Y元素对Cu-Al-Ni高温形状记忆合金性能的影响

图1 Cu-13Al-4Ni-xY (x = 0.2、0.5)合金的OM和SEM像

Fig.1 OM (a, b) and SEM (c, d) images of Cu-13Al-4Ni-xY alloys with x = 0.2 (a, c) and x = 0.5 (b, d)

图2

图2 Cu-13Al-4Ni-xY (x = 0.2、0.5)合金的XRD谱

Fig.2 XRD spectra of Cu-13Al-4Ni-xY (x = 0.2 and 0.5) alloys

XRD结果表明,Cu-13Al-4Ni-0.2Y合金与Cu-13Al-4Ni-0.5Y合金的相组成基本一致,且Cu-13Al-4Ni-0.5Y合金中的第二相颗粒较多(如图1中箭头所示),因此对Cu-13Al-4Ni-0.5Y合金进行TEM观察,以进一步确定Cu-13Al-4Ni-xY合金的相组成,结果如图3所示。由图3a~c可知,类似六方Cu₄Y的第二相粒子嵌在18R马氏体中,尺寸约为500 nm。高分辨图像(图3d)显示,在(1 $\bar{1}$ 4)晶面上,第二相的面间距为0.29 nm。第二相与基体之间没有明显位向关系。EDS面扫描图(图3e~h)证明第二相为富Y相。Cu、Al、Ni和Y在第二相中的原子分数分别为66.51%、7.51%、2.87%和23.09%,(Cu + Al + Ni)和Y的原子比接近4∶1。因此,第二相可以认为是六方结构的(Cu, Al, Ni)₄Y相。

图3

图3 Cu-13Al-4Ni-0.5Y合金的TEM明场像、SAED花样、HRTEM像及第二相区域EDS元素面扫描图

Fig.3 TEM bright field image (a), the corresponding selected area electron diffraction (SAED) patterns of 18R martensite (b) and second phase (c) in the Cu-13Al-4Ni-0.5Y alloy; high resolution transmission electron microscope (HRTEM) image of area d in Fig.3a (Inset shows the fast Fourier transform) (d); EDS element maps of Cu (e), Al (f), Ni (g), and Y (h) for the second phase (d—interplanar spacing)

图4a为Cu-13Al-4Ni-xY (x = 0.2、0.5)合金的压缩应力-应变曲线。可见,当x = 0.2时,压缩断裂强度为978 MPa,应变为15.6%；当x = 0.5时,压缩断裂强度和应变分别提高到1185 MPa和19.3%,远高于Cu-13Al-4Ni合金(580 MPa和10.5%)^[24]。在多晶Cu-Al-Ni合金发生形变时,合金中的晶粒位向发生变化,为了保证界面上的应变连续性,在晶界处会发生应力集中,因此Cu-13Al-4Ni合金的断裂形式是沿晶断裂^[21]。由于晶粒细化,Y元素大大提高了合金的强度和塑性。此外,第二相在基体中随机分布,导致位错和应力在第二相附近积累,所以Cu-13Al-4Ni-xY合金的断裂形式由未掺杂合金的沿晶断裂变为穿晶断裂,如图4b和c所示。基体中脆性2H马氏体的消失也是力学性能提高的另一个重要原因^[26]。

图4

图4 Cu-13Al-4Ni-xY (x = 0.2、0.5)合金的压缩应力-应变曲线及断口形貌

Fig.4 Compressive stress-strain curves (a), and fracture morphologies of the Cu-13Al-4Ni-xY alloys with x = 0.2 (b) and x = 0.5 (c)

图5为Cu-13Al-4Ni-xY (x = 0.2、0.5)合金的应变恢复特性曲线。当预应变为8%时,x = 0.2和0.5合金的形状记忆效应分别为4.8%和4.2%,远高于Cu-13Al-4Ni合金的2.6%^[24]。当预应变增加到10%时,x = 0.2合金的形状记忆效应为5.5%,x = 0.5合金的形状记忆效应为5.1%。Y掺杂提高了Cu-13Al-4Ni-xY合金的力学性能,增强了其抵抗不可逆变形的能力。因此,Cu-13Al-4Ni-xY合金的形状记忆效应得到了相应的改善。Cu-13Al-4Ni-0.5Y合金的可逆应变低于Cu-13Al-4Ni-0.2Y合金的原因在于,随着Y含量的增加,合金基体中的(Cu, Al, Ni)₄Y相含量随之增多,损害了合金的形状记忆效应。

图5

图5 Cu-13Al-4Ni-xY (x = 0.2、0.5)合金分别在8%和10%预应变下的恢复特性曲线

Fig.5 Recovery characteristic curves of the Cu-13Al-4Ni-xY alloys with x = 0.2 (a) and x = 0.5 (b) under pre-strains of 8% and 10%, respectively (The arrow lines represent the recovery strain after heating to 350^oC for 1 min. SME—shape memory effect)

图6为Cu-13Al-4Ni-xY (x = 0、0.2、0.5)合金的电势极化曲线,计算得到的腐蚀参数如表1所示。极化曲线分为阴极支路和阳极支路。阴极支路表示氧还原反应,中性溶液中的反应公式为^[27]：

图6

图6 Cu-13Al-4Ni-xY (x = 0、0.2、0.5)合金在3.5%NaCl溶液中的动电位极化曲线

Fig.6 Potential polarization curves for the Cu-13Al-4Ni-xY (x = 0, 0.2, 0.5) alloys in the 3.5%NaCl solution

表1 Cu-13Al-4Ni-xY合金在3.5%NaCl溶液中的腐蚀参数

Table 1 Corrosion parameters of Cu-13Al-4Ni-xY alloys in 3.5%NaCl solution

Alloy	E_corr (vs Ag/AgCl)	i_corr	R_p	v_corr
	V	μA·cm^-2	kΩ·cm²	mm·a^-1
Cu-13Al-4Ni^[25]	-0.264	1.47	2.29	0.017
Cu-13Al-4Ni-0.2Y	-0.271	2.25	1.40	0.026
Cu-13Al-4Ni-0.5Y	-0.272	3.03	1.56	0.035

Note:E_corr—corrosion potential, i_corr—corrosion current density, R_p—polarization resistance, v_corr—corrosion rate

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1 / 2 (a + b + c) O_{2} + (a + b + c) H_{2} O +

2 (a + b + c) e^{-} \to (a + b + c) O H^{-}

(1)

式中,a、b、c分别为Cu²⁺、Al³⁺、Ni²⁺的系数。

阳极支路表示Cu的溶解。首先Cu溶解为Cu⁺,然后Cu⁺被氧化为Cu²⁺^[28]：

C u \to C u^{+} + e^{-}

(2)

C u^{+} \to C u^{2 +} + e^{-}

(3)

Cu-Al-Ni合金的总溶解反应表达式为：

(C u + A l + N i) + 1 / 2 (a + b + c) O_{2} +

(a + b + c) H^{+} \to a C u^{2 +} + b A l^{3 +} + c N i^{2 +} +

(a + b + c) H_{2} O

(4)

众所周知,材料表面形成的氧化保护层可以保护金属不受腐蚀。由图6可知,随着施加电压的增加,阳极支路出现3个电流峰值。Chang等^[29]发现Ni²⁺的选择性溶解浓度是Cu²⁺和Al³⁺的3倍。所以第一次电流密度降低是由于Ni²⁺的选择性溶解造成的。Al₂O₃的稳定性几乎是Cu₂O^[20]的11倍。因此,电流密度第二次下降的现象可能是Cu⁺/Cu²⁺的溶解造成的,第三次是Al³⁺的溶解造成的。在图6和表1中,添加Y后,电极电位向负方向移动。同时,随着Y含量的增加,腐蚀电流密度升高,合金的腐蚀速率加快,表明Y元素对Cu-13Al-4Ni合金的耐蚀性稍有损伤。

3 结论

(1) Y元素掺杂后,Cu-13Al-4Ni合金从由2H和18R马氏体组成变为单一的18R马氏体,并且其中伴随着晶粒细化以及六方(Cu, Al, Ni)₄Y相的出现。

(2) 合金的力学性能大幅提升,Cu-13Al-4Ni-0.5Y合金的压缩断裂应变和压缩断裂应力分别达到了19.3%和1185 MPa。断裂形式也从沿晶断裂变为穿晶断裂。力学性能的提高促进了形状记忆效应的改善,Cu-13Al-4Ni-0.2Y合金在预应变10%加热后,可得到5.5%的可逆应变。Cu-13Al-4Ni-xY合金的耐蚀性相较于Cu-13Al-4Ni合金稍有下降。

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