L a_{3}} σ_{y m}

(7)

Orowan的强化效果可表示为^[29]

∆ σ_{A l_{2} O_{3}}^{O r o} = \frac{0.13 G b}{d_{p} [{(\frac{1}{2 V_{A l_{2} O_{3}}})}^{\frac{1}{3}} - 1]} l n \frac{d_{p}}{2 b}

(8)

其中 $d_{p}$ 为增强颗粒尺寸，G为铝的剪切模量(26.4 GPa)，b为柏氏矢量(0.286 nm)。根据相关研究，Al₂O₃和Al₁₁La₃的几何必需位错强化适用公式^[25]

∆ σ_{A l_{2} O_{3}}^{G N D} = α G b \sqrt[]{\frac{8 V_{A l_{2} O_{3}} ϵ_{y}}{b B}}

(9)

∆ σ_{A l_{11} L a_{3}}^{G N D} = α G b \sqrt[]{\frac{8 V_{A l_{11} L a_{3}} ϵ_{y}}{b B}}

(10)

其中 $α$ 为常数1.25，B可近似为增强颗粒的平均直径， $ϵ_{y}$ 为屈服应变0.2%。计算结果(列于表2)表明，原位引入的Al₂O₃和Al₁₁La₃对复合材料强度的贡献值为76 MPa，与实际屈服强度差值84 MPa十分接近。对表2中数据的分析结果表明，Al₂O₃的Orowan强化和几何必需位错强化作用对材料的贡献较大。本文实验中的高球磨强度和高烧结温度保障了原位反应的充分进行，但是烧结温度较高使Al₁₁La₃明显粗化，大大降低了强化效率。而材料的延伸率较高，表明适当提高颗粒的含量以优化增强相的数量有利于其高温力学性能的提高。

表2 复合材料和铝基体的屈服强度计算

Table 2 Calculation of yield strength of the composite and Al matrix

/MPa

∆ σ_{y m}

Al₂O₃

Al₁₁La₃

∆ σ_{L - T}

/MPa

$∆ σ_{O r o}$

/MPa

$∆ σ_{G N D}$

/MPa

∆ σ_{L - T}

/MPa

$∆ σ_{O r o}$

/MPa

$∆ σ_{G N D}$

/MPa

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2.4 材料断口的形貌

材料样品的室温和350℃高温拉伸断口，如图7所示。可以看出，复合材料的室温拉伸断口(图7a)表现出典型的复合材料断口形貌，在Al基体区域出现大量的韧窝和撕裂棱，表明基体区域的断裂韧性很好。还发现许多较大的Al₁₁La₃颗粒。图7b给出了图7a中白色方框区域的放大图像，可观察到Al₁₁La₃颗粒上的裂纹。这表明，在拉伸过程中载荷传递使Al₁₁La₃开裂，并可能进一步导致材料断裂。高温拉伸断口与室温断口的不同特征是，未观察到Al₁₁La₃颗粒，表明在高温下微米级颗粒的载荷传递作用减弱，难以发生界面脱粘或颗粒断裂^[30]。同时， Al基体的性能在高温下明显恶化^[31]并在拉伸过程中发生晶界滑动，因此晶界不再有强化作用。但是，细小的纳米相可抑制晶界的滑动，从而提高材料的高温强度。在没有增强相的Al基体中位错不能形成堆积，大部分位错在晶界处湮灭。而复合材料中细小的纳米相可与位错产生交互作用，阻碍位错运动。因此，需要更多的能量才能使材料进一步的变形，从而提高了材料的强度。但是，纳米相的钉扎作用使位错堆积和应力集中，从而产生材料的沿晶断裂。高温断口呈现出明显的沿晶断裂特征，且表面存在大量的纳米颗粒(图7c，d)，与上述文献报道相同。

图7

图7 复合材料的室温和350℃拉伸断口的SEM照片

Fig.7 SEM fractographs at RT (a, b) and 350℃ (c, d) for composites

3 结论

(1) 在铝粉中添加质量分数为8%的La₂O₃颗粒并用原位反应可制备(Al₁₁La₃+Al₂O₃)/Al复合材料。较高的球磨强度是激活原位反应的重要因素，在630℃烧结过程中原位反应基本完全，生成了微米级Al₁₁La₃和纳米级Al₂O₃。

(2) (Al₁₁La₃+Al₂O₃)/Al复合材料的室温强化机制主要为Al₁₁La₃和Al₂O₃的位错强化和载荷传递强化，而在高温下Al₁₁La₃的载荷传递作用减弱，强化机制是纳米Al₂O₃对晶界的钉扎。

(3) (Al₁₁La₃+Al₂O₃)/Al复合材料的350℃抗拉强度达到119 MPa并具有良好的延伸率，其性能优于目前应用在高温下的Al-Cu-Mg-Ag和Al-Si-Cu-Mg铝合金。

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This study was conducted to obtain a type of aluminum matrix composites exhibiting a good strength and certain ductility at high temperature. The 25 vol% TiO2-75 vol%2024 Al systems were selected to fabricate the (Al3Ti+Al2O3)/2024 Al composites with residual similar to 32 vol% Al matrix through powder metallurgy. The (Al3Ti+Al2O3)/2024 Al exhibits a good strength and certain ductility at high temperature as in the design. The microstructure of (Al3Ti+Al2O3)/2024 Al composites was investigated. It was discovered that the in-situ Al3Ti reinforcement was in coarse block-shaped particles of approximately 6.9 mu m in size and the Al-Al3Ti interface was clean. The Al2O3 particles were in the nano-scale and distributed in the Al matrix in a cluster form. The high temperature compression testing of the composites was conducted at the temperatures of 573 K, 623 K, 673 K, 723 K and 773 K with the strain rate of 10(-3) similar to 0.42 s(-1). The results demonstrated that the composites exhibited higher strength at the same high temperature than the other Al matrix composites with a similar volume fraction. The massive Al3Ti and Al2O3 phases played a load bearing role at high temperatures. The residual similar to 32 vol% Al matrix led the composites to acquire certain ductility. (C) 2018 Elsevier B.V.

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