Orientation dependence of the shape memory effect and superelasticity in ferromagnetic Co49Ni21Ga30 single crystals with γ’-phase particles

This paper reports on the orientation dependence of the shape memory effect and superelasticity in [001] and [1̅23] single-phase and aged at 623K, 1 hour single crystals of ferromagnetic Co49Ni21Ga30 (at.%) alloy with B2-L10 martensitic transformation. It was demonstrated that in single-phase crystals the values of reversible strain and the values of thermal and stress hysteresis depend on the crystal orientation. Precipitation of γ'-phase particles reduces the value of the shape memory effect and superelasticity and reduces their orientation dependence, increases the thermal and stress hysteresis in comparison with singlephase crystals.


Introduction
Currently, the ordered CoNiGa alloys with B2-L1 0 thermoelastic martensitic transformation (MT) (B2 ordered phase based on volume-centered cubic lattice, L1 0tetragonal martensite based on a facecentered tetragonal lattice) are promising ferromagnetic alloys with high strength and good ductility [1][2][3][4][5][6][7][8]. In CoNiGa crystals, firstly, a high stress level of the high-temperature phase can be achieved by choosing the orientation. It is known [9] that in alloys with B2 structure the dislocation slip takes place along the <100> direction on {110}, {100} slip planes. Under tension/compression deformation the [001] crystals are characterized by high stresses level of B2-phase because of the Schmid factor for operating slip systems is zero in contrast to other orientations, in which Schmid factors for these systems are high. Therefore crystals oriented along the [001] direction should show a wider temperature interval of stress-induced MT compared with other crystal orientations. Secondly, a high stress level of high-temperature phase in CoNiGa alloy can be achieved by precipitation of γ'-phase particles (γ'-phasean ordered face-centered cubic with L1 2 structure) [1][2][3][4]10]. Precipitation of γ'phase particles will allow to expand the temperature interval of superelasticity (SE) not only in the [001]-oriented crystals, but also in other orientations, result from increased stress levels of hightemperature phase and suppression of local plastic flow processes during the formation of martensite crystals under stress and to receive alloys with high-temperature of SE, a manifestation which will not depend on the crystal orientation. To date there have been no systematic investigations of the simultaneous influence of the crystal orientation and dispersed γ'-phase particles on the functional properties of single-crystal of Co 49 Ni 21 Ga 30 alloys. Therefore, in this paper presented the results of investigations on the effect of crystal orientation and γ'-phase particles on the value of shape memory effect (SME), ε SME , and SE, ε SE , thermal ΔT σ and stress Δσ hysteresis under stress, the temperature Secondly, in the [001]-oriented single crystals the contribution of detwinning strain of L1 0 -martensite, ε detw , to the total lattice deformation at B2-L1 0 MT is zero, and in the 23]-oriented single crystals L1 0 -martensite detwinning takes place and the associated strain of ε detw =0.8% [6] . This choice of orientations will allow not only to trace the influence of orientation on the functional properties due to different values of ε 0 , but also reveal the influence of L1 0 -martensite detwinning on energy dissipation processes.

Materials and methods
Single cryslals of Co 49 Ni 21 Ga 30 (at.%) alloy were grown by the Bridgman technique in inert gas atmosphere. Samples for compression tests were in parallelepiped form in size of 3x3x6 mm 3 with the compression axis oriented along the [001] and 23] directions. For single-phase state the samples were kept at T=1430 K for 30 minutes in quartz tubes in inert gas atmosphere followed by water quenching. To precipitate the nanometric dispersed γ'-phase particles selected low temperature aging at 623 K for 1 hour. After this aging the γ'-phase particles are spherical of size d=5 nm, and the volume fraction of f=15% [1]. Shape memory effect was measured using a specially designed installation when cooling/heating at different levels of external stresses. SE effect of single crystals were examined by the Instron 5969 at the temperature interval from 273 K to 623 K under compression. MT temperatures in free state were determined by differential scanning calorimetry (DSC). In the single-phase crystals peaks of direct and reverse transformation are observed by the DSC method and B2-L1 0 MT is characterized by low thermal hysteresis: ΔT=A f -M s =296-272=24K (M sstart temperature of the forward MT on cooling; A fthe finish temperature of the reverse MT on heating). At precipitation of nanometric γ'-phase particles after aging at T=623 K, 1h the MT temperatures shifted to lower temperatures: M s =165K, A f =274K, and the thermal hysteresis ΔT=A f -M s =109К increased in 4.5 times in comparison with single-phase crystals without γ'-phase particles. Figures 1 and 2 presents the results of a study of SME recorded during cooling/heating experiments under different external stresses from 2.5 MPa to 350 MPa for crystals oriented along the [001] and 23] directions in a single-phase state and after aging at 623K, 1 h. Such experiments allow us to determine the B2-L1 0 MT temperatures, the value of thermal hysteresis, ΔT σ , and transformation strain, ε SME , depending on external applied stresses.

Results and discussion
It is seen that in single-phase crystals oriented along the [001] and 23] directions one-stage B2-L1 0 MT is realized, which is fully reversible by heating and thus SME is observed. The value of SME in single-phase crystals depends on the crystal orientation. Thus, in crystals oriented along the [001] direction, at σ ext =2.5 MPa the ε SME =3.5%, and this value is close to theoretically calculated value of the lattice deformation ε 0 =4.5% for a given crystal orientation at B2-L1 0 MT [6]. Consequently, even at minimal external stresses of σ ext =2.5 MPa in single-phase crystals oriented along the [001] direction, occurs the destruction of the self-accommodated L1 0 -martensite microstructure and growth oriented twinned L1 0 -martensite [4]. In [001]-oriented single crystals at σ ext =20 MPa reaches a maximum reversible strain ε SME =4.2%, which is equal to the value of ε 0 =4.5% for a given orientation of the crystals at B2-L1 0 MT [6]. With increasing σ ext >60 MPa the value of ε SME [001] decreases slightly. In crystals oriented along the 23] direction at σ ext =2.5 MPa ε SME [123] =2.5% and this value is close to theoretically calculated value of the lattice deformation ε 0 =3.2% for the 23] crystals at B2-L1 0 MT as well [6]. This means that in 23] crystals at the minimum stress at σ ext =2.5 MPa selfaccommodated L1 0 -martensite structure is destroyed and there is a growth oriented L1 0 -martensite containing no twins as the value of ε SME =2.5% at σ ext =2.5 MPa exceeds value of ε 0 =ε CVP =2.4% (the contribution of detwinning strain of L1 0 -martensite in 23] crystals at B2-L1 0 -MT is 0.8% [6]). When σ ext >20 MPa the ε SME =2.9% reaches a maximum value close to the value of ε 0 for a given orientation of the crystals at B2-L1 0 MT [6] and with the increasing of σ ext the ε SME remains practically constant.
In both crystal orientations with minimal external compressive stresses the start temperature of the forward B2-L1 0 MT on cooling under stress is identical and M s σ =263 К. This temperature is close to M s , that determined by DSC for single-phase crystals in the free state. However, the value of the thermal hysteresis ΔТ σ =A f σ -M s σ , characterizing the energy dissipation at MT under stress, in [001]oriented single crystals has smaller value than in 23]-oriented single crystals, and in stress interval of σ ext =2.5÷40 MPa decreases from ΔT σ =27K to ΔТ σ =15K and then remains constant, and in 23]oriented single crystals, on the contrary, increases from ΔТ σ =42K to ΔТ σ =55K (figure 1). In 23]oriented crystals with increasing applied external stresses the position of habit plane of L1 0 -martensite under detwinning is changed relatively non-detwinning state. This leads to additional internal stresses and energy dissipation under unloading and it associated with the increase of ΔТ σ in the 23] crystals. In [001]-oriented single crystals, where the detwinning strain of L1 0 -martensite suppressed because of equality to zero Schmid factors, habit plane of L1 0 -martensite does not change its position [11], energy dissipation when unloading there is no and ΔT σ in [001]-oriented single crystals does not increase.   figure. 4), which is described by the Clausius-Clapeyron relation [11]: here ΔH and ΔSrespectively enthalpy and entropy change at B2-L1 0 MT; ε 0lattice deformation which depends on the crystal orientation; T 0temperature of chemical phase equilibrium which may be calculated as T 0 =1/2(M s +A f ) and ρmass density. From figure 3 shows that in the single-phase [001] and 23] crystals value of α=dσ cr /dT depends on the crystal orientation: α [001] =1.74 МPа/K, α [123] =2.95 МPа/K. Orientation dependence of α=dσ cr /dT is due to the orientation dependence of the ε SME =ε 0 . From (1) that smaller value of α=dσ cr /dT should corresponds to larger value of ε 0 and, in contrast, the smaller value of ε 0 should corresponds to larger value α=dσ cr /dT. This is confirmed experimentally.  At precipitation of γ'-phase particles in crystals oriented along the [001] and 23] directions does not change the nature of the MT and one-stage B2-L1 0 MT is observed as well as in single-phase crystals, which completely reversible after heating. Consequently, SME is realized. As can be seen from figure 2 at the precipitation of γ'-phase particles the SME under stress at σ ext exceeds of σ ext for single-phase crystals without the particles and the value of SME becomes smaller. Thus, in the [001]oriented single crystal of ε SME =1.5% is observed when σ ext =18 MPa, and in 23]-oriented sungle crystals ε SME =1.6% at σ ext =75 MPa. When σ ext =100 MPa values of SME are close: ε SME [001]=2.2%≈ε SME 23]=1.9% and with increasing of σ ext values of SME vary slightly (Figure 3, b,  curves). Therefore, the dispersed γ'-phase particles creates significant resistance to the movement of interphase and twin boundaries. Decrease of ε SME at precipitation of γ'-phase particles may be due to several factors: 1) a decrease in the volume fraction of the matrix, which undergoes MT with precipitation of γ'-phase particles, which do not undergo the MT and 2) difficulties of detwinning deformation of L1 0 -martensite [12]. At precipitation of γ'-phase particles after aging at T=623K, 1 h in both orientations there is a decline of M s σ temperature on ~110K as compared to single-phase state (Figure 1, 2). That also correlated with the data obtained by DSC [1]. At that the value of ΔТ σ is larger than in single-phase crystals and with growth of σ ext in both orientations the value of ΔТ σ is decreased. For example, in [001]-oriented single crystals with γ'-phase particles at minimal σ ext =18 MPa the ΔТ σ =100К and with increasing stress up to σ ext =300 MPa the ΔT σ is reduced to 50 K. In the 23] -oriented single crystal with γ'-phase particles at minimum σ ext =75MPa value of ΔТ σ =110K with increasing of σ ext to 350 MPa the ΔТ σ is reduced by 2 times. As well as in single-phase state in the crystals with particles ΔT σ 23]>ΔT σ [001] (figure 2). The physical reason of the increase of ΔT σ in crystals with particles compared with single-phase crystals without the particles is associated with resistance of particle to movement interphase and twins boundaries, with interaction of martensitic variants (interaction of martensite arising near the particles with stress-induced martensite) and with increasing density of twins martensite as has previously been shown in [1]. When the stress increases selection variant of stress-induced martensite is advantageous and the interaction between martensite variants makes a smaller contribution to the energy dissipation and this may be due to a decrease of ΔT σ with growth of external applied stress [4].
With an increase in the level of external applied stress σ ext in [001]-and 23] -oriented single crystals with γ'-phase particles as well as single-phase crystals the M s σ temperature rise is observed and with increasing of M s σ the stresses of σ ext =σ cr are falling on linear dependence. As seen from figure 4 the value of α=dσ cr /dT in the crystals with γ'-phase particles becomes smaller than a single-phase state for corresponding orientation and weakly dependent on orientation: α [001] =1.49 МPа/K и α [123] =1.55 МPа/К. According to relation (1), when the value of SME decreases the values of α=dσ cr /dT must increase and, therefore, must be larger than in single-phase crystal for corresponding orientation. This contradiction of the experimental data with the relation (1) is due to the dependence of the ε SME and ΔT σ and, accordingly, energy dissipation on the level of externally applied stresses, that by us it was shown in [2]. The weakening of the orientation dependence of α=dσ cr /dT in crystals with the γ'phase particles is due to degeneration orientation dependence of ε SME value and the ratio of ε SME -oriented single-phase crystals the SE is observed a wide temperature interval from A f =283K to 573K, ΔT SE =290К that is associated with the development of reversible stress-induced B2-L1 0 MT. Therefore, the studied alloys exhibit high-temperature SE. In 23] -oriented single-phase crystals SE is observed in the temperature interval of T=318÷473K and ΔT SE =155К. Precipitation of γ'-phase particles leads to an expansion of temperature interval of SE in both orientations: in [001]-oriented single crystals the SE observed from Т=273К to Т=623К and ΔТ SE [001]=350К and in 23]from Т=298К to Т=573К and ΔТ SE 23]=275К that exceeds the ΔT SE in single-phase crystals for the corresponding orientations. In the crystals oriented along the 23] direction SE also has a place at T=573K. Consequently, the precipitation of γ'-phase particles provides a large temperature interval of SE including at high temperature at T>573K, not only in [001]-orientation, but also in other orientations due to increasing the stresses level of high-temperature phase and suppressing local plastic flow processes during stress-induced martensite. Figure 4 shows the «σ-ε» curves at T=323K for single-phase and aged at T=623K, Δσ=73MPa. Precipitation of γ'-phase particles leads to a decrease of ε SE , increase of Δσ and weakening their orientation dependence: in [001]-oriented single crystals at T=323K ε SE =2.2%, Δσ=90MPa, and in 23]-oriented single crystals ε SE =2.1% and Δσ=95MPa. It should be noted that the value of SE equals to the value of SME for corresponding crystals orientation both in single-phase state and with the γ'-phase particles.

Summary
It was established experimentally that in single-phase Co 49 Ni 21 Ga 30 (at. %) crystals the values of SME and SE, the temperature interval of SE, the values of thermal and stress hysteresis depend on the crystal orientation. In [001]-oriented single-phase crystals the value of SME and SE has a maximum value equal to 4-4.5%. SE is observed over a wide temperature interval ΔT SE =290K and high temperature at T=573K and characterized of narrow stress hysteresis. Precipitation of γ'-phase particles leads to a decrease values of SME and SE, an increase of the temperature interval of SE, of the values of stress and thermal hysteresis under stress and a weakening them depending on the crystal orientation. It is shown that in crystals with the γ'-phase particles due to changes in the level of external applied stresses can control the value of thermal hysteresis from 100-110K up to 50 K.
It is shown that an increase in strength properties of high-temperature phase due to the γ'-phase precipitate can be obtained the SE over wide temperature interval and at high temperatures not only in the crystal of [001] orientation but in 23] -oriented single crystals.