纳米压入下镁基非晶合金的间歇性塑性流动

Intermittent Plastic Flow of Mg-Based Metallic Glass under Nanoindentation

镁基非晶合金通常表现出显著的宏观脆性,因此用常规拉伸、压缩等方法对该合金的变形行为进行研究具有很大困难。本研究利用具有高时间分辨率和高空间分辨率的纳米压痕技术观察了不同加载速率下镁基非晶合金的锯齿流变行为。结果表明,低加载速率促进锯齿的形成,而高加载速率则抑制锯齿的形成。其原因是在低加载速率下,单一剪切带足以耗散外加应变;而在高加载速率条件下,由于单一剪切带不能将应变耗散掉,因此需要更多的剪切带参与变形。为了进一步解释这一锯齿流变行为,本研究采用遍历处理对每个锯齿的应变突变进行了统计分析。结果表明,在不同的加载速率下,小的应变突变服从幂律分布,且幂指数为1.45;而大的应变突变则呈现指数衰减规律。最后,借助硬度对应变速率的敏感性,估算了镁基非晶合金在纳米压入条件下剪切转变区的体积,为4.5 nm3。

Mg-based bulk metallic glass under tension test or compression test will show significant brittleness in macroscope, leading to the difficulty to study the plastic deformation behavior. With high spatial and temporal resolution instrumented nanoindentation, an experimental investigation into the serrated flow of Mg-based metallic glass at different loading rate was conducted. The results showed that low loading rate promotes conspicuous pop-in events and high loading rate suppresses pop-in behavior. The reason is that a single shear band is sufficient to render the plastic strain at low loading rate but multiple shear bands are required at high loading rate. To gain a better understanding of the mechanism despite the characteristic lack of periodicity in the intermittent pop-in events, ergodic processing was introduced to demonstrate the statistic distributions of the strain burst size. The results showed that the smaller strain bursts are more probable and follow a power-law distribution and the exponent is constantly about 1.45 at all experimental loading rates. For a given loading rate, with the strain burst size increasing, the distributions of the strain burst sizes do not follow a power-law distribution but decrease exponentially in probability. Finally, based on the strain rate sensitivity on hardness, we estimated the volume of the shear transformation zone, its value is 4. 5 nm^3.