Comparison of the transformation behavior of cold rolling with aging and hot extrusion with aging processed Ni50.3Ti29.7Hf20 high temperature shape memory alloy

H Onat Tugrul; Hasan H Saygili; Mustafa S Velipasaoglu; Benat Kockar

doi:10.1088/1361-665X/ab39f3

1. Introduction

Ni–Ti alloys are the most promising shape memory alloys (SMAs) due to their high strength and good cyclic stability properties. However, their transformation temperatures (TTs) which are lower than 100 °C limit their usage for most of the high temperature aerospace applications. Adding a ternary alloying element such as Au, Pt, Pd, Zr and Hf to Ni–Ti based SMAs is the method to achieve high TTs [1–4]. Au, Pt and Pd are not suitable since they are relatively more expensive than that of Zr and Hf. Zr has a high oxygen affinity, thus makes the alloy brittle. Therefore, Hf would be the better choice in order produce high temperature NiTi based SMAs. Additionally, higher actuation strains can be achieved by the addition of Hf instead of Zr [5].

In Ni-rich NiTi and NiTiHf high temperature shape memory alloys (HTSMAs), micro and nano-scale precipitates can be formed via aging and these precipitates affect chemical and mechanical properties of the alloy. An increase in critical stress for slip such that huge stress levels can be applied to obtain higher actuation strain with the formation of nano-scale precipitates and this is the effect of precipitates to the mechanical properties of the alloy [6]. Formation of these precipitates also increases the TTs with the decrease of Ni content in the matrix which can be called as the effect of precipitates on the chemical properties of the alloy [7].

Up to now, researchers work on the aging effect on the actuation properties and the cyclic stability of Ni-rich Ni_50.3Ti_29.7Hf₂₀ alloy with typical formation of nano-precipitates in the matrix [8–13]. Different research studies have been dedicated to the microstructural investigation of precipitate size as well as the twinning formation in Ni-rich NiTiHf alloys [6, 8, 10]. Addition to the microstructural investigations, Karaca and his co-workers showed the aging effect via running load-biased heating-cooling experiments and revealed that aging at 550 °C for 3 h led to an increase in the strength of the material and a decrease in the plastic deformation during martensite–austenite tranformation under constant stress magnitudes [11]. Moreover, Karaman's group investigated the cyclic stability of this Ni-rich alloy via running functional fatigue experiments [14, 15]. They showed the effect of upper cycle temperature and the applied stress magnitude to the functional fatigue life and stated that the increase in upper cycle temperature and the applied stress magnitude led to a decrease in the number of cycles during fatigue experiments. It has been also shown that the functional fatigue life of Ni_50.3Ti_29.7Hf₂₀ can be enhanced by 3 times via conducting aging treatment at 550 °C for 3 h due to the formation of nano-precipitates [16]. Ni_50.3Ti_29.7Hf₂₀ alloy used in the aforementioned studies were hot extruded after casting to improve the chemical and microstructural homogenity before the aging heat treatment. Karaman's group has recently worked on Ni₅₀Ti₃₀Hf₂₀ near equiatomic HTSMAs and demonstrated the effect of cold rolling and subsequent annealing effect on the TTs by conducting stress free heating-cooling cycles and on the transformation and irrecoverable strain magnitudes by conducting load-biased heating cooling experiments [17]. Up to our best knowledge, there has been no study on the effect of cold rolling and subsequent aging process to the TTs and to the actuation and irrecoverable strain levels of Ni-rich Ni_50.3Ti_29.7Hf₂₀ alloy. Previous studies on Ni_50.3Ti_29.7Hf₂₀ HTSMA have shown that the TTs of this alloy can be increased via aging for 3 h above 500 °C. Aging Ni_50.3Ti_29.7Hf₂₀ alloy at 550 °C for 3 h gives optimum results in the literature if the magnitude of the TTs and cyclic stability behaviors are considered. Additionally, it is also known that the cyclic stability of NiTi based alloys can be increased with the application of plastic deformation techniques such as equal channel angular extrusion (ECAE) and/or cold rolling due to the decrease in the grain size and the increase in dislocation density as well as critical stress for slip [18–21]. For instance, Gall and his group worked on the effect of hot rolling, cold rolling and the subsequent heat treatments on the transformation properties of Ti-50.9at%Ni binary SMAs and concluded that the influence of dislocation density increase with thermo-mechanical processes to prevent the plastic flow is as similar as the influence of small coherent precipitates on preventing the plastic flow [19]. In another study, the pseudoelasticity behavior of polycrystalline Ti-50.9at%Ni alloy was examined to understand the effect of heat treatment after hot rolling and cold drawn processes to the cyclic stability and the crack growth with the number of cycles of this alloy [20]. It was observed that the resistance to cyclic degradation in the material which was hot rolled with 300 °C–1.5 h heat treated and cold drawn with 300 °C–1.5 h heat treated was excellent and the lowest resistance to the degradation was experienced in the cold drawn material due to the residual stresses induced during the cold deformation process [20]. Additionally, Karaman group investigated the effect of severe plastic deformation via ECAE and 30% cold drawing with 300 °C post annealing processes to the cyclic stability of near equiatomic NiTi alloy during the heating cooling under constant stress experiments [22]. In this study, ECAEd and cold drawn samples were heated and cooled under 100 and 200 MPa for 10 cycles. It was revealed that the recoverable strain values of the ECAEd and cold drawn samples were very stable throughout the cycles but the total irrecoverable strain values under 200 MPa of the cold rolled sample was higher than that of the ECAEd sample. However, both of the processes increased the cyclic stability of the near equiatomic NiTi alloy [22]. Miller and Lagoudas found that cold working with subsequent annealing processes increased the dimensional stability of NiTi binary alloys via observing less irrecoverable strain values during heating cooling under increasing constant stress cycles and they also stated that the dimensional stability increased further with the increase in cold working amount [23]. Therefore, it can be concluded from the aforementioned studies on NiTi alloys that the thermal and the thermomechanical processes increase the dimensional and TT stability of the NiTi based alloys due to the dislocation density increase with work hardening such that the mobility of the dislocations mitigates during thermal cycling which leads to the stability of the shape memory properties [24].

In this study, one set of Ni_50.3Ti_29.7Hf₂₀ alloy batch was hot extruded and then aged at 550 °C for 3 h and the other set was solutionized at 1050 °C for 2 h and then cold rolled for 10% at room temperature and then 550 °C for 3 h age treated as a final step. The main aim was to determine and to compare the transformation behavior of this alloy with two different thermo-mechanical processing conditions such as the actuation and irrecoverable strain, TTs thermal hysteresis magnitudes under increasing constant stress for actuator applications at high temperatures. This comparison was done to eliminate the hot extrusion step for easier processing since extruding the hard to deform materials such as NiTiHf at relatively higher temperatures generally requires expensive and complicated tools.

2. Experimental procedures

In this study, Ni, Ti and Hf elements having high purity levels were used to produce Ni_50.3Ti_29.7Hf₂₀ (at%) alloy via vacuum induction melting under high purity argon atmosphere. One set of the as cast Ni_50.3Ti_29.7Hf₂₀ alloy was sealed in a mild steel can and hot extruded at 900 °C with an area reduction of 4 to 1 and then aged at 550 °C for 3 h. The name conventions 'HE' and 'HE-A' will be used for hot extruded and hot extruded and aged samples, respectively throughout the text. The other set was solutionized at 1050 °C for 2 h. This set was also separated into 2 parts. One part was characterized in the as solutionized condition and the other part was cold rolled for 10% at room temperature and then aged at 550 °C for 3 h. Before solutionizing and aging heat treatments, the samples were wrapped in tantalum foil to diminish possible oxidation at relatively higher temperatures and both of the heat treatments were done in a vertical cylindrical furnace under high purity argon atmosphere and ended with water quenching. The name conventions 'S' and 'S-CR-A' will be used for the solutionized and solutionized-cold rolled and aged samples throughout the text for simplicity. The TTs of all samples were measured using differential scanning calorimetry (DSC) with a heating-cooling rate of 10 °C min⁻¹.

Dog bone shape tensile test samples which were used in load biased heating-cooling experiments with a gage length of 16.6 mm, a width of 2.25 mm and a thickness of 1 mm were cut using wire electrical discharge machine and shown in figure 1. Load biased heating-cooling experiments under increasing stress magnitudes starting from 100 MPa and increasing by 100 MPa after each cycle with a heating-cooling rate of 10 °C min⁻¹ were conducted till fracture in order to compare the shape memory characteristics such as actuation strain, irrecoverable strain, TTs and thermal hysteresis.

**Figure 1.** Tension test samples which were cut using WEDM (the dimensions were given in mm).
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Additionally, a JEOL transmission electron microscope (TEM) was utilized to investigate the twinning, precipitation and dislocation structures before and after the applied aforementioned processes. TEM samples were prepared using a focused ion beam equipment for thinning the samples down to 10–20 nm.

3. Experimental results

3.1. DSC analysis

DSC results of HE, HE-A, S and S-CR-A samples are shown in figure 2. The TTs were drawn from the DSC curves and are summarized in table 1. DSC results of HE and HE-A samples were shown in our previous study [16], however, the DSC results which were obtained from all samples were re-plotted in one graphic and all the TTs which were drawn from the DSC curves were re-tabulated for better comparison.

Table 1. TTs which were drawn from the DSC curves in figure 2.

Sample	Cycle	Transformation temperatures (°C)
		M_f	M_s	A_s	A_f
HE [16]	1	33.2	79.7	77.3	109.9
	2	30.2	78.2	70.4	106.5
	3	27	75.1	67	103.6
HE-A [16]	1	100.9	142	137.7	171.3
	2	99.9	141.3	136.4	168.9
	3	99.2	139.8	135.4	168.3
S	1	155.9	165.3	188.4	196.6
	2	153.3	163.1	180.8	190.6
	3	152.4	161.7	178.4	186.9
S-CR-A	1	—	187.5	85.1	244.5
	2	—	186.4	86	242.7
	3	—	185	85.1	241.9

Three cycles were run to determine the shift in the TTs. TTs of the solutionized sample were found to be the highest. DSC experiment was also run on the cold rolled sample; however, no visible transformation was detected since the TTs were shifted to very low temperatures. This result is consistent with the previous studies for highly deformed NiTi binary alloys [19, 21, 25]. Aging after cold rolling increased the TTs to higher temperatures such that A_s, A_f and M_s temperatures were determined and higher than that of the TTs of the solutionized sample. On the other hand, it was not possible to detect M_f temperature of the S-CR-A sample since the finish part of the martensitic transformation curve is not very well delineated. Additionally, hot extrusion also led to a decrease in the TTs since noticeable deformation was applied to the alloy during the extrusion process. After aging heat treatment of the hot extruded sample, TTs again increased to higher levels which were previously shown in the literature [11]. Additionally, aging after hot extrusion and cold rolling with subsequent aging after solutionizing heat treatment led to observe stable TTs starting from the second cycle due to the formation of precipitates via aging and dislocation formation via cold rolling. As stated in the literature, the TTs of all the samples can be stabilized as the number of cycles increases [24]. However, the number of cycles was kept as three in this study to see the direct effect of aging and cold rolling processes on the stability of the TTs of the ternary alloy.

3.2. Isobaric heating-cooling experiments

Isobaric heating-cooling experiments were performed, during which the samples were held under constant tensile stress while they were cooled and heated once at a rate of 10 °C min⁻¹. The stress magnitude was increased by 100 MPa till the fracture was observed for all samples while the samples were kept in the austenitic state. Figure 3 demonstrates how the irrecoverable and actuation strain values as well as the martensite start (M_s) temperature and thermal hysteresis were determined from a single thermal cycle. Figures 4(a) and (b) exhibit the comparison of thermal cycling response of the HE and S samples and HE-A and S-CR-A samples under different stress levels, respectively. Actuation and irrecoverable strain magnitudes extracted from the strain-temperature curves are shown in figure 5 as a function of applied stress. Additionally, M_s temperatures which were drawn from the strain versus temperatures graphs are presented in figure 6 to show the shift with the increasing stress magnitudes for all samples. The slopes which are representing dσ/dT ratio were given on the same figure. Lastly, the evolution of the thermal hysteresis with the applied stress levels for all samples can be seen in figure 7. It should be noted that, the isobaric cooling-heating experiments of hot extruded and hot extruded with aging processed Ni_50.3Ti_29.7Hf₂₀ alloy were shown previously in the literature [11]. On the other hand, the transformation properties of SMAs may differ from batch to batch due to very small deviations in the composition, therefore, the cooling-heating under stress experiments were also conducted on the batch which was used in this study and the results are presented below for comparison.

**Figure 3.** Schematic illustration which shows how to find out the M_s, thermal hysteresis, actuation and irrecoverable strain magnitudes from strain versus temperature curves.
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**Figure 4.** The comparison of strain versus temperature curves obtained from heating-cooling cycles under different stress levels of (a) solutionized versus hot extruded samples (b) hot extruded and aged versus solutionized, cold rolled and aged samples.
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**Figure 5.** The comparison of irrecoverable and actuation strain magnitudes with respect to the applied stress level for all samples.
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**Figure 6.** The comparison of M_s temperatures which were drawn from heating-cooling cycles under different stress levels of all samples.
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**Figure 7.** The comparison of thermal hysteresis values which were drawn from the heating-cooling cycles under different stress levels of all samples.
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The main observations from figures 4 and 5 can be summarized as:

(1)
HE-A and S-CR-A samples fractured under the stress level of 700 MPa without showing noticeable irrecoverable strain levels. On the other hand, HE and S samples fractured under 600 MPa and 400 MPa during cooling step, respectively. Therefore, thermo-mechanical treatments such as rolling and aging increased the fracture strength of the alloy which are also experienced in previous studies conducted on binary NiTi [22, 23, 26].
(2)
The actuation strain values increased with the increase of the stress magnitude for all samples. Additionally, the actuation strain values under all stress levels of the S-CR-A sample were determined to be very much lower than that of the other samples. These observations are consistent with the results which were observed in the cooling-heating under increasing constant stress experiments of binary NiTi alloy [23].
(3)
Only irrecoverable strain magnitudes of the HE sample were noticeable after 200 MPa.
(4)
The undercooling magnitudes of S-CR-A sample which can be interpreted with the slope of the cooling curves of the strain versus temperature diagrams were very much higher than that of the other samples.

Figure 6 shows the dependence of M_s temperature with the applied stress magnitudes. As the temperature increases, the stress necessary to induce martensite which is called as σ_SIM increases as well. This behavior is the inverse temperature dependence of yield strength behavior of many metallic materials. In other terms, the stress magnitude for the yield of conventional metallic materials decreases as the temperature increases. σ_SIM depends on temperature with a positive slope according to the modified Clasius–Clapeyron (CC) relation. CC relation is [26, 27]:

$\begin{eqnarray*}&&\displaystyle \frac{d\sigma }{dT}=-\displaystyle \frac{{\rm{\Delta }}H}{{T}_{0}{\varepsilon }_{act}}\end{eqnarray*}$

ΔH and T₀ are the transformation enthalpy and the chemical equilibrium temperature, respectively and ε_act is the actuation or transformation strain which is the difference in between the total strain and the irrecoverable strain values. T₀ strongly depends on chemistry of the alloy. Since the alloy is the same, T_o can be accepted as constant for all the samples. The variables in CC equation are the transformation enthalpy and strain magnitudes and these values affect the slope of the curves, in other terms the ratio of $\tfrac{d\sigma }{dT}.$ The slopes of HE, HE-A, S and S-CR-A samples were found as 5.8 MPa °C⁻¹, 6.8 MPa °C⁻¹, 8.6 MPa °C⁻¹ and 11 MPa °C⁻¹, respectively. The slope magnitude increased steeply with the application of rolling with subsequent aging treatment after solutionizing heat treatment. The stability of the parameters in CC relation actually dictates the stability of the shape memory properties [24]. The rationale behind the differences between the slope magnitudes will be explained in the next section.

Figure 7 represents the thermal hysteresis response of all the samples with the increase of stress. The thermal hysteresis magnitudes of HE and S samples increased as the applied stress increased. However, there was no noticeable change in the thermal hysteresis magnitudes of HE-A and especially S-CR-A samples up to 500 MPa under increasing stress magnitude.

3.3. Microstructural evolution

TEM images which were taken from HE and S samples are presented in figure 8. Additionally, TEM images of HE-A and S-CR-A samples are presented in figure 9. Figure 8 (a) shows the unique microstructure which is called as herring-bone morphology in the literature [28]. Twinned lamellae structures and the junction planes between the twin variants are demonstrated by full lines and dashed line, respectively. On the other hand, solutionized sample shows only martensitic plates.

**Figure 9.** TEM images of (a) HE-A and (b) S-CR-A Samples.
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After aging of the hot extruded sample, high volume fraction of nano-sized precipitation formation is observable and the distribution of these precipitates is shown in figure 9(a). Figure 9(b) is representative of the microstructure of solutionized, cold rolled and aged sample. As can be seen in the 1st circled area, the boundaries of the martensitic plates are not very well delineated due to the dislocation density increase via cold rolling process. The 2nd circled area represents the very fine nano-precipitates which were formed after aging process.

4. Discussion of the results

The main goal of this study was to compare the actuation behavior of the HE, HE-A, S and S-CR-A treated Ni_50.3Ti_29.7Hf₂₀ alloy to eliminate the hot extrusion step for easier processing since extruding the hard to deform NiTiHf materials at relatively higher temperatures generally requires expensive and complicated tools.

The TTs without applying stress and with applying stress and actuation-irrecoverable strain values with cooling-heating under increasing stress magnitudes together with the microstructural evolution via following different thermo-mechanical processes were revealed. The rationales behind all the results which were shown in the previous section are explained in the subsections of this chapter.

4.1. The effect of thermo-mechanical treatments on the TTs

10% of cold rolling of the solutionized Ni_50.3Ti_29.7Hf₂₀ alloy sample showed no transformation peaks in DSC experiment due to the dislocation density increase with the rolling process. After aging the cold rolled sample at 550 °C for 3 h, the forward and reverse transformation peaks were observed, however, the aging treatment was still not enough to determine the martensite finish temperature. Cold rolling induces high amount of dislocation to the alloy and this led to martensite stabilization [22, 23]. On the other hand, aging treatment led to the formation of Ni-rich nano-precipitates such that the Ni concentration of the matrix decreased and the TT increased. Karaman group also revealed the annealing effect of thermal treatment after cold and warm rolling processes which led to an increase in TTs of near equiatomic Ni₅₀Ti₃₀Hf₂₀ alloy [17]. The aging treatment in this study may also anneal the sample but this might not be the main reason of the increase in TTs since the martensite finish temperature of the S-CR-A sample was still not able to be determined.

Another intriguing finding is the observation of considerably higher TTs in the solutionized sample comparing with the TTs of the hot extruded sample. Although hot extrusion process was conducted at 900 °C, the TTs decreased considerably due to the refinement of the microstructure as it was shown in figure 8(a). The decrease in the TTs can be attributed to the very fine twin structure and the herring bone morphology. The aging treatment after hot extrusion process increased the TTs due to precipitation formation and this mechanism has been already explained in the literature, extensively [11].

Additionally, the TTs of HE-A and S-CR-A samples were observed to be highly stable from the beginning of the thermal cycling as can be seen in table 1. Moreover, the thermal hysteresis of the S-CR-A sample which were obtained from the cooling-heating under constant stress experiments up to 500 MPa were more stable than that of the other samples due to the increase in the dislocation density and the precipitation formation.

4.2. The effect of thermo-mechanical treatments on the actuation and irrecoverable strain and thermal hysteresis

The effect of thermo-mechanical treatments on the dimensional stability of Ni_50.3Ti_29.7Hf₂₀ is one of the most important issues that should be discussed since dimensional stability is the main design criterion for actuator applications. Therefore, in this section the actuation and irrecoverable strain magnitudes which were gathered from all cooling-heating under constant stress experiments are discussed.

The actuation strain magnitudes of all thermo-mechanically treated samples increased as the applied stress levels were increased since the externally applied stress helped the formation of martensitic variant with the most favorable orientation which led to an increase in the external shape change. The actuation strain at each stress level for the S-CR-A sample were determined to be lower than that of the other samples due to the increase strength of the sample with cold deformation. The critical stress level to induce phase transformation in S-CR-A must be higher such that higher stress magnitudes should be necessary to achieve higher actuation strain levels. The same behavior was observed previously in ECAE processed NiTi binary alloy [26]. Moreover, cold rolling might induce texture to the NiTiHf alloy and this texture led to achieve less actuation strain values since it has been already known that cold drawn polycrystalline NiTi alloys exhibit a strong texture of [111] type at which the worst shape memory behavior is expected [29].

It was observed that the irrecoverable strain magnitudes of all the samples at each stress levels were found to be very low. Only 0.4% of irrecoverable strain was observed for HE sample under 300 MPa and then decreased down to lower values as the applied stress was increased since the sample might be further strengthened with the externally applied stresses. The HE-A and S-CR-A samples have been already strong due to nano-precipitate formation and inducing dislocation via cold rolling process. It is difficult to comment on the mechanical behavior of solutionized (S) sample since this sample showed early fracture, however S sample did not reveal any irrecoverable strain up to 300 MPa. Actually, these results are very well consistent with the results obtained from figure 6. Figure 6 shows the CC slopes of all samples. S-CR-A sample which revealed the lowest actuation strain showed the highest CC slope. Although CC slopes of the HE and HE-A samples differed they showed almost the same actuation strain magnitudes at each stress levels. This might be attributed to the formation of nano-precipitates which can enhance the phase transformation with the stress fields around the coherent precipitates.

Figure 7 presents the thermal hysteresis comparison of all the samples at each stress levels. It can be concluded from the graph that aging after hot extrusion and cold rolling with subsequent aging treatment after solutionizing enhance the stability of the thermal hysteresis of the alloy. The positive effect of aging on the cyclic stability of the NiTiHf alloys in terms of thermal hysteresis has been already shown and discussed in the literature [11, 14–16]. However, the enhancing effect of cold rolling with subsequent aging was shown in this study and the stability of the thermal hysteresis of S-CR-A sample up to 500 MPa was more pronounced. Since the strength of HE-A and S-CR-A samples was increased with the precipitate and dislocation formation, the thermal hysteresis was not affected with the increase of the externally applied stress. Actually, the thermal hysteresis values of NiTi based alloys increase due to the increase in dislocation density with the increased stress magnitudes and with the number of cycles [26].

The thermal hysteresis values of HE-A sample and S-CR-A sample change between 20 °C–25 °C and 30 °C–35 °C. The increase in the thermal hysteresis of the S and HE samples might be attributed to the increase in dislocation density with the increase in the applied stress. Additionally, the slopes of the cooling and heating curves of S-CR-A sample which were shown in figure 4(b) became shallow such that the forward and reverse transformations can be completed via applying more supercooling and superheating respectively due to the high dislocation density which hinders the martensitic transformation.

4.3. The effect of thermo-mechanical treatments on the microstructural evolution

TEM images in figures 8(a) and 9(a) reveal the microstructures of HE and HE-A samples. High volume of nano-precipitate formation was obtained after aging heat treatment and precipitate density was shown in figure 9(a). On the other hand, figure 8(a) reveals the very fine twin structure and herring bone morphology after hot extrusion process. Hot extrusion was conducted at 900 °C and it was not expected to refine the microstructure at this high deformation temperature but the percentage of the deformation which was 4 to 1 area reduction might lead to this refinement.

Figures 8(b) and 9(b) represent TEM images of S and S-CR-A samples. The characteristic martensitic structure was obtained after solutionizing treatment; however, cold rolling and aging led to an increase in dislocation density and nano-precipitate formation, respectively.

5. Summary and conclusions

In this study, the effect of thermo-mechanical treatments such as hot extrusion with subsequent aging and cold rolling with subsequent aging of solutionized Ni_50.3Ti_29.7Hf₂₀ alloy was investigated. The TTs, actuation and irrecoverable strain magnitudes and thermal hysteresis values of all thermo-mechanically treated samples were compared. The main findings and conclusions via analyzing the experimental results can be summarized as follows:

(1)
The TTs can be stabilized with the number of cycles not only by hot extrusion with subsequent aging treatment but also by cold rolling with subsequent aging treatment of the solutionized alloy.
(2)
It has been already proved in the literature that hot extrusion with subsequent aging treatment increases CC slope of the Ni-rich NiTiHf alloys and in this study, the highest CC slope was achieved by cold rolling with subsequent aging treatment of solutionized Ni_50.3Ti_29.7Hf₂₀ alloy due to the increase in dislocation density and hence the strength of the alloy such that the dimensional and thermal hysteresis stability under increased stress levels were maintained. However, due to the increase in CC slope of the S-CR-A sample, the actuation strain values were less than that of the other samples.
(3)
Cold rolling with subsequent aging of solutionized sample and hot extruded with aging processed sample showed very low irrecoverable strain magnitudes even at very high stress magnitudes in cooling-heating under stress experiments. Therefore, cold rolling and subsequent aging processes after solutionizing heat treatment can be an alternative thermo-mechanical treatment path to aging treatment after hot extrusion process. However, it should be noted that the actuation strain values of S-CR-A sample at each stress levels were found to be the lowest.
(4)
The thermal hysteresis was not affected by the increased applied stress and the number of cycles after hot extrusion with subsequent aging of the alloy and cold rolling with subsequent aging treatment of the solutionized alloy.
(5)
In the light of the experimental results, it can be concluded that hot extrusion is not a mandatory step to get improved resistance to plastic deformation. One can also follow a different thermo-mechanical route to achieve higher strength as well as better shape recovery properties in NiTiHf alloy. Hot extrusion is very useful in obtaining chemical and microstructural homogeneity but additional thermal and/or thermo-mechanical treatments can be the processing steps in enhancing the shape memory behavior of the alloy.

Acknowledgments

This study was supported by the Turkish Aerospace Industries under Grant no. DKTM/2015/10.

Comparison of the transformation behavior of cold rolling with aging and hot extrusion with aging processed Ni_50.3Ti_29.7Hf₂₀ high temperature shape memory alloy

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Abstract

1. Introduction

2. Experimental procedures

3. Experimental results

3.1. DSC analysis

3.2. Isobaric heating-cooling experiments

3.3. Microstructural evolution

4. Discussion of the results

4.1. The effect of thermo-mechanical treatments on the TTs

4.2. The effect of thermo-mechanical treatments on the actuation and irrecoverable strain and thermal hysteresis

4.3. The effect of thermo-mechanical treatments on the microstructural evolution

5. Summary and conclusions

Acknowledgments

Comparison of the transformation behavior of cold rolling with aging and hot extrusion with aging processed Ni50.3Ti29.7Hf20 high temperature shape memory alloy

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Submit

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Share this article

Author e-mails

Author affiliations

ORCID iDs

Dates

Peer review information

Abstract

1. Introduction

2. Experimental procedures

3. Experimental results

3.1. DSC analysis

3.2. Isobaric heating-cooling experiments

3.3. Microstructural evolution

4. Discussion of the results

4.1. The effect of thermo-mechanical treatments on the TTs

4.2. The effect of thermo-mechanical treatments on the actuation and irrecoverable strain and thermal hysteresis

4.3. The effect of thermo-mechanical treatments on the microstructural evolution

5. Summary and conclusions

Acknowledgments

Comparison of the transformation behavior of cold rolling with aging and hot extrusion with aging processed Ni_50.3Ti_29.7Hf₂₀ high temperature shape memory alloy