The R-phase transformation presents low thermal hysteresis, high stability against thermal cycling and ultrahigh internal friction (IF). However, a clear explanation for the occurrence of the R-phase after low temperature aging and systematic characterization of the condition are still lacking. In this study, the evolution of the phase transformation behaviors and IF values after aging at 250 °C for different times are investigated in a Ni_50.9Ti_49.1 alloy. Direct experimental evidence for the microstructure evolution of the B2 phase and the corresponding R-phase is provided utilizing in-situ transmission electron microscopy (TEM) techniques, together with geometric phase analysis (GPA). The reason for the occurrence of a nanodomain-structured R-phase after low temperature precipitate-free aging and the mechanism of ultrahigh intrinsic IF are illustrated. The results show that Ni segregation indicated by the localized strain fields is responsible for the formation of the R-phase after low temperature aging. Longer aging time causes larger element heterogeneity in the matrix, as a result, a wider existing temperature window for the R-phase. The ultrahigh intrinsic IF plateaus (IF_Int = 0.120 ∼ 0.183) found in NiTi shape memory alloys (SMAs) are predominantly determined by the volume fraction of nanodomain boundaries. These findings provide basic insights into the R-phase formation mechanism and provide a simple way to adjust ultrahigh IF performance suitable for different application scenarios.

R相转变具有低热滞后、高热循环稳定性和超高内摩擦（IF）。然而，仍然缺乏对低温老化后 R 相发生的明确解释和对该条件的系统表征。_{在这项研究中，在 Ni 50.9} Ti _49.1中研究了在 250 °C 时效不同时间后相变行为和 IF 值的演变。合金。利用原位透射电子显微镜 (TEM) 技术和几何相分析 (GPA) 提供了 B2 相和相应 R 相的微观结构演变的直接实验证据。阐述了低温无析出物时效后产生纳米畴结构R相的原因和超高本征IF的机理。结果表明，由局部应变场指示的Ni偏析是低温时效后R相形成的原因。较长的老化时间导致基体中较大的元素异质性，因此，R 相的现有温度窗口较宽。超高固有 IF 平台（IF _Int = 0.120 ∼ 0.183) 在 NiTi 形状记忆合金 (SMA) 中发现主要由纳米域边界的体积分数决定。这些发现提供了对 R 相形成机制的基本见解，并提供了一种简单的方法来调整适合不同应用场景的超高 IF 性能。