Transformation of the TiNi Alloy Microstructure and the Mechanical Properties Caused by Repeated B2-B19′ Martensitic Transformations

doi:10.1007/s40195-015-0317-6

Abstract

Abstract:

The influences of thermal cycling treatment in the temperature range of B2-B19 martensitic transformations (-150 to 150 °C) on the TiNi alloy structure and properties were studied. Different states named the initial coarse-grained (CG) state, the ultrafine-grained (UFG) state after ECAP (with a grain size of 200 nm), the state after ECAP and cold upsetting by 30% were considered. The results show that the microhardness and the strength increase in all the three states. According to the XRD analysis, a more significant increment in the dislocation density, resulting from thermal cycling, is observed in the UFG alloy than in the CG alloy.

Key words: Thermal cycling, TiNi alloy, Equal-channel angular pressing, Martensitic transformation

Anna Churakova, Dmitry Gunderov, Alexander Lukyanov, Niklas Nollmann. Transformation of the TiNi Alloy Microstructure and the Mechanical Properties Caused by Repeated B2-B19′ Martensitic Transformations[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(10): 1230-1237.

Figures/Tables 9

Fig. 1 Structures of Ti49.3Ni50.7 in different states: a CG, b CG + TC (OM), c CG (TEM), d CG + TC (n = 20), e CG + TC (n = 50), f CG + TC (n = 100)

Fig. 2 Microstructures of Ti49.3Ni50.7 in the UFG state: a UFG, b UFG + TC (n = 20), c UFG + TC (n = 50), d UFG + TC (n = 100)

Fig. 3 Structures of Ti49.3Ni50.7: a ECAP + cold upsetting, b ECAP + cold upsetting and TC (n = 20), c ECAP + cold upsetting and TC (n = 50), d ECAP + cold upsetting and TC (n = 100)

Table 1 Parameters of the Ti49.3Ni50.7 structure according to X-ray analysis

State	FWHM (110)	CSR (nm)	E (%)	ρ × 10¹⁵ (m^-2)
CG	0.282	158	0.122	0.8
CG + TC	0.347	49	0.221	1.8
Δ	0.065	109	0.099	1
UFG	0.327	43	0.250	5.8
UFG + TC	0.440	40	0.465	9.6
Δ	0.113	3	0.215	3.8
UFG + upsetting	0.96	31	0.647	16
UFG + upsetting + TC	1.13	27	0.854	22
Δ	0.17	4	0.207	6.2

Fig. 4 X-ray diffraction patterns of the alloy Ti49.3Ni50.7: 1 CG, 2 CG + TC; 3 UFG, 4 UFG + TC; 5 UFG + cold upsetting, 6 UFG + cold upsetting + TC

Table 2 Characteristic temperatures of martensitic transformations

State	M_s	M_f	A_s	A_f	M_s - M_f	A_s - A_f
CG	-32	-62	-29	-5	30	24
CG + TC*	-6	-41	-39	-2	35	37
UFG	-23	-47	-11	3	24	14
UFG + TC*	27	-12	-3	32	39	35

Fig. 5 Microhardness plots under different states (1 CG, 2 UFG, 3 UFG + cold upsetting)

Table 3 Results of mechanical tensile tests of Ti49.3Ni50.7 in different states

State	UTS (MPa)	σ_m(MPa)	YS (MPa)	δ (%)	HV(MPa)
CG	820 ± 20	440 ± 10	520 ± 15	36 ± 2	2695 ± 70
CG + TC^*	900 ± 15	620 ± 10	620 ± 10	31 ± 2	3150 ± 75
UFG	1310 ± 20	450 ± 10	1130 ± 10	29 ± 1	3550 ± 90
UFG + TC^*	1360 ± 10	530 ± 15	1200 ± 15	31 ± 1	4070 ± 80
UFG + upsetting	1750 ± 20	-	1410 ± 20	22 ± 2	4860 ± 120
UFG + upsetting + TC^*	1910 ± 25	-	1550 ± 15	26 ± 3	5425 ± 95

Fig. 6 Stress-strain curves for Ti49.3Ni50.7: 1 CG, 2 CG+ TC; 3 UFG, 4 UFG + TC; 5 UFG + cold upsetting, 6 UFG + cold upsetting + TC

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