Low-cost,(ZrCu)50-xTax,high,temperature,shape,memory,alloys,showing,excellent,shape,memory,effect

Weihong Go ,Xioyng Yi ,Bin Sun ,Yudong Fu ,Xinglong Meng

aCollege of Materials Science and Chemical Engineering,Harbin Engineering University,Harbin,150001,China

bCollege of Nuclear Equipment and Nuclear Engineering,YanTai University,YanTai,264005,China

cSchool of Materials Science and Engineering,Harbin Institute of Technology,Harbin,150001,China

Keywords:ZrCu-Based alloys High-temperature shape memory alloys Microstructure Shape memory effect Martensitic transformation

ABSTRACT ZrCu-based alloys,as one of the most potential high-temperature shape memory alloys,show quite high martensitic transformation temperature,relatively low price for the raw materials,and good casting fluidity.In this work,the martensitic transformation(MT)and shape memory effect of(ZrCu)50-xTax high-temperature shape memory alloys were systematically investigated.Both X ray diffraction and SEM backscattered electron image demonstrated the coexistence of the main phase monoclinic martensite phase and the second phase Ta2Cu.SEM results also revealed that increasing Ta content resulted in a larger volume fraction of Ta2Cu phase.Differential scanning calorimetry results showed that the MT temperatures remarkably increased with Ta addition.The ductility of the (ZrCu)50-xTax alloys was reduced to some degree with increasing Ta content.The shape memory effect was improved significantly,in which(ZrCu)98Ta2 alloy showed the 6.19%maximum recovery strain during 8% pre-strain under compression at 25 °C.

The low martensitic transformation temperature (Ms) (less than 100°C) of NiTi-based shape memory alloys (SMAs) limited the ceiling temperature,unable to be utilized in the high-temperature environment[1–3].In order to meet these special requirements,it is,therefore,highly imperative to develop high-temperature SMAs (HTSMAs) [4,5].One of the promising HTSMAs is ZrCu alloy that shows a lower cost and excellent thermal cycle stability associated with not less than 3% shape recovery[6].Furthermore,proper alloying in ZrCu-based HTSMAs enables to achieve higher shape recovery and simultaneously ensures the required martensitic transformation (MT) temperature,as well as some special crystal structure to avoid plastic deformation [7–9].Therefore,the study of ZrCu-based alloys is currently one of the most active research directions in the area of HTSMAs.

It should be noteworthy that ZrCu-based alloys usually exhibit a wide transformation hysteresis and poor thermal stability.Most preliminary studies indicate that the martensitic transformation start temperature(Ms) of ZrCu intermetallics decreases from 140 to 10°C after 50 cycles whereas the transformation hysteresis can increase up to 300°C by the thermal cycling.Several representative elements (i.e.Co,Al,Cr,Ti,Ni,and Hf) have been added to study the corresponding effect on the MT,thermal stability and mechanical properties [10–13].Among these elements,Co,Al,Cr,and Ti elements decrease the MT temperatures,in which a composition with around 7.5 at.% cobaltMswill be tuned around room temperature [12].In addition,the addition of Co element improves thermal cycle stabilities and reduces thermal hysteresis as well.In contrast,the addition of Hf and Ni elements leads to the increased MT temperatures accompanied by the reduced shape recovery effect.Specifically,a remarkable decline of shape recovery ratio is observed above 5 at.%hafnium[8,14].

As reported,a few refractory metallic elements in Ti–Ni-based SMAs result in the formation of ductile phases,e.g.,β-Nb phase and Ta-rich solid solution phase,which improve the machinability [15].However,no related work has been done yet regarding the effect of the Ta addition on the MT temperature and shape memory properties of ZrCu-based alloys.Therefore,here we presented a systematic study of the phase structure,martensitic phase transformation behavior,and shape memory properties of (ZrCu)100-xTaxalloys.It was found that both MT temperatures and shape memory effect remarkably increased with appropriate Ta addition,which can be valuable for future material design and performance optimization of HTSMAs.

Fig.1.XRD patterns of(ZrCu)100-xTax(x=0,2,4,6,8,10,and 15)alloys after solution treated.

(ZrCu)100-xTaxingots (x=2,4,6,8,10,and 15 mol.%) were synthesized by high vacuum arc-melting furnace (5.0 × 10-5Pa) with pure elements of Zr (99.9%),Cu (99.9%),and Ta (99.7%) in an argon atmosphere.These ingots were sealed in the vacuum quartz tubes (vacuum:3.0 × 10-3Pa) and solution-treated at 800°C for 4 h,followed by ice water-quenching.The test samples were cut by wire cutting.The phase crystal structure was determined by X-ray diffraction analysis with Cu Kα radiation and the test step is 8°/min.The microstructural morphologies were measured using a Quanta 200FEG scanning electron microscope(SEM)with the backscattered electron(BSE)SEM images and the Energy Dispersive X-ray (EDX) analysis.The temperatures of phase transformation of these alloys were determined by a DSC(Diamond PE)with a 20°C/min heating/cooling rate,and the weight is lower than 20 mg.The mechanical properties were measured by an Instron 5569 testing system with the compression cycle test at a crosshead displacement speed of 0.2 mm/min with cylindrical samples of 3 mm in diameter and 5 mm in thickness.After deformation,the compressive samples were heated to 500°C for complete shape recovery,and then cooled to room temperature.The heights of the samples were measured before loading(L0),after loading(L1),and after heating to 773 K for 5 min(L2)by extensometer.Shape recover ratio(η)was calculated by η=(L2－L1)/(L0－L1)×100%.

Fig.2.(a,b) the morphologies of martensite in (ZrCu)98Ta2 alloy;The illustration shows the corresponding electron diffraction patterns.

Fig.3.Backscattered electron images of (ZrCu)100-xTax (x=2,4,6,8,10,and 15) alloys(a) x=2,(b) x=4,(c) x=6,(d) x=8,(e) x=10,(f) x=15.

Fig.1 shows the XRD patterns taken from(ZrCu)100-xTax(x=2,4,6,8,10,and 15) alloys at 25°C after solution treatment.The crystal structures of all the samples were mainly indexed as the monoclinic ZrCu martensite structure,which implies that (ZrCu)100-xTaxalloys possessed the MT temperature higher than 25°C.Besides,the intensity of the diffraction peak at 38.7°gradually increased.Some diffraction peaks indexed as Ta2Cu phase at 38.7°and 69.7°appeared from(ZrCu)98Ta2to(ZrCu)85Ta15alloys.The intensity of these peaks became higher with more Ta content.These results imply that Ta element has a low solid solubility in ZrCu alloy.

Fig.2 shows the morphologies of martensite in (ZrCu)90Ta10alloy,and the illustration shows the corresponding electron diffraction patterns,respectively.The results show that the addition of Ta didn’t change the martensite orientation relationship.The relationship bettwen the variants was also(021)type I twin and the microtwins also showed(001)compound twin relationship.There was big difference of variants in size.The size of some variants was up to 500 nm,while some variants were only several nanometers.While the size of microtwins was very thin,only nanometers.

Fig.4.Bcackscattered electron(BSE)SEM images of the precipitates and the select region is(a)the white area,(c)the Matrix,and(e)the grey area;the EDX of(b)the white area,(d) the matrix,and (f) the grey area of (ZrCu)85Ta15 alloy.

The backscattered electron(BSE)SEM images of(ZrCu)100-xTaxalloys are shown in Fig.3.In terms of pristine ZrCu alloy,no second phase was observed in our previous report[7].As the Ta content was fixed at 2 mol.%,few second phases(the white one embedded in the matrix)existed in the matrix.With further increasing the Ta content,more second phases could be found,based on the SEM images,shown in Fig.3(a–f).The shape of the second phase gradually changed from globular to rod-like.In addition,the composition analysis taken from (ZrCu)85Ta15alloy by SEM-EDX shown in Fig.4(a,b) confirms the second phase as Ta2Cu,consistent with the above mentioned XRD result.Fig.4(c–f) exhibit the matrix and the grey area EDX results.It can be found that there was a little different in the contant of Ta.The EDX results of the grey area was almost the same with the matrix.

The DSC heating and cooling curves in Fig.5(a,b)are to evaluate the effect of Ta addition on the MT temperature.The only one step feature of phase transformation means the typical B2↔B19′character for our present materials.The extracted MT temperatures as a function of Ta content from the DSC curves are shown in Fig.5(c)and(d),respectively,whereMpandApare the corresponding peak of MT temperature and reverse MT temperature,respectively.Compared to Zr50Cu50alloy (Ap: 266°C andMp: 83°C),Ta addition remarkably increased bothMpandAptemperatures.

The utilization of valence electron concentration can be a good tool for understanding the mechanism of the varied martensitic transformation temperature in shape memory alloys area.The valence electron concentrationcvdecreased from 0.2174 (Zr50Cu50alloy) to 0.189((ZrCu)45Ta15alloy).TheMptemperature remarkably increased after Ta addition.It is consistent with our previous results that theAstemperature increases with the decreasedcv[8,16–18].However,Aptemperature increased first for the Ta content of 2%,then decreased,and increased with increasing the Ta content to 16%.Mptemperature increased first for the Ta content of 2%,then decreased,and increased with increasing the Ta content to 15%.It is due to the fact that the different content and shape of the second phase Ta2Cu may affect the composition or the mobility of the interface during martensitic transformation [19].

Fig.5.(a)Heating DSC curves of(ZrCu)100-xTax(x=2,4,6,8,10,and 15)alloys,(b)cooling DSC curves of(ZrCu)100-xTax(x=2,4,6,8,10,and 15)alloys,and(c,d)effect of Ta content on martensitic transformation temperatures (Ap, Mp) of (ZrCu)100-xTax alloys (Mp and Ap means the forward and reverse transformation peak temperature,respectively).

Fig.6.(a) Compressive stress-strain curves of (ZrCu)100-xTax (x=2,4,6,8,10,and 15) alloys,(b) effect of Ta content on fracture stress and strain.

Fig.7.Compressive cycle stress-strain curves under different pre-strain.(a)(ZrCu)98Ta2 alloy;(b)(ZrCu)96Ta4 alloy;(c)(ZrCu)94Ta6 alloy;(d)(ZrCu)92Ta8 alloy;(e)(ZrCu)90Ta10 alloy;(f) (ZrCu)85Ta15 alloy.

Fig.6(a)shows the compressive stress-strain curves of(ZrCu)100-xTaxalloys at 25°C,and Fig.6(b) extracts fracture stress and strain of(ZrCu)100-xTaxalloys as a function of Ta content.Compared to Zr50Cu50alloy,the improvement of the compressive strength in(ZrCu)98Ta2alloy is mainly ascribed to the solution strengthening and refinement strengthening caused by the Ta addition.It should be mentioned that both fracture stress and strain decreased first and then increased with the increasing Ta content from x＞2%.This is attributed to the different shape,size,and distribution of the Ta2Cu precipitate phase that plays a different role in the mechanical properties.A big difference was that the size of the Ta2Cu phase became smaller with more homogeneous distribution for the higher Ta content (x ≥6%),in contrast with alloys with low Ta content(much bigger size with an obvious aggregation).This phenomenon leads to the lowest fracture stress observed in(ZrCu)94Ta6alloy.No stress plateau was observed on the compressive stress-strain curves of (ZrCu)100-xTaxalloys,like Zr50Cu50alloy and some TiNiHf alloys,but this is distinct from other typical SMAs.According to the previous investigations,the combination of twinning mechanism and plastic deformation during deformation lead to the disappearance of the stress plateau[7,20].

Fig.8.Effect of Ta content on shape recovery rate.

Fig.7(a–f) shows the loading-unloading stress-strain curves of(ZrCu)100-xTax

alloys obtained from cyclic compression testing.The black line represents elastic recovery after unloading.The red line represents SME strain caused by the reverse transformation.Fig.8 exhibits the variation of the shape recovery ratio under different pre-loading stress.As the Ta content x ≤6%,(ZrCu)100-xTaxalloys showed a perfect recovery strain of 3%.Under the larger pre-strain than 3%,the completely recoverable strain first decreased and then increased with increasing Ta content.(ZrCu)98Ta2alloy had the best completely recovery strain of 6.51%under 8%pre-strain,significantly higher than Zr50Cu50alloy(a recovery strain of 5.91% under 8% pre-strain).This may be due to the precipitation behavior of a limited number of Ta2Cu phase with a small size that enhances the matrix strength and avoids plastic deformation.

The reason for the increasing Martensitic transformation and its reverse transformation is complex and not clear after Ta addition,but it can be related to the microstructure.The forward-transformation temperature intervals and hysteresis is related to the elastic strain energy and irreversible energy.With the increment of the Ta content,the large amount of Ta2Cu phase precipitate act as obstacles to hinder the mobility of the martensite interface during deformation.This probably gives rise to the increasement of martensitic transformation and the observed deterioration of the shape memory effect.In addition,with the increased stress the shape recovery rate decreases because of the plastic deformation leading to the unrecoverable strain.

The effect of Ta content in (ZrCu)100-xTaxalloys on the microstructure,martensitic transformation,and the shape memory effect has been investigated.

1.(ZrCu)100-xTaxalloys show the composite structure consisting of Ta2Cu phase in the matrix of the monoclinic ZrCu martensite phase,where the larger amount of Ta2Cu phase has been observed with increasing the Ta contents.

2.Ta addition effectively increases the martensitic transformation temperatures,the corresponding peak of martensitic transformation temperature(Mp)of (ZrCu)85Ta15alloy is boosting up to 150°C.

3.The gradually decreased ductility of(ZrCu)100-xTaxalloys is due to the precipitation behavior of the Ta2Cu second phase.

4.Compared with Zr50Cu50alloy the shape recovery of (ZrCu)100-xTaxalloys is improved for appropriate Ta addition but becomes worse for higher Ta content (more strictly as Ta2Cu precipitation).The maximum recovery strain of 6.19% with 8% pre-strain has been obtained in(ZrCu)98Ta2alloy under compression at 25°C.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

Project (51701045) supported by the National Natural Science Foundation of China,project (3072021CFJ1003) supported by Harbin Engineering University basic scientific research business expenses of central universities,project (61429090307) supported by Key Laboratory of Precision Thermal Processing of Metals Fund project of Harbin Institute of Technology,and project supported by"Ten Thousand Million" Engineering Science and Technology Major Special Project of Heilongjiang Province.