Strategic Method to Enhancing the Formability of Nitinol Foils via Micro- Incremental Sheet Forming Processes and Evaluation of Structure – Property

doi:10.21203/rs.3.rs-4623830/v1

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Strategic Method to Enhancing the Formability of Nitinol Foils via Micro- Incremental Sheet Forming Processes and Evaluation of Structure – Property

https://doi.org/10.21203/rs.3.rs-4623830/v1

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The Nitinol alloy foils were deformed incrementally using the Single point micro incremental forming (SPMIF), in which the foils are deformed with the help of a hemispherical end tool, where no specific die is required and the forming limits are more due to the local deformation. Micro incremental forming is much needed process to develop the miniature components. The present study reveals the formability and fracture behaviour of Nitinol alloy foils under the SPMIF process. The formability studies are carried out with the spindle speeds. At higher forming speeds the formability is more as compared to the lower spindle speeds due to the presence of shear strains during the incremental forming. The XRD analysis is carried out for the phase fractions. EBSD and TEM analysis is carried out for texture analysis and dislocation density. The fracture behaviour is investigated through the void coalescence method and the parameters of void coalescence such as void (L/W) ratio, void size (µm), d-factor and thickness of ligament (LT) are measured through the ImageJ software. The relation between spindle speed and void coalescence parameters is studied.

Nitinol

Single point incremental forming

Tensile properties

Void Coalescence

Forming limit diagram

Shape memory alloy

The Nickel-Titanium (Ni-Ti, Nitinol) alloy demonstrates shape-memory (SMA) characteristics, enabling it to change shape when subjected to heat treatment, along with superelastic (SE) properties that enhance its flexibility and durability. In contrast to various other metals and alloys, Nitinol can undergo deformation ranging from 10 to 30 times more and still recover its initial shape. Normally, Nitinol contains a nickel percentage ranging between 50 and 51 atomic per cent (equivalent to 55 to 56 per cent weight per cent) [1]. Nitinols have found successful applications as biomaterials and in diverse engineering fields such as the creation of stents, guide wires, and orthodontic archwires [2]. Shape-memory alloys (SMAs) derived from these materials have been utilized as thermal actuators in the automotive industry for various functions, including engine control, fan clutches, radiator shutters, brake ventilation, fuel management, transmission control, climate control, and minimizing noise from rattling [3]. Nitinol is additionally employed in numerous roller and ball bearing uses owing to its exceptional resistance to corrosion and relatively high durability.

There is a wide array of applications for sheet metal, spanning from basic trays to intricate components in construction, automotive, aviation, boilers, kitchenware, office supplies, and various household and food-beverage appliances. Through the use of a die and punch, sheet metal gets shaped into the specific geometries required for these applications. Despite their natural ductility, sheet metals have limits in their production, with failures such as fracture and necking occurring beyond a certain point, known as the forming limit. The forming limit diagrams (FLDs) serve as a valuable tool for accessing the formability of sheet metals. Strains are measured at the points of component failures, and these measured strains are used to construct FLDs. The formability of sheet metal is influenced not only by friction but also by factors such as blank holding pressure, lubrication, strain pathways, and the severity of the forming process [4].

Conventional sheet metal forming relies on punch and die mechanisms but has its limitations, including reduced strain capacity, challenges related to friction between the metal and the die, difficulties in lubrication, and the need for highly intense shaping. Moreover, the cost associated with the die and punch is considerably high. As a result of issues like necking, wrinkling, fracture, and earing, the traditional press-forming methods for sheet metal encounter limitations. Besides, in small-scale manufacturing, traditional press forming methods tend to become costlier due to the need for dedicated punch and die sets, hydraulic presses, and specialized tool designers. Conventional farming also tends to reduce the formability of complex structures because of varying strain paths and excessive strains involved.

To address these challenges, incremental sheet formation presents a solution. In single-point, incremental forming (SPIF), a ball-ended forming tool follows specified pathways determined by the user. This gradual deformation process used in SPIF allows for the gradual shaping of the sheet, resulting in increased limits to strains and overcoming some of the limitations associated with conventional forming [5].

The production of micro components is gaining significant importance across various industries like medical engineering, optics, bioengineering, and engineering surfaces with specific qualities. To fabricate these diverse micro components, a range of techniques has been developed, including micro-EDM, the LIGA process, precision-micro cutting, nano-imprinting, and various beam processing methods. Even within different forming procedures like micro-deep drawing [6], micro extrusion [7], micro indentation for surface texture modification [8], micro punching [9], and micro-grooving in spinning [10], the focus has been on product miniaturization.

Factors like friction and grain size play crucial roles in product miniaturization within these processes [11, 12]. Among these techniques, the single point micro incremental forming (SPMIF) process stands out as particularly suitable for developing miniature products. It requires less tooling and ensures a good surface finish, making it an ideal choice for creating these small-scale components.

The adoption of SPMIF for developing microdevices and robots with thicknesses less than 1 mm was explored by Saotome and Okamoto et al. [13]. Y. H Kim et al. [14] conducted research focused on Al 1050 to evaluate its response to incremental sheet forming, examining the impact of process variables. Meier et al. [15] utilized a robot cell to incrementally deform aluminium material, achieving higher precision in their outcomes. Jackson et al. [16] identified that stretching and shear, both perpendicular and parallel to the tool direction respectively, were the primary causes of deformation in the incremental sheet forming (ISF).

Given the wide applications of micro-structured components, efforts have been made to scale down ISF to the micro level. Zhao and Zhu [17] demonstrated SPMIF using modified CNC machinery to deform multi-stage foils and analyzed formability during the process. Sekine and Obikawa [18], in their pursuit of developing miniature components, constructed a dedicated CNC machine for experimental SPMIF and studied the formability of different aluminium alloys. Further studies focused on specific materials and techniques: Sharma et al. [19] experimented with Ni-Ti sheets, examining parameter effects on formability during laser forming. Prasad et al. [20] delved into the microstructural, mechanical, and formability aspects of a newly developed Nitinol alloy containing Hf and Ta. Yoganjaneyulu et al. [21] investigated the influence of tool diameter and speeds on the fracture behaviour (void coalescence) of titanium grade 2 sheets, noting higher spindle speeds enhanced formability and larger tool diameters resulted in greater deformation and fracture strain. Vigneshwaran et al. [22] studied the relationship between strain triaxiality and void coalescence in aluminium alloys during cryorolling. Narayanasamy et al. [23–25] explored forming behaviour using sheets of various thicknesses, connecting properties like forming limit, fracture limit, wrinkling limit, void coalescence, and length-to-width (L/W) ratio with shear forces and material variables for a comprehensive understanding.

It appears that while there is a considerable focus within SPIF research on aluminium, copper and steel alloys, there's a noticeable lack of literature when it comes to SPIF applications involving Nitinol. Nitinol has extensive usage in various fields such as biomedical implants, automotive actuators, electrical devices, and aerospace applications. Additionally, there's an absence of direct literature specifically addressing SPIF and SPMIF techniques applied to Nitinol.

The current study aims to shed light on the mechanisms that dictate formability, fracture behaviour, and the overall formation process during SPMIF involving Nitinol. The research also delves into the influence of spindle speeds on formability during SPMIF. Furthermore, the investigation into fracture behaviour involves analyzing strain triaxiality and establishing correlations with void coalescence and intercrystalline separation for a comprehensive understanding of the material behaviour during the SPMIF process.

Nitinol foils, with a thickness of 250 µm, were acquired for the investigation involving SPMIF. The obtained Nitinol material comprised 50.5 wt. % Ni and 49.5 wt. % Ti. Employing standard metallographic procedures, the foils underwent etching in a solution composed of 4% HF, 14% HNO₃, and 82% H₂O for a duration of 90 seconds. Surface morphology examinations were conducted using a Metzer optical microscope. To facilitate SPMIF experimentation, these foils, each measuring 150 mm², were marked with laser-assisted grid circles, featuring a diameter of 2 mm and a depth of 25 µm.

The SPMIF tool, constructed from F6 tool steel with a hemispherical end, was employed on a vertical milling centre (VM 200). Coconut oil served as the lubricant to ensure an optimal surface finish. CNC programming was utilized to set parameters such as spindle speed, vertical step depth, and tool path feed rate. Specifically, spindle speeds of 100, 150, and 200 rpm were selected, while the vertical step depth and tool path feed rate were consistently set at 100 µm and 300 mm/min, respectively.

The SPMIF experiments, illustrated in Fig. 1(a), resulted in the transformation of Nitinol foils into frustum cups (refer to Fig. 1(b)). The process was halted upon the appearance of cracks. To quantify major and minor strains for constructing the Forming Limit Diagram (FLD), measurements were taken using the deformed grid circle. Strain measurement at the necking zone was conducted using a probe microscope (VMM, standard gauge visual 250) with an error margin of 3 µm.

To evaluate the durability of the frustum cups, tensile samples were extracted from the cup walls, as shown in Fig. 1c. Tensile properties were assessed using a Tinus Olsen tensile testing unit (HKL50) at a strain rate of 1 mm/min. The surfaces of the fractured tensile specimens were examined through Vega TSCAN™ scanning electron microscopy. The obtained micrographs were then utilized for void coalescence analysis, encompassing parameters such as void area fraction, void size, ligament thickness (LT), L/W ratio, and d-factor. These analyses were performed using AutoCAD software.

The microstructure examination involved the use of a field emission scanning electron microscope (FEGSEM) equipped with electron-backscattered diffraction (EBSD). Samples were extracted from the walls of deformed foils and subjected to electropolishing in an 80:20 methanol and perchloric acid solution at 38 V for 20 seconds using a Struers-ElectropolTM-IV unit. Orientation images were captured with a 0.3 µm step size utilizing the FEITM-Quanta 3D FEGSEM. The acquired data was processed using TSL OIM 4.6 software to generate grain size plots, orientation plots, and pole figures.

Specimens, measuring 0.5 mm², were extracted from the deformed area post-surface Mechanical Impact Treatment (SPMIF) and mechanically reduced to approximately ~ 100 µm thick foils. These foils underwent electropolishing using a twin-jet polishing unit. Microscopic investigations were carried out utilizing the JEOL JEM 2100 transmission electron microscope (TEM). For X-ray diffraction (XRD) analysis, samples from the deformed foils were exposed to Cu-Kα radiation (λ = 1.5406 Å) at a scanning rate of 2° per minute, employing the Rigaku Ultima III™ apparatus.

3.1 Forming limit diagram of Nitinol foils processed at various spindle speeds

In Fig. 2, the FLD illustrates the changes in major strain (ε₁) and minor strain (ε₂) of the Nitinol alloy subjected to deformation at spindle speeds ranging from 100 rpm to 200 rpm. The SPMIF process applied to the foil involves solely tensile forces, with no contribution from compressive forces or a combination of both. Consequently, the FLD displays positive major and minor strain values. Upon examination of the FLD, it becomes evident that the forming limit strain of Nitinol foil increases with higher spindle speeds. Notably, the highest spindle speed of 200 rpm yields a greater forming limit strain compared to its counterparts. This improvement can be attributed to the marginal reduction in downward force at lower spindle speeds, leading to sliding friction. As the spindle speed increases, sliding friction diminishes, generating frictional heat along the contact area, ultimately enhancing formability, as articulated by Uheida et al. [26]. The reduced sliding friction and localized heating contribute to the increased forming limit strain of Ti-6Al-4V formed at 200 rpm. Furthermore, the higher spindle speed induces shear stress on the foil, leading to additional plastic deformation [21].

3.2. Effect of spindle speed on microstructure

The micrographs in Fig. 3 (a-d) illustrate the Nitinol base metal (BM), as well as Nitinol after subjecting to SPMIF at spindle speeds of 100 rpm, 150 rpm, and 200 rpm, respectively. In the BM micrograph (Fig. 3a), austenitic phases are prominently visible, characterized by large equiaxed grains. Following SPMIF, the micrographs of Nitinol exhibit a reduction in grain size, and this reduction is proportionally associated with an increase in spindle speed. Additionally, the micrographs reveal the presence of needle-like structures, identified as Ti₂ Ni (B19’) martensitic phases. The absence of significant martensite formation in the present study is evident, and this can be attributed to the fact that the small amount of deformation applied did not induce stress-induced martensite. The SPMIF depth and strain imparted on the Nitinol were relatively low. Consistent with these findings, similar observations were reported in Nitinol subjected to a burnishing operation with a depth of a few microns, for instance, 0.5 µm. In such cases, the resulting microstructure showcased a higher presence of austenitic phases with only a few traces of martensitic phases, which can be attributed to the limited cold work imposed during the process [27].

To assess the impact of deformation speed on grain morphology and orientation of the Nitinol foils, EBSD was conducted on samples subjected to incremental deformation at various speeds. All EBSD samples were extracted from the deformed region. The Inverse Pole Figure (IPF) displayed in Fig. 4(a) illustrates the crystallographic orientation using different colour coding in the EBSD maps. The orientation image maps of the base material and samples deformed at different speeds (100 rpm, 150 rpm, and 200 rpm) are displayed in Fig. 4(b–d), providing insights into the B2 grain features. The microstructure of the Nitinol alloy predominantly consists of coarse B2 grains with discernible grain boundaries. In the sample deformed at 100 rpm, the majority of crystals exhibit a bluish colour in the analysis, signifying that the deformation is along the (111) direction for B2 grains. Additionally, a small number of grains with red and green colours, representing (001) and (101) orientations, respectively, undergo deformation (Fig. 4(c)). The preference for < 111 > orientation parallel to the deformed direction is commonly acknowledged as the most effective path for deformation [28]. The average grain size of the base material and samples deformed at 100 rpm, 150 rpm, and 200 rpm was measured to be 22.49 µm, 21.10 µm, 19.32 µm, and 16.53 µm, respectively. The average grain size decreases with increasing deformation speed. It has been reported that as the speed increases, the average grain size tends to decrease.

Figure 5 illustrates the pole figures of Nitinol for the base material and deformed components. These pole figures represent the orientation changes of deformed components concerning spindle speeds. The poles are distributed in various planes, such as {001}, {111}, and {110}, in the Nitinol alloy. The pole figure demonstrates that before deformation, the crystal orientations of the grains remained relatively constant. However, due to shear strain and tension during deformation, the points captured in the pole figure become dispersed and distributed randomly. The scattered distribution of data points in the pole figures at higher deformation rates indicates that grains underwent rotation and changes throughout the deformation process [29, 30].

To evaluate the correlation between the misorientation angle distribution and deformation speed, low- and high-angle grain boundaries (LAGBs: 2°-15°, HAGBs: > 15°) were selected to analyze the misorientation angles of specimens deformed at different speeds, as depicted in Fig. 6(a–d). The volume fraction of LAGBs increased with deformation speed. This rise in LAGB content at higher speeds is attributed to the straining of the material [31].

In Fig. 6(a), the misorientations of grains in the Nitinol alloy are revealed, with the majority of grain boundaries exhibiting high angles. The analysis demonstrates a variation in the percentage of HAGBs and LAGBs before and after deformation at various spindle speeds. The percentage of HAGBs is larger in the base material compared to the material deformed at higher speeds, and the percentage of LAGBs increases with spindle speed, reaching its maximum at 200 rpm and its peak at a misorientation angle of 5.63 (Refer to Fig. 6).

Low-angle grain boundaries (LAGBs) were generated as a consequence of crystallographic orientation changes or grain rotations occurring during the deformation process. The shear stress-induced deformation led to the creation of more sub-grains or LAGBs. Conversely, the low-angle grains were formed by the separation of high-angle grains. As observed in Fig. 6, the grain boundary map consolidates clusters of low-angle boundaries at higher speeds, clearly indicating that LAGBs increase with the rise in spindle speeds [32–34].

After the original grain deformed at 100 rpm, a few LAGBs were identified within the segment grains, indicating that the segment grains also underwent some low-angle misorientation in addition to deformation (Fig. 6(b)). The formation of high LAGBs results from the grain being progressively split into smaller pieces as spindle speeds increase. Hence, it appears more advantageous for the grains to change orientation during deformation rather than undergo division [35].

In Fig. 7, the Kernel Average Misorientation (KAM) maps for Nitinol are presented as colour bars ranging from 0 to 5, with red and blue indicating the highest and lowest values, respectively. In the KAM map of the non-deformed microstructure (Fig. 7(a)), a small portion of the green colour exhibits a uniform distribution, suggesting that the scattered form has a low density of dislocations. With increasing deformation speeds, the proportion of green and yellow colours also increases (refer to Fig. 7(b)). The rise in dislocation density and the material's resistance to plastic flow both contribute to the increase in dislocation mobility. This augmented dislocation density may lead to the formation of LAGBs. As plastic deformation increases, intergranular dislocations tend to move toward the grain boundaries. In the KAM analysis map shown in Fig. 7(c), deformation heterogeneity is preferentially concentrated adjacent to the grain borders rather than in the grain interior. Furthermore, the distribution of KAM values varies across locations, supporting previous findings and highlighting the non-uniform change of grains with higher deformation rates [36–38].

Transmission electron microscopy (TEM) images of the base material (BM) and deformed Nitinol foils after the SPMIF process at various spindle speeds are depicted in Fig. 8(a-d). According to the TEM results, the BM exhibits very few dislocations, primarily in the form of lines. However, in deformed Nitinol foils after SPMIF, dislocation entanglement occurs [26]. The dislocation density increases proportionally with the spindle speed during SPMIF, and a dense dislocation density is observed for the deformed foil at a spindle speed of 200 rpm. These results are evident in the KAM maps (Fig. 7), where the dislocation density increases with the spindle speed. The anticipated enhancement of mechanical properties in the formed products after SPMIF is attributed to the increase in dislocation density with increasing spindle speed.

3.3 X-Ray Diffraction analysis

Phase analysis of the Nitinol alloy was conducted using X-ray diffraction (XRD) on a Rigaku Ultima III diffractometer, employing Cu-kα radiation with a wavelength of 1.54 Å. Figure 4 presents the XRD spectra of the base material (BM) and incrementally deformed samples of the Nitinol alloy. The spectra of both the BM and deformed samples only exhibit peaks associated with the austenite B2 phase. The peaks strongly indicate the major volume fraction of the B2 phase, and the noise and broadening at the base of the peaks suggest the presence of secondary phases with low fractions [39, 40].

The identification of these phases in XRD is corroborated by the microstructure (refer to Fig. 9), where prominent peaks are observed for the B2 phase, contrasted by lower intensity peaks corresponding to the B19 phase. Variation in peak intensities is likely attributed to the presence of different grain orientations, which vary with deformation.

From the XRD analysis, it is observed that the width of peaks increases with the spindle speed. Narrow peaks are observed in the BM compared to the deformed samples. The grain size, microstrains, and dislocation density were calculated using the Scherrer relation, and the results are reported in Table 1.

The analysis reveals that the strain increases, and the grain size reduces concerning various spindle speeds during the SPMIF process. The XRD analysis clearly shows that the strength increases due to the formation of strains during the SPMIF process (refer to Table 1).

Table 1

Represents the Crystal size, micro strains and dislocation density for various spindle speeds.
S. No	Condition	Crystal Size (µm)	Micro strains (10^− 3)	Dislocation density ( 10¹²,m²)
1	BM	39.06232717	0.110393	1.84
2	100 RPM	27.49400425	0.167947	2.36
3	150 RPM	21.0183071	0.216273	3.80
4	200 RPM	16.83867121	0.275411	6.59

3.4. Effect of spindle speed on tensile properties of deformed Nitinol foil

The mechanical properties were analyzed from the tensile plots, and the results are reported in Table 2. From the tensile plot, the maximum Ultimate Tensile Strength (UTS) is observed in the material when the sheet is formed at 200 rpm. In the tensile testing of the base material (BM), initially, it shows linear elastic behaviour, indicating an increase in stress to strain. At a certain stress level in the range of 400 MPa, the stress-induced austenite transforms into the martensite phase. Due to stress, the single lattice austenite starts to shear and transform into martensite. This effect causes the specimen to gradually elongate at a constant stress, leading to the plateau region in the tensile curve [41, 42] (refer to Fig. 10). At 8% elongation, martensite transformation is completed, marking the end of the plateau region. After the completion of the transformation, the material elongates with stress, and failure occurs at 19% elongation. The material deformed at a higher spindle speed (200 rpm) exhibits a higher ultimate tensile strength and lower total elongation. The impact of dense dislocations during the SPMIF process is responsible for the increase in strength, which is proportional to the rotational spindle speed [28].

In the deformed sample, the phase transformation takes place at higher stress compared to the base material, and the stress increases with increasing speed. The increment in strength is attributed to strain hardening and the development of strains within the material during the SPMIF process. The material fails at lower strains in the case of the deformed sample due to the strain-hardening effect. The tensile strength increases due to the increase in dislocation density, as these dislocations act as obstacles to the movement of dislocations [29]. The predominant increase in strength is likely due to the pile-up of dislocations and grain refinement during the SPMIF process.

Table 2

Tensile properties and microhardness of Nitinol.
S. No.	Process condition	Yield strength (MPa)	Ultimate tensile strength (MPa)	Elongation (%)	Microhardness (HV_0.1)
1	BM	406 ± 3	592 ± 3	19	261
2	100 rpm	456 ± 3	676 ± 3	17	284
3	150 rpm	536 ± 3	785 ± 3	12	312
4	200 rpm	656 ± 3	995 ± 3	9	337

3.4 Effect of spindle speeds on fracture (void coalescence) behaviour

The study of the fracture surface using Field Emission Scanning Electron Microscopy (FESEM) analysis reveals a combination of ductile and brittle fracture characteristics, as shown in Fig. 11. The ductile fracture is attributed to the rotational shear forces generated during the Surface Mechanical Impact Treatment (SPMIF) process, leading to plastic deformation [21]. The coexistence of both ductile and brittle features on the fracture surface indicates the complex nature of the deformation and failure mechanisms involved in the material subjected to SPMIF.

The examination of the fracture behaviour, specifically void coalescence, in Ni-Ti alloy components subjected to incremental deformation is conducted through FESEM analysis. Various void coalescence parameters, including void area fraction (%), void size, d-factor, ligament thickness (LT), and L/W ratio, are quantified using AutoCAD Software across all deformed components. The relationship between void coalescence parameters and spindle speed is illustrated in Fig. 12. The observed relationship indicates that as the spindle speed increases, there is a reduction in void size, attributed to the heightened generation of shear stress and strain during the incremental forming process at elevated spindle speeds. Concurrently, void area fraction, ligament thickness, and void area fraction exhibit an increase with escalating forming speed, a consequence of enhanced formability through plastic deformation. The L/W ratio of voids increases with spindle speed, signifying the elliptical transformation of voids due to their convergence at higher spindle speeds. This transformation is indicative of a potential rise in rotational shear strain, leading to void coalescence. At elevated forming speeds, there is a likelihood of increased rotational shear strain, resulting in the coalescence of voids and their conversion into intercrystalline separations, ultimately culminating in necking [22]. The fracture surface analysis depicted in Fig. 11 illustrates microvoids and provides insights into the nature of fracture. Notably, an increase in spindle speed correlates with a higher number of microvoids. The component deformed at 200 rpm exhibits a greater concentration of microvoids on its fracture surface. As the material undergoes deformation at higher speeds, the pronounced strain effects lead to the expansion of microvoids. Consequently, the expansion of microvoids contributes to an increase in ligament thickness with spindle speeds. The heightened presence of shear stress during the SPMIF process further amplifies the void area at higher spindle speeds.

After completing the study on the formability and fracture (void coalescence) behaviour of Nitinol alloy foils during the SPMIF process, the following results were reached

The forming limit strains are increased with increasing the spindle speeds and the conclusions are shown in the FLD plots. The maximum plastic deformation is achieved with higher forming speeds. The Mechanical properties of components deformed with higher forming speeds give higher values; this is due to the increase in dislocation density and is proved with the help of TEM micrograph analysis. From the fracture analysis using FESEM micrographs, it is concluded that the void size is decreasing and the void area is increasing with forming speeds. Orientation analysis is carried out with EBSD and from the EBSD analysis it can be concluded that the grain size and orientation have been changed with forming speeds. At higher forming speeds the fraction of low angle boundary is more. From the present research work, it can be concluded that the forming speeds are influencing the formability and strength of the material.

Conflict of interest:

The authors solemnly declare no conflict.

Contribution statement

GY formulated the idea of this investigation and also the PI of this sponsored work. SV was involved in the analysis of results and preparation of the final draft. SS contributed to the XRD and final review of this manuscript. AB and TN involved in the data interpretation and characterization.

The authors solemnly declare no conflict. Besides, the authors also declare that this work is not published or under communication with any other journal.

Acknowledgements

The author (Dr Gunda Yoganjaneyulu) is thankful to the DST–SERB, Government of India for sponsoring the Start–up Research Grant (SRG) Project (File No: SRG/2020/000988) for carrying out this research investigation. The authors are also thankful to Professor IndradevSamajdar, Department of Metallurgical Engineering and Materials Science, National facility, OIM and Texture lab, IIT-Bombay, India, for providing EBSD & TEM facility.

Data availability:

The dataset will be provided by the corresponding authors upon reasonable request.

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Strategic Method to Enhancing the Formability of Nitinol Foils via Micro- Incremental Sheet Forming Processes and Evaluation of Structure – Property

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Abstract

Figures

1. Introduction

2. Experimental procedure

3. Results and Discussion

3.1 Forming limit diagram of Nitinol foils processed at various spindle speeds

3.2. Effect of spindle speed on microstructure

3.3 X-Ray Diffraction analysis

3.4. Effect of spindle speed on tensile properties of deformed Nitinol foil

3.4 Effect of spindle speeds on fracture (void coalescence) behaviour

4. Conclusions

Declarations

Conflict of interest:

Acknowledgements

Data availability:

References

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Version 1