Effect of Severe Torsion Deformation on Structure and Properties of Titanium–Nickel Shape Memory Alloy

Abstract

In the present work, the possibility of applying severe torsion deformation (STD) to a bulk near-equiatomic NiTi shape memory alloy in order to accumulate super-high strain and improve mechanical and functional properties was studied. STD was performed using the multidirectional test system “BÄHR MDS-830” at a temperature of 500 °C (the upper border temperature for the development of dynamic polygonization) in 14 and 30 turns with accumulated true strain values of 4.3 and 9.1, respectively. Structural phase state and properties were studied using differential scanning calorimetry, X-ray diffractometry, transmission electron microscopy, hardness measurements, and thermomechanical bending tests. STD at 500 °C allowed for the accumulation of high strain without failure. As a result of STD in 30 turns, a submicrocrystalline structure with an average grain/subgrain size of about 500 nm was formed. This structure ensured the achievement of high maximum completely recoverable strain values of 6.1–6.8%. The results obtained show the prospects of applying severe torsion straining deformation to titanium nickelide in terms of forming an ultrafine-grained structure and high properties.

1. Introduction

Several intelligent devices operating on the basis of the shape memory effect are widely implemented in various applications in the fields of engineering and medicine [1,2,3]. Binary NiTi alloys provide remarkable shape memory characteristics in combination with extraordinary mechanical properties; therefore, they are the most widely used shape memory alloys (SMAs) for industrial purposes. Severe plastic deformation (SPD) is commonly applied to NiTi SMA for the formation of an ultrafine-grained (UFG) structure, e.g., submicrocrustalline (SMCS) and nanocrystalline structures (NCSs), and to improve the combination of its properties [4,5,6,7,8,9,10]. However, the formation of an NCS in bulk NiTi samples after SPD is a difficult scientific challenge [11,12]. Some SPD modes (high-pressure torsion, multipass cold rolling, and MaxStrain deformation) allow for the formation of NCSs in small samples only suitable for investigative purposes [7,8,13,14]. Other SPD modes, including equal channel angular pressing (ECAP), warm rotary forging, etc., allow for the formation of only SMC or NC structures with an average size of their structural elements (grains/subgrains) near 100 nm [14,15,16]. The application of common SPD modes is limited by another issue consisting of a noticeable decrease in ductility along with the improvement of strength characteristics. In addition, some special techniques such as friction welding and laser welding, which only affect the surface layer, with improved functional characteristics were applied to various Ti- and Ni-based alloys, including NiTi SMA [17,18,19,20].

One of the SPD modes that allows for the accumulation of high plastic strain before fracture in combination with a low impact on the shape and dimensions of a sample is severe torsion deformation (STD) [21,22,23,24]. The main drawback of deformation via torsion is the inhomogeneous strain distribution resulting in non-uniform work hardening and grain refinement. STD allows for the accumulation of high strain and the acquisition of a gradient hierarchical structure with considerably refined structural elements, which, as shown in [25,26,27], simultaneously provide high strength and ductility, demonstrating potential with respect to designing high-strength and -ductility materials. STD has been successfully applied to various metals and alloys, including Mg, Ni, Ti, steel, and high-entropy and medium-entropy alloys [19,20,21,22,23]. However, the application of STD to NiTi SMA with the accumulation of high strain at low deformation temperatures has not yet been studied.

Therefore, in this study, STD is applied to an NiTi SMA for the first time at a deformation temperature of 500 °C in attempt to achieve the maximum number of turns and thus accumulate super-high strain. This temperature was chosen because it is close to the upper border temperature for the formation of a dynamically polygonised dislocation substructure [5] that must provide the highest number of turns and facilitate the formation of a UFG structure. Finally, the features of structural formation and its effect on the mechanical and functional properties of near-equiatomic NiTi SMA after STSD are studied.

2. Materials and Methods

In the present study, a near-equiatomic NiTi shape memory alloy (50.0 at.% Ni) was used. The chemical composition of the alloy is shown in

Table 1. Chemical composition of the studied NiTi alloy [mass %].

Ni	Ti	C	O	N	H	Other
54.94	balance	<0.041	<0.040	<0.002	<0.003	<0.1

. NiTi rods with a diameter of 20 mm to 6 mm were rotary-forged at 800–900 °C at “Industrial Center MATEK SMA” Ltd., Moscow, Russia After the rotary forging, post-deformation annealing at 700 °C for 30 min followed by water cooling was employed, which served as a reference treatment (RT).

To induce torsional deformation, cylindrical specimens with a reduced gauge section for strain localization were prepared (Figure 1). The specimens, with a length of 130 mm and a central part with a length of 20 mm and a diameter of 4.5 mm, were cut from hot-forged rods with a diameter of 6 mm on a turning lathe.

STD was carried out on the multidirectional test system “BÄHR MDS-830” [28] at a temperature of 500 °C and a torsional rate of 0.32 turn/s, leading to a maximum strain rate of 0.1 s⁻¹ in 14 and 30 turns. Specimens were induction-heated with a heating rate of 5 °C/s and cooled by applying air from the compressed air supply to the deformed area. It was observed that the failure of the NiTi sample or the appearance of cracks did not occur, even after 30 turns of torsional deformation. The accumulated true strain e was about 4.3 and 9.1 for 14 and 30 turns, respectively. However, as it was already noted in the introduction, torsion deformation is not uniform. To study its structure and properties, a zone of the deformed NiTi sample corresponding to a half of a radius was selected.

The phase composition after STD was examined via X-ray diffractometry and differential scanning calorimetry (DSC). The X-ray diffraction studies were performed using a “DRON-3.0” diffractometer (U = 30 kV, I = 25 mA) employing CuKα radiation at room temperature. Starting and finishing temperatures of forward and reverse martensitic transformations (MTs)—A_s, A_f, M_s, and M_f after RT and STD—were estimated using DSC 3+ Mettler Toledo calorimeter at a cooling–heating rate of 10 K/min. The microstructure after deformation was directly observed using a transmission electron microscope (TEM) “JEM 200CX”. The analysis of brightfield images was performed at a magnification of ×15,000–×50,000.

The maximum complete recovery strain ${}_{r,1}^{max}$, which is the main shape recovery parameter, was determined using a thermomechanical method. This involved the induction of strain through bending at room temperature and heating for shape recovery, leading to the complete recovery of the original shape without residual strain [5,29,30]. The amount of induced strain was slowly increased from 0.5 % at steps of about 1% in order to define maximum completely recoverable strain.

To evaluate the change of mechanical properties after torsional deformation, the Vickers hardness HV₁ was measured. The measurements were performed at room temperature using the stationary hardness tester “Metolab 421” with a 1 kg load and a 10 s dwell time.

3. Results and Discussion

3.1. Phase Composition Study

X-ray diffractograms of the samples after the reference treatment (RT) and deformation via torsion for 14 and 30 turns at 500 °C are presented in Figure 2. The X-ray diffraction analysis shows that after the RT was applied, only monoclinic B19′-martensite was present, with some indications of the presence of residual B2-austenite and/or an intermediate R-phase. Generally, the application of torsional deformation did not change the phase composition at room temperature. Some background noise in the 2𝜃 angle range from 42° to 43°, which can be ascribed to the {110} peak of B2-austenite and the $\left(330\right)-\left(3\overline{3}0\right)$ doublet rhombohedral of the R-phase, was detected, but the main phase was still B19′ -martensite.

The widths of (020) _B19′ and (111)_B19′ -martensite X-ray lines’ degrees of angular shifting were examined to evaluate the extent of crystal lattice defects after deformation.

Deformation at 500 °C led to an increase in the (002)_B19′ and (111)_B19′ -martensite X-ray lines’ half-height widths from 0.5 and 0.52 to 0.64 and 0.67, respectively. The X-ray line width remained constant, with an increase in the number of turns that may indicate a steady-state stage of deformation reflecting an equilibrium between the dynamic hardening and softening processes. The positions of the X-ray peaks remained constant after deformation.

Calorimetric curves of the NiTi SMA before and after deformation are shown in Figure 3. In the initial state, both forward and reverse martensitic transformations proceeded in one stage: B2 → B19′ for cooling and B19′ → B2 for heating. Torsion deformation at 500 °C in 14 and 30 turns led to an increase in internal stresses in the alloy, which stimulated the flow of the forward MT through the intermediate R-phase according to the following two-stage mechanism: B2 → R → B19′ (Figure 3). The change in the stages of transformations was the result of an increase in the defectiveness of the structure due to phase and deformation hardening [31,32,33,34,35]. The reverse MT after deformation via torsion, however, proceeds in one stage, namely, B19′ → B2, as in the initial state after the RT (Figure 3).

The DSC data fully correspond to the results of the XRD study at a temperature of 20 °C. The characteristic temperatures of forward and reverse MTs are presented in

Table 2. Characteristic temperatures of MT after RT and STD.

Treatment	Cooling				Heating		Mpeak	Rpeak	Apeak
Reference treatment	Ms		Mf		As	Af	64	-	97
	78		53		83	102
500 °C 14 turns	Cooling				Heating		43	52	85
	B2 → R		R → B19		B2-B19′
	Ms	TRs	TRf	Mf	As	Af
	59	50	45	32	78	90
500 °C 30 turns	Cooling				Heating		24	40	67
	B2 → R		R → B19′		B2 → B19′
	Ms	TRs	TRf	Mf	As	Af
	48	33	28	17	60	73

. A change in the staging of the course of the forward MT led to an increase in the width of the temperature range of the forward transformation after STD. An increase in the number of turns during torsion led to an increase in the peak-to-peak distance between M_peak and R_peak, as follows: M_peak shifts towards lower temperatures (by 19 °C) than those of R_peak (by 12 °C). Thus, the preliminary deformation expands the range of existence of the R-phase during the forward MT.

Torsional deformation led to a shift in the temperature range of the reverse MT towards lower temperatures. The finishing temperature of the reverse MT A_f after torsional deformation for 14 and 30 turns decreased by 12 and 29 °C, respectively.

3.2. Structural Study

Structural images obtained using transmission electron microscopy are shown in Figure 4. Groups of parallel martensitic crystals with a width of 30–60 nm can be observed in the bright-field images. At the same time, submicron B2-austenite grains and individual structural elements (both equiaxed and non-equiaxed) with a size of about 200–300 nm are also observable. Compared to the structure after the RT formed through high-temperature deformation with a grain size of 20–30 μm, the resulting structure is characterized by greater dispersion and a much higher dislocation density (of approximately 10¹⁰ cm⁻²) [4,5].

As a result of the analysis of the structure formed post-deformation using the linear intercept method, it was found that the average size of the structural elements is 483 ± 19 nm, which allowed for the characterization of the resulting structure as a submicrocrystalline structure [11,12,25].

The deformation of titanium nickelide via ECAP and abc-pressing under similar deformation–temperature conditions (T = 450 °C, e = 5–10) resulted in the formation of a B2-austenite structure with an average grain/subgrain size of 200–600 nm [36,37,38,39,40]. This finding correlates well with the results obtained and indicates the effectiveness of the STD mode in terms of obtaining an ultrafine-grained structure. Further structural refinement can provisionally be associated with a gradual decrease in the deformation temperature.

The most characteristic reflections of phases are indexed in the selected area electron diffraction (SAED) pattern taken from the area in the top of Figure 4 and shown in the bottom of Figure 4. Analysis of the SAED pattern after STD indicates that the main phases in the alloy at room temperature are B19′-martensite and the R-phase. The presence of B2-austenite is not clear because its reflections are overlapped by the reflections of the R-phase.

In the SAED pattern, numerous point reflections belonging to various orientations of B19′- and R-phases can be observed. The azimuthal distances between them show mainly high-angle misorientations between corresponding structural elements. Thus, the majority of the equiaxed structural elements are grains separated by high-angle boundaries.

It should also be noted that the majority of the reflections present low-angle azimuthal broadening due to the imperfection of the lattice orientation inside individual grains caused by an increased dislocation density. Note that the low-angle azimuthal broadening of reflection can be induced by closely oriented neighboring subgrains. Thus, the torsion deformation of the bulk NiTi samples at a temperature of 500 °C is accompanied by the formation of a mixed grained and subgrained submicrocrystalline structure with an average size of its elements of about 500 nm, which was formed as a result of dynamic recrystallization and polygonization.

3.3. Functional Properties and Hardness

The strain distribution and deformation hardening that occurred during torsional deformation were not uniform over the radius of the deformed section of the rod. As a result, the hardness changed from the center to the outer surface of the specimen. The hardness values are up to 40 HV higher near the surface of the specimen.

The average hardness values after the RT and after deformation at 500 °C in 14 and 30 turns are shown in

Table 3. Hardness and maximum completely recoverable strain of NiTi SMA after STD.

Treatment	$\overline{\mathbf{H}{\mathbf{V}}_1}$	${}_{\mathit{r},1}^{\mathit{m}\mathit{a}\mathit{x}},\%$
Reference treatment	178 ± 4	2.5 ± 0.3
T = 500 °C, 14 turns	209 ± 9	6.1 ± 0.3
T = 500 °C, 30 turns	212 ± 7	6.8 ± 0.3

. STD at 500 °C led to an increase in hardness values from 178 to 209 and 212 HV as compared to the RT due to the enhancement of hardening processes. Hardness after torsion at 500 °C in 14 and 30 turns remained unchanged due to the induction of a steady-state stage of the flow stress.

Since the main phase at room temperature is B19′-martensite, when deformation (bending around the mandrel) was induced, the reorientation of martensite variants occurred, and the alloy exhibited the shape memory effect during subsequent heating. When the induced strain was 2.5%, the shape recovery rate was 100% both for samples after torsion and the RT. An increase in the induced strain to 3% and higher resulted in the appearance of residual strain in the samples after the RT. The maximum completely recoverable strain of the samples after STD in 14 and 30 turns at 500 °C is significantly higher compared to the RT and amounts to 6.1 and 6.8%, respectively. The obtained values of maximum completely recoverable strain after STD were slightly lower than the properties of the samples obtained after ECAP in a quasi-continuous mode in seven passes at 450 °C—${}_{r,1}^{max}$= 8.4% [15]. This can be explained by deformation at a lower temperature and a different deformation scheme.

The reason for the observed increase in the maximum completely recoverable strain after STD as compared to the reference treatment is the grain refinement down to a submicron size and an increase in the dislocation density. Both factors lead to an increase in the transformation yield stress (which, in our case, is the critical stress for martensite’s reorientation, i.e., not a phase but an in-phase structural transformation) and dislocation yield stress. However, the latter is much larger; thus, the difference between the dislocation and transformation yield stresses grew [9]. This led to the subsequent involvement of the irreversible dislocation slip in the deformation process and the appearance of residual strain.

Thus, applying the STD in 14 and 30 turns at 500 °C leads to the improvement of the mechanical and functional properties of Ti-50.0 at.% Ni compared to such properties in an annealed, statically recrystallized state. These high values of maximum completely recoverable strain were achieved via STD due to the formation of a mixed submicrocrystalline structure as a result of the dynamic recrystallization and polygonization corresponding to a more complete course of MT.

The results obtained show the prospects of applying torsion deformation to titanium nickelide alloys in terms of the formation of an ultrafine-grained structure providing excellent properties.

4. Conclusions

In the present study, severe torsion deformation was applied for the first time to a bulk near-equiatomic NiTi shape memory alloy (SMA) at a temperature of 500 °C in 14 and 30 turns with accumulated true strain values of 4.3 and 9.1, respectively. Structure, mechanical, and functional properties were investigated. The following conclusions were drawn:

• Severe torsional deformation (STD) at 500 °C was applied to bulk Ti-50.0 at.% Ni specimens accompanied by the accumulation of high strain without failure.
• As a result of STD in 14 and 30 turns at 500 °C, a mixed submicrocrystalline structure with an average grain/subgrain size of 500 nm and an increased dislocation density of ~10¹⁰ cm⁻² was formed as a consequence of dynamic recrystallization and polygonization processes. This structure ensured the implementation of high values of maximum completely recoverable strain of 6.1–6.8%.
• STD at 500 °C led to a reduction in the temperature ranges of forward and reverse martensitic transformations compared with those of an annealed statically recrystallized state due to an increase in crystal lattice defect concentration and the transition to a two-stage transformation upon cooling.
• The results obtained show the prospects of applying severe torsion deformation to titanium nickelide in terms of forming an ultrafine-grained structure with remarkable properties. Further refinement of the structure and an increase in the combination of properties was primarily associated with a gradual decrease in the deformation temperature.