Heat treatment of hot-isostatic-pressed 60NiTi shape memory alloy: Microstructure, phase transformation and mechanical properties

doi:10.1016/j.jmst.2021.10.005

Abstract

Abstract:

60NiTi alloy is considered to be a promising material for specialized bearing and gear applications due to its high hardness, strength, and low modulus. However, fabricating 60NiTi through conventional processing methods is challenging due to the brittleness and poor workability. In this study, 60NiTi with high relative density was successfully fabricated directly from pre-alloyed powder through hot isostatic pressing. The effects of solution and aging treatments on microstructure and mechanical properties were systematically studied by advanced characterization techniques. The hot-isostatic-pressed 60NiTi showed low average hardness and elastic strain due to the formation of a soft Ni₃Ti phase and B2 NiTi matrix. Solution treatment above 1000 ℃ dissolved the Ni₃Ti phase and promoted the formation of nanoscale Ni₄Ti₃ precipitates, which significantly improved the hardness, strength, and elastic strain of 60NiTi. The formation of the Ni₄Ti₃ phase can be mainly attributed to the driving forces induced by the chemical supersaturation and mechanical stress concentration. Finally, the phase transformation mechanisms during heat treatment and compression test were discussed.

Key words: Ni-rich 60NiTi, Hot isostatic pressing, Heat treatment, Phase transformation, Mechanical properties

Yinghao Zhou, Xiyu Yao, Wenfei Lu, Dandan Liang, Xiaodi Liu, Ming Yan, Jun Shen. Heat treatment of hot-isostatic-pressed 60NiTi shape memory alloy: Microstructure, phase transformation and mechanical properties[J]. J. Mater. Sci. Technol., 2022, 107: 124-135.

Figures/Tables 17

Table 1 Chemical composition of the 60NiTi.

Ti	Ni	N(ppm)	O(ppm)
Bal.	59.8	36	381

Fig. 1. Binary phase diagram of Ni-Ti.

Table 2 The heat treatment parameters adopted in this study.

Sample	Solution condition	Cooling way	Aging condition	Cooling method	Sample state
ST1000	1000 ℃/1 h	WQ	-	-	Free from cracks
ST1050	1050 ℃/1 h	WQ	-	-	Cracked
ST1100	1100 ℃/1 h	WQ	-	-	Cracked
ST1000+AT400	1000 ℃/1 h	WQ	400 ℃/4 h	WQ	Free from cracks

Fig. 2. Characterization of the 60NiTi pre-alloyed powder: (a)-(b) SEM morphology and XRD spectrum of the 60NiTi pre-alloyed powder, respectively, (c) TEM bright field image showing the microstructure inside the powder, and (d) SAED pattern obtained along [11¯1] B2 NiTi matrix, (e) high-resolution TEM showing the lattice fringe around Ni4Ti3 precipitate, (f) FFT image of the high-resolution TEM image shown in Fig. 2(e).

Fig. 3. Characterization of 60NiTi fabricated through HIP: (a) optical micrograph showing the pore defect, (b) XRD data showing the phase constitution, (c)-(d) SEM-BSE images showing the phase morphology.

Table 3 Element compositions and possible phases of the regions marked in Fig. 3 (at.%).

Position	Ni (at.%)	Ti (at.%)	Possible phase
A	50.73	49.27	NiTi
B	75.21	24.79	Ni₃Ti
C	75.74	24.26	Ni₃Ti
D	75.09	24.91	Ni₃Ti
E	34.61	65.39	Ti₂Ni
F	59.26	40.74	Ni₃Ti₂

Fig. 4. EBSD analyses of HIP-fabricated 60NiTi: (a) EBSD orientation map of 60NiTi, the color codes representing the crystal orientations, (b) phase map showing the distribution of different kinds of phases and their contents, (c)-(e) grain sizes of NiTi, Ni3Ti, and Ni3Ti2, respectively.

Fig. 5. Mechanical properties of HIP-fabricated 60NiTi: (a) nano-indentation curves of the phases in 60NiTi, (b) compression stress-strain curve, (c) strain distribution maps under different strains obtained through DIC.

Table 4 Summary of the elastic modulus, nano-hardness, and Vickers hardness of the phases in 60NiTi.

Phase	Elastic modulus (GPa)	Nano-hardness (GPa)	Calculated Vickers hardness	Vickers hardness (HV0.5)
NiTi(B2)	72.83	4.07	376.23	366.25
Ni₃Ti	85.80	4.70	434.47	395.40
Ni₃Ti₂	94.17	5.89	544.47	487.93

Fig. 6. Characterization of heat-treated 60NiTi: (a) macroscopic morphology showing cracks after heat treatments, (b) XRD spectra showing the phase constitution of the heat-treated 60NiTi samples, (c) phase content of heat-treated 60NiTi calculated through XRD refinement.

Fig. 7. Microstructure of heat-treated 60NiTi: (a)-(c), (d)-(f) SEM-BSE images, EBSD orientation maps, and phase maps of the ST1000 and ST1000+AT400 60NiTi samples, respectively.

Fig. 8. STEM analyses of the solution-treated 60NiTi: (a)-(b) HAADF and corresponding BF images of the solution-treated 60NiTi, (c) high-magnification HAADF image showing the dislocation distribution, (d) SAED pattern taken from [11¯1]B2 NiTi matrix phase. The yellow circles highlight Ni4Ti3 reflections, and red circles highlight the R-phase reflections.

Fig. 9. STEM analyses of 60NiTi subjected to age treatment: (a)-(b) HAADF and corresponding BF images of the 60NiTi sample subjected to age treatment, (c) high-magnification HAADF image showing the morphology of the Ni4Ti3 precipitates, (d) SAED pattern obtained from [1$\bar{1}$1] B2 NiTi matrix phase. The yellow circles highlight Ni4Ti3 reflections.

Fig. 10. Mechanical properties of heat-treated 60NiTi: (a) Variation in Vickers hardness at different Ni4Ti3 precipitate contents (the black line represents Vickers hardness, and the red line represents the Ni4Ti3 precipitate content), (b) compression stress-strain curves, (c) and (d) strain distribution maps of the ST1000 and ST1000+AT400 samples subjected to different levels of strain obtained through DIC.

Fig. 11. DSC curves of the HIP-fabricated and heat-treated 60NiTi samples.

Table 5 Summary of elastic modulus, yield strength, and compression strength of 60NiTi.

Sample		Elastic modulus (GPa)	Yield strength (MPa)	Strain(%)	Compression strength (MPa)
HIP	Stage I	93.72	430.57	0.70	-
	Stage II	25.96	2058.67	7.45	-
	Stage III	-	-	9.44	2683.82
ST1000		99.59	2362.76	4.10	2900.96
ST1000+AT400		88.01	2731.69	4.83	3005.88

Fig. 12. STEM analyses of the 60NiTi after fracture: (a) STEM-HAADF image of the HIP-fabricated 60NiTi, the inset showing the SAED pattern of B19’ phase, (b) SAED patterns obtained from the areas in Fig. 12(a) by the red circles, (c)-(d) STEM-HAADF and BF images of the ST1000 sample after fracture, (e) STEM-HAADF image of the ST1000-AT400 sample after fracture, (f)-(g) SAED patterns of the ST1000 and ST1000-AT400 samples after fracture, respectively.

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