Elsevier

Intermetallics

Volume 131, April 2021, 107114
Intermetallics

Improved superelastic stability of NiTi shape memory alloys through surface nano-crystallization followed by low temperature aging treatment

https://doi.org/10.1016/j.intermet.2021.107114Get rights and content

Highlights

  • Functional stability is improved by ultrasonic shot peening and subsequent aging.

  • Ultrasonic shot peening generates dislocation networks in grain interior.

  • Ultrasonic shot peening generates nano-grains at the surface region.

  • Nano-grains and dislocation networks assist the precipitation process.

  • Homogeneous distribution of Ni4Ti3 particles is expected after aging.

Abstract

In this work, we proposed a process to improve the functional stability of Ni-rich NiTi alloys, which combines surface nano-crystallization by means of ultrasonic shot peening and subsequent low-temperature (523 K) aging treatment. The as-annealed coarse-grained NiTi plates were fist subjected to ultrasonic shot peening, which generates nano-grains at the surface region, and introduces dislocation networks in the interior of the NiTi sample. Both the grain boundaries and the dislocations could serve as the heterogenous nucleation sites for Ni4Ti3 nanoprecipitates during subsequent low-temperature aging treatment. Therefore, with the presence of nano-grains and dislocation networks, a homogeneous distribution of Ni4Ti3 nanoprecipitates in the sample is expected, which will strengthen largely the NiTi matrix and resist the plastic activities during the repetitive martensitic transformation. As a result, the improved stability during cyclic superelastic loading is achieved.

Introduction

Near-equiatomic NiTi shape memory alloy undergoes a reversible thermo-elastic martensitic transformation between a B2-structured austenite phase (A) and a B19′-structured martensite phase (M), leading to the unique shape memory effect and superelasticity, which enable the NiTi alloys to recover from large deformation of up to 10% [1,2]. Due to the unique functional properties and their excellent mechanical properties [3], NiTi shape memory alloys are attracting increasing interest in practical applications [[4], [5], [6], [7]].

NiTi devices normally undergo a cyclic A↔M transformation during practical utilization, especially in the occasions that NiTi alloys work as actuators [[8], [9], [10]]. The plastic activities associated with the martensite transformation give rise to the plastic strains, which could not be recovered during subsequent heating or unloading. The plastic strain will accumulate with the increase of the number of A↔M transformation cycles, leading to the degradation of functional properties (i.e. functional fatigue) [[11], [12], [13], [14], [15], [16]]. Therefore, suppressing the plastic strain during A↔M transformation is essential to improve the functional stability of NiTi alloys. Strengthening the NiTi matrix to improve yield strength is considered as the main approach to postpone the plastic deformation during loading. Among all existing methods to strengthen metallic materials, grain refinement and precipitation hardening have been addressed to improve the functional stability of NiTi alloys [[17], [18], [19], [20], [21], [22]].

Grain refinement has been frequently employed to improve the yield strength of metallic materials [[23], [24], [25], [26]]. Many researchers have shown that ultrafine grains with average diameter of <100 nm are necessary to improve the functional stability of NiTi alloys during superelastic cycling [20,[27], [28], [29]]. Generally, nano-grained NiTi alloys are obtained through sever plastic deformation (e.g. equal channel angular extrusion [30], cold drawing [18], cold rolling [31]) followed by post-deformation annealing [18,[30], [31], [32], [33], [34]]. However, it is quite challenging to apply severe plastic deformation on NiTi alloys, due to the rapid work hardening, high strength and toughness associated with NiTi alloys [35,36]. Moreover, it is rather difficult to perform severe plastic deformation on the large-scale NiTi structures or the NiTi structures with complex geometry, e.g. fabricated by welding or additive manufacturing [37].

Precipitation hardening is another efficient method to strengthen metallic materials [[38], [39], [40]]. In Ni-rich (i.e. Ni content >50.0 at.%) NiTi shape memory alloys, Ni4Ti3 precipitates could be introduced by aging between 473 and 873 K [1,[41], [42], [43], [44], [45], [46], [47], [48]]. The efficiency of precipitation hardening depends highly on the size, density and distribution of Ni4Ti3 particles in the matrix. Generally, homogeneously distributed dense nano-precipitates are optimal for strengthening NiTi matrix. However, in previous studies, a high temperature (e.g. 1073–1273 K) solution treatment were normally required before the subsequent aging treatment [37,42,49]. The high temperature heat treatment will coarsen the microstructure, leading to the formation of grains with average size of several tens of microns [37,42,50]. According to the classical theory of solid-solid phase transformation [51], grain boundaries could act as the nucleation sites and promote the precipitation of the secondary phase in the vicinity of grain boundaries. Therefore, a heterogeneous distribution of Ni4Ti3 precipitates has been frequently observed in coarse-grained Ni-rich NiTi alloys, due to the different precipitation behavior between grain boundary region and grain interior [37,41,42,49,50]. The low density of precipitates in grain interior will undermine the precipitation hardening effect. Moreover, the heterogeneously distributed of Ni4Ti3 precipitates could disturb the transformation sequence, leading to the appearance of multiple-step transformation [[52], [53], [54]]. For instance, a three-step transformation including an A→R transformation and two B19’ martensite with different transformation temperature has been frequently reported in Ni-rich NiTi alloys after aging at intermediate temperatures (e.g. 673–823 K) [37,42,49]. The complex transformation sequences will lead to a stepwise shape recovery over a wide temperature range. This will limit the application of NiTi alloys as actuators, which normally requires a complete shape recovery at certain temperature.

In our previous studies [19,50,55], we found that grain size could influence largely the aging microstructure and thus the phase transformation behavior and functional properties. The microstructure with homogeneously distributed Ni4Ti3 nanoprecipitates has been obtained by low-temperature (523 K) aging of Ni-rich NiTi alloys with small grains (e.g. average grain size of 1.7 μm) [19,50,55]. It was found that the function stability benefits largely from the homogeneously distributed Ni4Ti3 nanoprecipitates [19].

As discussed above, refined grains seem to be critical to improve the functional stability of NiTi alloys by means of either grain refinement or precipitation hardening. Conventionally, cold deformation combined with the subsequent post-deformation annealing treatments are employed to refine the microstructure of NiTi alloys [50,56]. However, plastic deformation is not always practicable, for instance, on the complex NiTi alloy structures fabricated by additive manufacturing or by welding. In one of our previous work [57], an approach was developed to improve the functional stability of coarse-grained NiTi alloys, which combines repetitive A↔M transformation and subsequent low-temperature aging treatment. Dislocation networks were introduced in the grain interior by rapid repetitive A↔M transformation between boiling water and liquid nitrogen. The dislocations serve as the nucleation sites for Ni4Ti3 nanoprecipitates during low-temperature (523 K) aging treatment, assisting the formation of the microstructure with homogeneously distributed nanoprecipitates. The stability of superelasticity was improved largely based on the unique microstructure [57]. However, this method is not easy to conduct on the large scale NiTi structures, since a rapid cooling-heating process is required to generate dislocation networks in grain interior.

Inspired by the above findings, in this work, we propose a feasible approach to improve the functional stability of coarse-grained NiTi alloys, which combines a surface nano-crystallization process by means of ultrasonic shot peening and subsequent low-temperature (523 K) aging treatment. Ultrasonic shot peening could generate nano-grains at the surface region and introduce dislocations in the interior of coarse grains underneath the surface. Both the nano-grains and dislocation networks could assist the precipitation process, leading to the homogeneous distribution of Ni4Ti3 nanoparticles. As a result, the improvement of functional stability of NiTi alloys could be expected.

Section snippets

Materials and methods

A commercial cold rolled NiTi plate with a nominal composition of Ti-50.8 at.% Ni and thickness of 0.5 mm was used in this work. The dog-bone tensile samples with gauge length of 20 mm and gauge width of 3 mm were cut from the as-received NiTi plates using wire electrical discharge machining. The tensile samples were annealed at 973 K for 1 h in argon atmosphere, followed by water quenching at room temperature (around 300 K). Afterwards, the annealed samples were subjected to ultrasonic shot

Microstructure

The etched microstructure of the as-annealed and USP-treated Ti-50.8 at.% Ni samples are shown in Fig. 2. After annealing at 973 K for 1 h, the sample contains mainly coarse equiaxed grains, as shown in Fig. 2a. According to the SEM images, ultrasonic shot peening treatment does not affect the grain size in the interior of the sample. Although the USP treatment mainly influences the surface of the NiTi plate, the microstructure in the interior of the sample may be also affected, since the

Discussions

The degradation of superelasticity is one of the main issues that limits the wide application of NiTi shape memory alloys, especially in the occasion requires cyclic actuations, e.g. actuators. It is well accepted that the plastic activities associated with the A↔M transformation is the main reason for the degradation of superelasticity in NiTi shape memory alloys [[13], [14], [15], [16]]. Therefore, suppressing the plastic deformation is essential to improve the functional stability of NiTi

Conclusions

In this work, we show that the functional stability of a coarse-grained Ni-rich NiTi alloy could be improved largely by a simple approach featured with ultrasonic shot peening treatment followed by low-temperature aging treatment. Apart from a thin layer with nanograins at surface, ultrasonic shot peening could also generate dislocation networks in the coarse grains in interior of NiTi plate. The presence of dislocation networks in coarse grains could assist the nucleation of nanoprecipitates,

CRediT authorship contribution statement

Xiaoqiang Li: Methodology, Investigation, Writing - original draft, Visualization. Hu Chen: Methodology, Investigation, Validation. Weimin Guo: Conceptualization, Methodology, Validation, Writing - review & editing. Yanjin Guan: Methodology, Resources, Writing - review & editing, Supervision. Zuocheng Wang: Conceptualization, Writing - review & editing, Supervision. Qingkai Zeng: Methodology, Resources, Writing - review & editing. Xiebin Wang: Conceptualization, Methodology, Validation,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by National Key R&D Program of China (Grant No.: 2018YFB1105100), the National Natural Science Foundation of China (NSFC, Grant No. 51905310), Shandong Provincial Natural Science Foundation, China (Grant No.: ZR2018QEM001), Natural Science Foundation of Jiangsu Province (Grant No.: BK20180231), Key Research and Development Program of Shandong Province (2019GGX104065), the Young Scholars Program of Shandong University (Grant No.: 2018WLJH24), the Foundation of Key

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