Triaxial tension-induced damage behavior of nanocrystalline NiTi alloy and its dependence on grain size

doi:10.1016/j.jmst.2020.10.041

Abstract

Abstract:

This study focused on the effect of grain size (GS) on dynamic damage performance of nano-crystalline nickel titanium (NC NiTi) alloy. Molecular dynamics simulations were conducted to triaxially expand it at a high strain rate (4 × 10⁹ s^-1), while the temperature and initial pressure remained 300 K and 0 bar, respectively. It was discovered that the superelastic NiTi alloy exhibited the similar damage response as ductile metallic materials, which was vividly characterized by void nucleation, growth, and coalescence. The stress-strain curves demonstrated that the void nucleations always occurred near the start of the strain softening region at various grain sizes. Interestingly, it was discovered that the void evolution was characteristic of an almost double-linear behavior, and the piecewise linearity became more prominent for the void volume fraction increase at larger grain size. More importantly, the fracture behavior was found to be strongly dependent upon the grain size in the NC NiTi alloy. For small grain size, the existing voids propagated along the grain boundaries and in the grains, leading to intergranular and transgranular fracture. Contrarily, the intergranular-dominated fracture was responsible for the void propagation in the large grain. In addition, the starting time, ending time, and threshold of void nucleation were found to be weak sensitivity to GS, and a reverse effect was appropriate to the void growth. The results highlighted that as the GS increased, more complete stress relaxation and shorter duration time were produced, leading to larger void volume fraction and faster growth rate.

Key words: NC NiTi alloy, Void volume fraction, Fracture behavior, Grain size effect, Molecular dynamics modeling

Fang Wang, Liu He, Xiangguo Zeng, Zhongpeng Qi, Bo Song, Xin Yang. Triaxial tension-induced damage behavior of nanocrystalline NiTi alloy and its dependence on grain size[J]. J. Mater. Sci. Technol., 2021, 77: 90-99.

Figures/Tables 18

Fig. 1. Weibull plot of the grain size of the NC NiTi alloy prepared by annealing treatment at 350℃.

Table 1. Mean grain size of various NiTi specimens.

Group number	1	2	3	4	5	6	7	8
P	0.03	0.10	0.24	0.40	0.60	0.87	0.94	0.99
Grain size (nm)	3.50	5.42	7.59	9.45	11.83	15.39	17.23	20.05

Fig. 2. Initial configurations of the NiTi alloy with average grain size of 9.45 nm. The atoms are colored by (a) particle identifiers and (b) common neighbor analysis (CNA).

Fig. 3. Flowchart of the post-process.

Fig. 4. Tensile stress and void volume fraction profiles for the NiTi alloy with average grain size of (a) 5.42 nm and (b) 20.05 nm, respectively.

Table 2. Starting time (Ts), ending time (Te), and void nucleation and growth thresholds of the NiTi alloys.

Average grain size (nm)		3.50	5.42	7.59	9.54	11.83	15.39	17.23	20.05
Nucleation stage	T_s(ps)	23.6	22.9	22.3	22.2	22.0	21.8	21.7	21.6
	T_e(ps)	26.5	26.0	25.4	25.2	25.1	25.0	24.5	24.4
	P_n(GPa)	-17.3	-17.4	-17.4	-17.4	-17.4	-17.4	-17.4	-17.4
Growth stage	T_s(ps)	26.5	26.0	25.4	25.2	25.1	25.0	24.5	24.4
	T_e(ps)	38.0	36.5	35.8	35.5	35.0	34.4	33.7	33.5
	P_g(GPa)	-2.5	-2.3	-2.2	-2.0	-1.4	-1.3	-0.9	-0.5

Fig. 5. Stress-strain response combined with dynamic damage performance of the NiTi alloy with average grain size of 5.42 nm under triaxial tension. Void volume fraction and applied strain are shown at different stages during the loading. The purple parts in the inset figures represent surfaces of the voids.

Fig. 6. Stress-strain response combined with dynamic damage performance of the NiTi alloy with average grain size of 15.39 nm under triaxial tension. Void volume fraction and applied strain are shown at different stages during the loading. The purple parts in the inset figures represent surfaces of the voids.

Fig. 7. Stress-strain response combined with dynamic damage performance of the NiTi alloy with average grain size of 20.05 nm under triaxial tension. Void volume fraction and applied strain are shown at different stages during the loading. The purple parts in the inset figures represent surfaces of the voids.

Fig. 8. Distribution of the strain in the materials with various grain sizes.

Fig. 9. Tensile stress profiles for the NiTi alloys with average grain size of (a) 3.50-9.45 nm and (b) 11.83-20.05 nm.

Fig. 10. Overall void volume fraction as function of time at different grain sizes.

Fig. 11. External snapshots of void distributions at 5.42 nm, 15.39 nm and 20.05 nm, respectively. Atoms of grains with different sizes were colored by particle identifiers.

Fig. 12. Cross-sectional snapshots of void distributions at 5.42 nm, 15.39 nm, and 20.05 nm, respectively. Atoms of grains with different sizes were colored by particle identifiers.

Fig. 13. Fracture performance involved in the materials with various grain sizes.

Fig. 14. Threshold difference and total duration time for void nucleation and growth at different grain sizes.

Fig. 15. Three-dimensional surface diagram of potential energy varying with grain size and time.

Table 3. Characteristic values of potential energy during the void evolution.

Average grain size (nm)		3.50	5.42	7.59	9.54	11.83	15.39	17.23	20.05
Potential energy (10⁶ eV)	Maximum	-9.146	-9.194	-9.248	-9.284	-9.302	-9.326	-9.328	-9.330
	Minimum	-9.581	-9.637	-9.701	-9.753	-9.798	-9.839	-9.861	-9.884
	Difference	0.435	0.443	0.453	0.469	0.496	0.513	0.533	0.554

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