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实验所用样品为Al-4%Ag合金,采用高纯度的Al(99.995%)和Ag(99.999%)通过真空电弧熔炼获得。首先经线切割得到大小为
$ 10\;\mathrm{m}\mathrm{m}\times 10\;\mathrm{m}\mathrm{m}\times 1\;{\rm{mm}}$ 的片状样品,利用金相砂纸对样品进行物理抛光。然后利用酒精和高氯酸体积比为5:1配置的抛光液对样品进行电化学抛光处理,去除其表面的氧化物和油污。最后将样品放入真空(10–5 Pa)石英管中,在823 K的温度下保温40 h消除样品在熔炼切割过程中所产生的加工硬化和残余应力达到均质化的效果。 -
正电子湮没寿命测量采用22Na源强度大约为1.0 MBq,将源放入两片相同的样品中间。采用快-慢正电子湮没寿命谱符合系统,半高宽时间分辨率(FWHM)为196 ps,寿命谱总计数为
$ 2.0\times {10}^{6} $ 。通过ProgramLT_9软件对测量得到的正电子寿命谱数据进行两组分解谱。扣除源成分(382 ps,20%)之后,将淬火样品的长寿命组分$ {\tau }_{2} $ 锁定在240 ps(Al基体中单空位的寿命),将形变样品的长寿命组分$ {\tau }_{2} $ 锁定在235 ps(Al基体中位错的寿命),短寿命组分$ {\tau }_{1} $ 设为自由拟合状态。平均正电子湮没寿命可以表示为
$$ {\tau }_{\rm mean}={\tau }_{1}{I}_{1}+{\tau }_{2}{I}_{2}, $$ 其中短组分寿命
$ {\tau }_{1} $ 主要是正电子在自由态湮没的寿命,长组分寿命$ {\tau }_{2} $ 主要是正电子在Al基体中单空位湮没的寿命。$ {I}_{1} $ 和$ {I}_{2} $ 分别对应的是短寿命组分和长寿命组分的强度。平均正电子湮没寿命可以表示为$ {\mathrm{\tau }}_{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}={\tau }_{1}{I}_{1}+ {\tau }_{2}{I}_{2} $ 。 -
准备淬火和形变样品。淬火样品,首先将均质化处理好的样品在真空状态下加热到823 K,保温30 min,随后将样品快速淬入冰水混合物中。然后等待样品完全冷却之后将样品取出擦干备用。形变样品,在室温下按照一个固定的方向轧制,每次轧辊下压量不超过0.02 mm,往复轧制10次以上方可继续下压,尽可能保证形变量一致。形变实验的样品所需的形变量为20%。由于在冷轧过程中样品内部会产生比较复杂的缺陷。因此样品达到20%形变量之后,需要在393 K的温度下退火1 h用以去除多余的空位型缺陷。分别对淬火和形变样品进行低温正电子湮没实验。低温实验过程在高真空(10–5 Pa)环境下进行,从10 K开始平均每间隔40 K收集一张正电子寿命谱(PALS)和符合多普勒展宽能谱(CDBS),测量温度点分别为10,50,90,130,170,210,250,273和293 K。
Study on Positron Annihilation Parameters of Low Temperature Segregation of Ag Atoms in Al-4%Ag Alloy
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摘要: 分别研究了823 K淬火处理和20%形变量的Al-4%Ag低温下Ag析出物对正电子的捕获行为的变化。采用正电子湮没寿命谱(PALS)技术和符合多普勒展宽能谱(CDBS)在温度范围10~293 K内对其进行表征。多普勒展宽能谱结果表明2种样品中均存在Ag析出物。正电子寿命谱的解谱结果中的各组分给出了Ag析出物随测量温度的变化规律。在170 ~273 K之间,正电子湮没行为具有较强的温度依赖性。但对于两个具有不同类型缺陷的样品,在低于170 K时观察到样品中Ag析出物捕获正电子能力出现了差异。随着测量温度的降低,淬火样品中的Ag析出物的正电子寿命和强度基本不变。在低于170 K的测量中,形变样品中的Ag析出物对正电子的捕获能力仍旧存在着较强的温度依赖性,但是变化幅度在逐渐减弱。当测量温度提升到室温(273~293 K),越来越多的正电子从Ag析出物中逃逸,逐渐回到自由状态或被其他深陷阱所捕获,失去了对温度的依赖性。Abstract: The positron trapping behavior of Ag precipitates at 823 K quenched and 20% deformation Al-4%Ag alloy at low temperature was studied respectively. It was characterized by positron annihilation lifetime spectroscopy (PALS) technology and coincident Doppler broadening spectroscopy (CDBS) in the temperature range of 10~293 K. The positions of the Ag precipitate peak in the Doppler broadening spectrum shows that there are Ag precipitates in both samples. The components of the lifetime spectrum analysis results show that the change of Ag precipitates with the measurement temperature has a strong temperature dependence at the range of 170~273 K. However, for two specimens with different types of defects, in the measurement data below 170 K, a difference in the positron trapping ability of the Ag precipitates in the sample was observed. With the decrease of the measurement temperature, both the positron lifetime and it’s intensity of the Ag precipitates in the quenched specimens begin to stabilize and lose their dependence on temperature. In the measurement below 170 K, the precipitating ability of the Ag precipitates in the deformed sample still has a strong temperature dependence, but the change amplitude is gradually weakening. In the end, when the measurement temperature increased to around room temperature (273~293 K), more and more positrons escaped from the Ag precipitate, gradually transitioned back to the free state or trapping by other deep traps, and lost the temperature dependence.
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Key words:
- Al alloy /
- low temperature /
- positron /
- Ag precipitate
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