Effect of adding Si on shape memory effect in Co–Ni alloy system

doi:10.1016/j.msea.2006.04.121

Materials Science and Engineering: A

Volumes 438–440, 25 November 2006, Pages 468-471

https://doi.org/10.1016/j.msea.2006.04.121 Get rights and content

Abstract

In this paper, the effect of adding Si to Co–31.5 mass% Ni alloys on fcc–hcp martensitic transformation is investigated. The Co–Ni–Si ternary alloys with different amount of Si from 1 to 5 mass% were prepared. The stacking fault probability of Co–Ni–Si polycrystalline alloys were determined by X-ray diffraction profile analysis and compared with the binary Co–Ni alloy. The results show that the stacking fault probability of the fcc phase of alloys increases with increasing Si content. The effect of Si on phase transformation and shape memory behavior is evaluated. The experimental results show that both the critical strength and the shape memory effect of the ternary alloys will increase by the addition of Si. The improvement mechanism of the shape memory effect by adding Si to binary Co–Ni alloys is discussed.

Introduction

Co–Ni alloys are interesting materials characterized by a magnetically induced shape memory effect due to its higher saturation magnetization in both parent and martensitic phases [1], [2], [3]. So far reversible strain can be induced under magnetic field in Co–Ni single crystals. However, the magnetically induced strain in these alloys is not very stable and can decay easily. This is due to the low yield strength in these alloys, which is caused by the perfect dislocations in the parent phase of these alloys that can be easily moved. According to our research work [4], the shape memory effect (SME) in binary Co–Ni alloys is limited. The studies [4], [5] shows that the SME of Co–Ni polycrystalline alloys decreased with the increase in Ni content. For those of which Ni content is higher than 30 mass%, no SME was detected. In Co–32Ni (mass%) single crystal only the sample with [0 0 1] direction displays obvious SME [3].

It is well known that Si plays an important role for the improvement of shape memory effect in Fe–Mn alloys. The Fe–Mn–Si alloys with 28–33% Mn and 4–6% Si exhibit a nearly perfect shape memory effect [6]. It is demonstrated that the SME is improved by increasing the yield stress of the parent phase and lowering the stacking fault energy and Neel temperature by adding Si in Fe–Mn alloys. However studies on the effect of Si on γ(fcc) → ɛ(hcp) martensitic transformation and shape memory behavior in Co–Ni alloys were seldom reported.

There exists a γ(fcc) → ɛ(hcp) martensitic transformation in binary Co–Ni alloy system of less than 35 mass% Ni [8]. The thermodynamic and mechanism of nucleation on γ → ɛ transformation in Co-based alloys have been investigated [7], [8], [9]. In this paper we chose Co–31.5% Ni-based polycrystalline alloys, which have no SME, and different amount of third element Si were added to the alloys. The effect addition of Si on stacking fault probability of γ-phase and shape memory effect in Co–Ni binary alloys was investigated. Thereby the role of Si on phase transformation and shape memory behavior is discussed.

Section snippets

Experimental procedure

Four kinds of alloys with different content of Ni are prepared by using metal elements Co, Ni, Si with the purity of more than 99.5%. After melted in non-consumable vacuum arc furnace and homogenization at 1373 K, the ingots were hot rolled into 1.2 mm thick plates at 1073 K. The specimens with the dimension of 60 mm × 2 mm were cut and quenched from 973 K into icy water. The chemical composition of the alloys are listed in Table 1.

The tested samples are electro-polished to insure that there exist no

Results and discussion

The determined M_s and A_s temperatures and stacking fault probabilities P_sf of four tested alloys are listed in Table 2.

From Table 2 we can find that the starting point of martensitic phase transformation M_s increases with increasing Si content but the starting temperature of reverse phase transformation A_s deceases, accompanied by a decrease of phase transformation hysteresis. At the same time the stacking fault probability of γ-phase of alloys increases with increasing Si content as shown in

Conclusion

Both the critical strength and SME of the ternary polycrystalline alloys will increase by the addition of Si. The improvement mechanism of SME by adding Si to binary Co–Ni alloys may be due to the strengthening of parent phase and the increase of stacking probability, which is favorable for the formation of stress-induced ɛ martensite.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 50171042) and the Fundamental Research Project of Shanghai Science and Technology Committee (No. 0452nm054).

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Significant improvement of shape memory effect in Co-Ni-based alloys through Si alloying
2019, Journal of Alloys and Compounds
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They ascribed the significant improvement of SME by Si addition to its resulting two beneficial effects: 1) to increase yield strength and 2) to decrease stacking faults energy of matrix [9,10]. Unexpectedly, the studies of Zhou et al. showed that the SME was only 13% in the Co-31.4Ni-4.7Si alloy after tensile deformation at RT by 2% although the Si addition also strengthened the matrix and lowered its stacking faults energy [18]. Our recent work showed that the SME in a Co-30.8Ni-3Si alloy was also only 20% when bent at RT by 2.4%, but it could remarkably increase to 59% when bent at 77 K [1].
We proposed that much lower forward martensitic transformation temperature (M_s) and lower yield strength were responsible for lower improvement of shape memory effect (SME) at room temperature (RT) in Co-based alloys than that in Fe-Mn-based alloys by Si alloying. To prove our opinion, the SME and martensitic transformation were investigated in Co-xNi-6Si (x = 15, 20, 25 wt%) alloys with higher Si content. The results confirmed that the Si addition could significantly improve the SME of Co-Ni-based alloys after adjusting the M_s temperature to around RT. The Co-20Ni-6Si alloy with M_s temperature close to RT showed the highest shape recovery of 80% after bent by 2.7%. This value was much higher than the ones in the Co-15Ni-6Si and Co-25Ni-6Si alloys whose M_s temperatures were much far from RT. The reduction in the M_s temperature by the Si addition augmented with increasing the Ni content.
Improvement of low-cycle fatigue resistance of Co-Ni alloys by silicon addition
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Ferromagnetic Co alloys showing mechanical twining of the austenitic matrix [14,15] or the shape memory effect (SME) caused by deformation induced ε-MT [16,17] were selected as promising candidates for this evaluation. Numerous studies dedicated to SME of Co alloys [17–19] noted similarities in phase transformation behavior of the Co alloys and the Fe-based high-Mn shape memory alloys. In an old report [15], LCF properties of Co–31Ni and Co–33Ni alloys have been discussed in light of dislocation slip character and strain-induced ε-MT. In that study, ordinary LCF resistance was observed to be comparable to that of the Fe-based high-Mn alloys with microstructure not satisfying the design criteria proposed in [11].
The effect of silicon alloying on the low-cycle fatigue (LCF) resistance and fatigued microstructure of the Co–31Ni, Co–31Ni–3Si, Co–33Ni, and Co–33Ni–3Si (in wt%) alloys was studied under an axial strain control mode at the total strain amplitude, Δε_t/2, of 0.01. The addition of 3 wt% of Si increases the fatigue life, N_f, of the examined Co alloys by more than 2.5 times. For instance, N_f of ~ 4000 and 10,000 cycles were achieved in the Co–31Ni and Co–31Ni–3Si alloys, respectively. It was observed that improved LCF resistance of the Si added Co alloys results from the moderate kinetics of the deformation-induced ε-martensitic transformation (ε-MT) and inhibited cross-slip of perfect dislocations provided by solid solution hardening of the austenite. The similarity in the fatigue behavior and fatigued microstructure of the Si added Co alloys and Fe-based high-Mn alloys is discussed.
Role of thermal martensite in shape memory effect of CoAl and CoNi alloys
2017, Transactions of Nonferrous Metals Society of China (English Edition)
To address the role of the HCP martensite in CoAl and CoNi shape memory alloys, the relationship between the shape memory effect (SME) and the content of the thermal and stress-induced HCP martensite was investigated in the solution-treated CoAl and CoNi alloys. In-situ optical observations were employed to investigate the contents of thermal HCP martensite before and after deep cooling and its influence on the stress-induced HCP martensite transformation and SME. The results show that the SME in both the CoAl and the CoNi alloys results from the stress-induced HCP martensite. The role of the thermal HCP martensite in both of them is the strengthening of the matrix. The much higher yield strength in the solution-treated CoAl alloy due to solution strengthening of Al is responsible for its better SME compared with the CoNi alloy.
Shape recovery increase in a Co-Al-W alloy realized by stress-induced hcp martensitic transformation after strengthening matrix
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Co-based alloys undergoing a face-centered cubic (fcc) ⇌ hexagonal close-packed (hcp) martensitic transformation, as a new kind of shape memory alloys, have drawn attention in the past decade [1–7]. The reason was that a good shape memory effect (SME) can be expected in them because the fcc ⇌ hcp martensitic transformation can result in a good SME in Fe-Mn-Si-based alloys [1–13]. On the other hand, both the fcc and hcp phases are ferromagnetic in them [1–5,14–16], while both of them are paramagnetic in the Fe-Mn-Si-based alloys [9,17].
The shape memory effect (SME), microstructures, and mechanical behavior after the deformation at different temperatures were investigated in a solution-treated Co-6.6Al alloy and solution-treated and aged Co-6.5Al-5.4W alloys. The SME in both the solution-treated and aged Co-6.5Al-5.4W alloys without thermal hcp martensite rose rapidly as lowering the deformation temperature, but it rose slightly in the solution-treated Co-6.6Al alloy containing 85% thermal hcp martensite. Regardless of the deformation temperature, the aged Co-6.5Al-5.4W alloy showed the highest SME, and a large recovery strain of 3.9% was obtained. This work clarifies that the SME in the Co-Al-based alloys results also from the stress-induced hcp martensitic transformation. The significant precipitation strengthening of β-CoAl phase is responsible for this remarkable improvement. A higher recovery strain can be expected in other Co-based alloys with hcp martensitic transformation by further strengthening matrix and promoting stress-induced hcp martensitic transformation.
Origin of shape memory effect in Co-Ni alloys undergoing fcc⇌hcp martensitic transformation
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Co–Ni alloys undergoing a face-centered cubic (fcc)⇌hexagonal close packed (hcp) martensitic transformation have drawn attention in the last over decade [1–9].
It is not clear about which of the coalescence of thermal hcp martensite and stress-induced hcp martensite is responsible for shape memory effect (SME) in Co–Ni alloys undergoing fcc⇌hcp martensitic transformation. To clarify this problem, the relationship was re-examined between the SME and the amount of thermal hcp martensite in a Co–30.3Ni alloy obtained by deep cooling. The relationship between the SME and the amount of stress-induced hcp martensite was also investigated in the Co–30.3Ni alloy and in a Co–30.8Ni–2.99Si alloy without thermal hcp martensite. The results show that the SME in the Co–Ni alloys results from the stress-induced hcp martensite, not from the coalescence of the thermal hcp martensite. The low yield strength leads to their poor SME. The increase in the SME with the rise in the amount of thermal hcp marteniste results from its strengthening of matrix and promoting of stress-induced hcp martenite. The SME is much better in the Co–30.8Ni–2.99Si alloy than in the Co–30.3Ni alloy because much more stress-induced hcp martensite were produced in the Co–30.8Ni–2.99Si alloy. Strengthening of matrix by other methods will improve the SME in Co–Ni alloys undergoing fcc⇌hcp martensitic transformation.
Remarkable improvement of shape memory effect in a Co-31Ni-3Si alloy by training treatment
2014, Materials Science and Engineering: A
The influence of training cycles consisting of deformation by 5% at room temperature and subsequent annealing at different temperatures on the shape memory effect was studied in a Co–31Ni–3Si alloy to improve its shape memory effect. The results showed that the optimal annealing temperature in training was 973 K. A large recovery strain of 2.3% was obtained after one training cycle at the optimal annealing temperature and then bend by 3.7% at 77 K. The training promoted the stress-induced epsilon martensitic transformation and suppressed the occurrence of slip deformation due to the increase in yield strength. These results revealed that the shape memory effect in Co–Ni-based alloys stems from the stress-induced epsilon martensitic transformation. Their low yield strengths account for their poor shape memory effect.

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Effect of adding Si on shape memory effect in Co–Ni alloy system

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusion

Acknowledgments

Acta Metall.

Acta Mater.

Appl. Phys. Lett.

Proceedings of the ICOMAT’02, International Conference on Martensitic Transformations

J. Phys. IV (France)

Appl. Phys. Lett.

Mater. Sci. Forum