Understanding of multi-stage R-phase transformation in aged Ni-rich Ti–Ni shape memory alloys
Introduction
Ti–Ni alloys of near-equiatomic composition are technologically important materials because they combine excellent functional properties (shape memory effect, superelasticity) with good mechanical strength and ductility [1]. It is known that near-equiatomic Ti–Ni alloys may undergo three different martensitic transformations depending on thermomechanical treatment condition: B2-R, R-B19′ and B2-B19′, which are the key to the above functional properties and have been extensively discussed in literature [2], [3], [4], [5], [6]. These three kinds of transformations are characterized by different features of transformation strain and hysteresis. B2-R, called R-phase transformation, exhibits small transformation strain (∼1%) and hysteresis (∼2–5 K). In contrast, R-B19′ and B2-B19′ are characterized by much larger transformation strain (∼10%) and hysteresis (∼20–70 K).
Unlike, fully annealed and quenched near-equiatomic Ti–Ni alloys, which transform from B2 to B19′ directly, the aged Ni-rich Ti–Ni alloys normally transform in two stages (B2-R-B19′), and thus, show two distinct peaks on differential scanning calorimetry (DSC) curves, which are well documented in literature [7]. In addition to the generation of R-phase transformation, an abnormal three-stage (one-stage R and two-stage B19′) martensitic transformation in intermediate-temperature-aged Ni-rich Ti–Ni alloys was reported from time to time. The debate about the origin of this three-stage martensitic transformation has been going on for a long time and several explanations were given based on different experimental observations [1], [8], [9], [10], [11], [12]. Recently, by further studies, Fan et al. proved that those preceding mechanisms are not the ultimate origin of this abnormal phenomenon and they attributed it to the inhomogeneous precipitate distribution between grain boundary and grain interior [13]. In addition, three-stage martensitic transformation are identified to be B2-R-B19′ in grain boundary region, and B2-B19′ in grain interior by doing partial differential scanning calorimetry cycles. This scenario successfully explained this abnormal phenomenon and is consistent with all the existing data reported in literature.
As mentioned in the above, both the normal two-stage martensitic transformation and the abnormal three-stage transformation of intermediate-temperature-aged alloys involve only one stage of R-phase transformation. However, an abnormal two-stage R-phase transformation was found in Ti–50.9 at% Ni polycrystals aged at low temperatures (250–300 °C). By investigation with DSC, X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques, Kim et al. attributed it to the local stress and composition heterogeneity between Ti3Ni4 particles [14]. Apparently, this mechanism expects that this strange phenomenon is intrinsic for all Ti3Ni4-containing samples and is independent of whether or not the sample is single crystal or polycrystal. As reported in the following, this mechanism is not consistent with the fact that 250 °C-aged Ti–50.6 at% Ni (this will be abbreviated as 50.6Ni, hereafter) single crystals all undergo normal one-stage R-phase transformation in our experiment.
Since, grain boundary plays an important role in the occurrence of abnormal three-stage martensitic transformation, as Fan et al. pointed out, we consider that this abnormal two-stage R-phase transformation may be due to the existence of grain boundary. Based on this idea, we designed the following experiment. At first, low-temperature aging is done with 50.6Ni single crystals, which is the crucial test for the mechanism based on the local heterogeneity of composition and stress between Ti3Ni4 particles. Then, the same aging treatment is done with 50.6Ni polycrystals to clarify the effect of grain boundary on this abnormal phenomenon. As can be seen in the following, our experimental results direct to a simple explanation for the abnormal two-stage R-phase transformation in low-temperature-aged alloys. In addition, it is suggested that there is a unified scenario for both the abnormal three-stage martensitic transformation in intermediate-temperature-aged alloys and the above two-stage R-phase transformation in low-temperature-aged alloys.
Section snippets
Experimental procedure
In the present study, 50.6Ni single crystals and polycrystals were used and Ni-content of the samples was determined by chemical analysis.
All the samples were small plates made by spark-cutting, which were about 3 mm × 3 mm × 1 mm. In order to remove the affected surface layer, samples were mechanically polished and chemically etched. Then they were sealed into quartz tubes filled with argon (about 1.5 × 103 Pa) and Ti-getter was also sealed into the tubes to avoid oxidation during the subsequent heat
DSC results of the 50.6Ni single crystals aged at 250 °C
Fig. 1 shows the transformation behaviors of 50.6Ni single crystals aged at 250 °C for different time. It can be seen that 50.6Ni single crystals undergo normal two-stage transformation (B2-R-B19′), both on cooling and heating, except for the one aged for 1 h, showing only one-stage transformation (B2-R). Therefore, all single crystal samples undergo normal one-stage R-phase transformation, not two-stage R-phase transformation. This important finding proves that the local heterogeneity of
Origin of the abnormal two-stage R-phase transformation behavior
Concerning the origin of two-stage R-phase transformation, Jim et al. speculated that the local heterogeneity of composition and stress between Ti3Ni4 particles is responsible for it. This mechanism implies that two-stage R-phase transformation is intrinsic for all Ti3Ni4-containing samples; no matter they are single crystals or polycrystals. However, all 50.6Ni single crystals, in our experiment, show normal one-stage R-phase transformation, independent of aging time. This important result
Conclusions
In order to find the origin of abnormal two-stage R-phase transformation in low-temperature-aged Ni-rich Ti–Ni alloys, we investigated the transformation behaviors of 50.6Ni single crystals and polycrystals, which were aged at 250 °C after solution treatment, with DSC technique. We obtained following conclusions.
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All 50.6Ni single crystals undergo normal one-stage R-phase transformation. This indicates that the local heterogeneity of composition and stress between Ti3Ni4 particles is not
Acknowledgements
The authors thank G. Ji, Y. Wang for fruitful discussions. This work was supported by Sakigake-21 of JST, Kakenhi of JSPS and a special fund for Cheungkong professorship, National Science Foundation of China, as well as National Basic Research Program of China under Grant No. 2004CB619303.
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