Abstract The study is focused to understand the micro-mechanisms that govern the deformation behavior of a Ni-rich NiTi alloy at the sub-micron scale. This alloy undergoes reversible stress induced martensitic transformation leading to pseudoelasticity. Correspondingly, significant strain recoverability is noted. In this investigation, nanoindentation is utilized as the primary experimental tool. Various parameters including the indenter tip configuration, size and applied load levels are varied systematically. It is observed that compared to the most commonly used sharp Berkovich indenter tip, blunt spherical tip imposes reduced amount of strain and strain-gradient within the indentation volume. This opens up the window for systematically varying the sequential deformation modes in NiTi. Further in-depth analysis revealed that optimum combination of indenter tip configuration, size and applied load level is prerequisite to appreciate the pseudoelastic mechanism in NiTi. Most importantly, a tailored and simplified protocol is formulated for converting nanoindentation load-displacement response to corresponding indentation-stress-strain behavior for pseudoelastic alloy. These indentation-stress-strain curves featuring the signature deformation characteristics of NiTi alloy are realized without utilizing any specialized experimental modes. In a nutshell, this study highlights the importance of choosing adequate nanoindentation parameters for assessing the functional characteristics of pseudoelastic NiTi system at sub-micron scale. Accordingly, overall localized deformation behavior of this unique alloy is investigated.

中文翻译：

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使用纳米压痕评估 NiTi 基形状记忆合金的小尺度变形特性和应力应变行为

摘要 本研究的重点是了解在亚微米尺度上控制富镍 NiTi 合金变形行为的微观机制。这种合金经历可逆的应力诱导马氏体转变，导致伪弹性。相应地，注意到显着的应变恢复性。在这项研究中，纳米压痕被用作主要的实验工具。包括压头配置、尺寸和施加的载荷水平在内的各种参数系统地变化。据观察，与最常用的尖锐 Berkovich 压头尖端相比，钝球形尖端在压痕体积内施加了减少的应变和应变梯度。这为系统地改变 NiTi 中的顺序变形模式打开了窗口。进一步的深入分析表明，压头尖端配置、尺寸和施加的载荷水平的最佳组合是了解 NiTi 伪弹性机制的先决条件。最重要的是，制定了一个定制和简化的协议，用于将纳米压痕载荷-位移响应转换为伪弹性合金的相应压痕-应力-应变行为。这些具有 NiTi 合金标志性变形特征的压痕-应力-应变曲线是在不使用任何专门的实验模式的情况下实现的。简而言之，本研究强调了选择足够的纳米压痕参数以评估亚微米级伪弹性 NiTi 系统功能特性的重要性。因此，研究了这种独特合金的整体局部变形行为。