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Cyclic ageing of Ti–50.8 at.% Ni alloy
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Intermetallics 16 (2008) 394e398 www.elsevier.com/locate/intermet
Cyclic ageing of Tie50.8 at.% Ni alloy Fei Jiang a, Li Li a, Yufeng Zheng a,b, Hong Yang c, Yinong Liu a,c,* b
a Centre for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, PR China Department of Advanced Materials and Nanotechnology, College of Engineering, Peking University, Beijing 10087, PR China c School of Mechanical Engineering, The University of Western Australia, Crawley, WA 6009, Australia
Received 5 October 2007; received in revised form 24 November 2007; accepted 27 November 2007 Available online 14 January 2008
Abstract This study investigated the effects of cyclic ageing on the transformation behaviour of Tie50.8 at.% Ni by means of differential scanning calorimetric analysis. It is found that cyclic ageing between two different temperatures causes reversible changes of the transformation behaviour and alternating increases and decreases of transformation temperatures. This is attributed to the temperature dependence of the phase equilibria between the B2 matrix and Ni-rich precipitates. Based on the experimental evidences, the solvus of the precipitates (Ti3Ni4) in B2eNiTi is estimated. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: B. Martensitic transformations; B. Shape-memory effects; C. Heat treatment; F. Calorimetry
1. Introduction Near-equiatomic NiTi alloys are attractive functional shape memory alloys for a wide spectrum of innovative applications, largely owing to their novel properties of shape memory effect and pseudoelasticity. It is well known that the transformation behaviour and thermomechanical properties of NiTi alloys with excess Ni to the equiatomic stoichiometry are sensitive to ageing treatment due to the formation of Ni-rich precipitates [1,2]. Three types of Ni-rich precipitates have been observed, including TiNi3, Ti2Ni3 and Ti3Ni4 [1e7]. TiNi3 [4] and Ti2Ni3 [2,5] are incoherent in B2eNiTi, thus ineffective to the transformation behaviour of the B2 matrix. In comparison, Ti3Ni4 is more influential to the transformation behaviour and thermomechanical properties of NiTi, including the R-phase transformation [8,9], all-round shape memory effect [10] and multiple-stage transformation behaviour [11]. Ti3Ni4 has a hexagonal structure and is coherent with the B2-matrix [6,7].
* Corresponding author. School of Mechanical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Tel.: þ61 8 6488 3132; fax: þ61 8 6488 1024. E-mail address: [email protected] (Y. Liu). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.11.012
Precipitation is a thermally reversible process. In 1988, Horikawa et al. [12] conducted a study of ‘‘short-time’’ cyclic anneals between 713 K and 783 K of a Tie51 at.% Ni alloy. They observed an interesting up-and-down movement of the transformation temperatures with anneal cycles. Zhang et al. [13] later conducted a similar study with long-time ageing and observed the same reversible change of transformation temperatures with repeated ageing cycles. They explained the phenomenon in terms of phase equilibrium between the NiTi matrix and the metalstable Ti3Ni4 precipitates at different ageing temperatures. Whereas the concept of the equilibrium between B2-matrix and Ti3Ni4 has been reasonably well established, actual data on the position of the solvus [14] and cyclic variation of the transformation behaviour [13] have been scarce in the literature. This study investigated the effect of cyclic ageing on the transformation behaviour of a Tie50.8 at.% Ni alloy. Based on the transformation temperatures determined, the solvus between Ti3Ni4 and the B2eTiNi matrix is estimated. 2. Experimental procedure A commercial Tie50.8 at.% Ni alloy wire of 1 mm in diameter was used in this study. Samples were solution treated
F. Jiang et al. / Intermetallics 16 (2008) 394e398
in vacuum at 1273 K for 3.6 ks. Two solution treated samples were further heat treated following different ageing routines. Sample A was cyclically aged at 673 K for 126 ks and 753 K for 45 ks. Sample B was cyclically aged at 753 K for 45 ks and 833 K for 10.6 ks. The ageing treatment durations were chosen to be long enough for the respective ageing temperatures to ensure full precipitation. After each ageing treatment, the samples were slightly chemically etched to remove surface oxide layer using an aqueous solution of 40 vol.% HNO3 and 10 vol.% HF. The transformation behaviour of the samples was characterized using a PerkineElmer Diamond Differential Scanning Calorimeter (DSC) with a heating and cooling rate of 20 K/min. 3. Results Fig. 1 shows DSC measurements of the transformation behaviour of the Tie50.8 at.% Ni alloy over three ageing cycles between 673 K and 753 K. Curve (a) was measured under the solution treated condition. The sample underwent a singlestage A 4 M transformation, at 240 K and 276 K on cooling and on heating, respectively. Curve (b) was measured after ageing at 673 K for 126 ks. The sample developed two-stage transformations both on cooling and on heating, which are identified to be A 4 R (1e10 ) and A 4 M (2e20 ) transformations. It is necessary to clarify that these are two independent transformation streams. The occurrence of such separate transformation streams is indicative to microstructural inhomogeneity of the matrix [11]. The A 4 M transformations occurred at significantly increased temperatures compared to the transformations in the solution treated sample, indicating depletion of Ni content in the B2 matrix due to formation of Ni-rich precipitates. Curve (c) was measured after a second ageing treatment at 753 K, 80 K above the initial ageing temperature, for a further 45 ks. The sample exhibited a transformation sequence of
TAeM ¼
DH AeM DH DH ReM þ TAeR TReM
where DH are the transformation enthalpies, which can be determined experimentally from the DSC curves, and TAeR and TReM are peak temperatures of the respective transformations. The TAeM calculated using Eq. (1) is also shown in the figure, as the dashed curve. It is seen that TAeR, TReM, TAeM and TMeA all show similar zig-zag patterns of increase and decrease in accord with the ageing temperatures. It is also noticed that there were slight increases of TMeA and TReM (and TAeM) with increasing ageing cycles for both ageing
2'
1' 2
Transformation Temperature (K)
(exothermic)
1
(c)=(b)+753Kx45ks 3' (d)=(c)+673Kx126ks 3 (e)=(d)+753Kx45ks
TM-A 340
320
TA-R
300
TR-M
TA-M
280 673 K 260
(f)=(e)+673Kx210ks
753 K 240 0.0
220
240
ð1Þ
AeR
(endothermic)
(b)=(a)+673Kx126ks
Heat Flow
A / R / M on cooling and M / A on heating. The critical temperatures of the transformations are obviously lower than those after the first ageing treatment. This is attributed to increase in the Ni content of the matrix caused by the dissolution of Ni-rich precipitates at higher temperatures [13]. Curve (d) was measured after a further ageing treatment at the same temperature as for curve (b), at 673 K for 126 ks. The sample exhibited a similar transformation sequence of A / R / M on cooling and M / A on heating as curve (c) but at similar temperatures as (b). Continued ageing cycles practically repeated the transformation behaviours shown in curves (c) and (d). Fig. 2 shows the transformation peak temperatures as a function of the number of ageing cycles between 673 K and 753 K. The data points appearing at ‘‘half’’ cycles represent ageing at 673 K and those at ‘‘full’’ cycles represent ageing at 753 K. The two initial data at ‘‘0’’ cycle represent the as-solution treated condition. The occurrence of the A / R / M transformation on cooling suppressed the A / M transformation, thus direct measurement of the transformation is impossible. However, the temperature of the prohibited A / M transformation may be estimated based on thermodynamic principles as [15,16]:
360
(a) solution treated
200
395
260
280
300
320
340
360
380
0.5
1.0
1.5
2.0
2.5
3.0
Cycle Number
Ageing Temperature (K) Fig. 1. Transformation behaviour of Tie50.8 at.% Ni alloy after cyclic ageing at 673 K and 753 K.
Fig. 2. Effect of cyclic ageing at 673 K and 753 K on the transformation temperatures of Tie50.8 at.% Ni alloy. The dashed curve indicates TAeM calculated using Eq. (1).
F. Jiang et al. / Intermetallics 16 (2008) 394e398
temperatures. This is attributed to the incompleteness of the ageing process at the temperatures up to the ageing times. In contrast to the martensitic transformations, TAeR showed little variation with ageing cycle, implying much weaker composition and precipitation dependences of TAeR compared to the temperatures of the martensitic transformations [17]. Fig. 3 shows DSC measurements of the transformation behaviour of the alloy over two ageing cycles between 753 K and 833 K. After ageing at 753 K, the sample showed a two-stage A / R / M transformation on cooling and a single-stage M / A transformation on heating, typical of fully aged condition (curve (b)). When the ageing temperature was raised to 833 K (curve (c)), the transformation behaviour was restored to becoming similar to that of the solution treated sample (curve (a)), i.e. a single-stage A / M transformation, implying a solutionised condition. It is also clear that repeating the alternative ageing process simply restored the transformation behaviour to those shown in curves (b) and (c) alternatively. Fig. 4 shows the transformation temperatures as a function of the number of ageing cycles between 753 K and 833 K. The data points appearing at ‘‘half’’ cycles represent ageing at 753 K and those at ‘‘full’’ cycles represent ageing at 833 K. The sample showed no R-phase transformation after ageing at 833 K and in this case the TAeM values were directly determined from the DSC measurements. When aged at 753 K, the sample exhibited a two-stage A / R / M transformation on cooling. In this case TAeM is calculated using Eq. (1). It is seen that the transformation temperatures of this sample exhibited similar alternating increases and decreases with cyclic ageing to the sample shown in Fig. 2. Fig. 5 shows the dependences of TAeM and TMeA on ageing temperature. The data shown are collected from the last ageing cycle from each sample, because the ageing process is regarded the most complete in these cycles (refer to Fig. 1). The data for ageing temperature of 753 K are from the second sample (shown in Fig. 3). TAeM values shown for ageing
340
Transformation Temperature (K)
396
320 833 K 300
280 TM-A 260
753 K TA-M
240
220 0.0
0.5
1.0
1.5
2.0
2.5
Cycle Number Fig. 4. Effect of cyclic ageing at 753 K and 833 K on the transformation temperatures of Tie50.8 at.% Ni alloy.
temperatures of 673 K and 753 K are calculated values from TAeR and TReM using Eq. (1). The data at above 900 K are from two additional samples, whose transformation behaviours are shown in the inset. It is seen that the ageing temperature may be divided into two distinctive regions, as indicated in the figure. In region I, TAeM and TMeA decreased with increasing ageing temperature to 833 K. For higher ageing temperatures in region II, TAeM and TMeA remained unchanged, implying that the ageing temperature has reached above the solubility limit. This is consistent with the behaviour of single-stage A 4 M transformation observed, as shown in the inset. 4. Discussion 4.1. Effects of matrix composition and internal stresses Ageing of Ni-rich B2eNiTi alloys causes complex variations to the transformation behaviour, as evident in this study
(a) solution treated
Heat Flow
(c)=(b)+833Kx10.6ks
(d)=(c)+753Kx45ks
(e)=(d)+833Kx10.6ks
200
220
240
260
280
300
320
340
360
380
Ageing Temperature (K) Fig. 3. Transformation behaviour of Tie50.8 at.% Ni alloy after cyclic ageing at 753 K and 833 K.
903K 1.8ks
Heat Flow
Transformation Temperature (K)
340 (b)=(a)+753Kx45ks
TM-A
320
933K 1.8ks
TA-M
300
200
250
300
350
Temperature (K) 280 I II
260
240 650
700
750
800
850
solution
900
950
Ageing Temperature (K) Fig. 5. Ageing temperature dependence of TAeM and TMeA temperatures of Tie50.8 at.% Ni alloy.
F. Jiang et al. / Intermetallics 16 (2008) 394e398
as well as being well documented in the literature [18,19]. These effects have been traditionally attributed to variations in the chemical composition of the B2 matrix and the influences of internal stresses associated with precipitates, in particular coherent Ti3Ni4 [20]. It is our view that claims of the effect of internal stresses on transformation temperatures are incorrect. The common perception of the effect of internal stresses on transformation temperatures is based on the wellestablished understanding of the effect of applied constant stresses on transformation temperatures, as expressed in the ClausiuseClapeyron relation [21]. The analogy of internal stress to applied stress is inappropriate. In the case of a constant applied stress, the stress remains effective throughout the process of the transformation. In the case of local internal stresses, which are contained in the form of elastic strains, the stresses are relaxed immediately with the nucleation of martensite, due to the fact that the first martensite variant is formed ‘into’ the stress field, thus are ineffective to the bulk of the process of the transformation. In this regard, we consider that the transformation temperatures are mainly affected by the variations of the matrix composition and that the effect of Ti3Ni4 is mainly to facilitate the nucleation of the R-phase [22]. Fig. 6 shows the Ni-content dependences of TAeM and TMeA using data from this study as well as from the literature [23,24], as under solution treated condition. The slopes are determined to be 106 K/(at.% Ni) and 101 K/(at.% Ni) for TAeM and TMeA, respectively. Based on these dependences, Ni content of the matrix after ageing may be estimated using the transformation temperatures experimentally measured. Using the TMeA values shown in Fig. 5, which are less affected by the occurrence of the A / R transformation (as compared to TAeM), the Ni contents of the matrix after ageing at the three different temperatures are estimated to be 50.11 at.%, 50.34 at.% and 50.76 at.%, as indicated in Fig. 6.
360
397
4.2. Solubility of Ni in B2eNiTi Using the Ni content determined in Fig. 6, the solvus of Ti3Ni4 in B2eNiTi is estimated as shown in Fig. 7. For the Tie50.8 at.% Ni alloy, the dissolution temperature for Ti3Ni4 is estimated to be 833 K, which is consistent with that reported by Zhang [13]. The data points of the samples heat treated at 903 K and 933 K clearly indicate that they are above the solvus of Ti3Ni4, i.e. correspond to the ‘‘solutionised’’ condition. Also shown in the figure are the tentative positions of the solvuses of Ti2Ni3 and TiNi3 in B2eNiTi (the solvus of TiNi3 is defined in the equilibrium NieTi phase diagram). This implies that for a given alloy composition, different solution treatment temperatures exist for the three different precipitates, as indicated by T1, T2 and T3. In this regard, the condition of the two samples heat treated at 903 and 933 K is more appropriately regarded as ‘‘solutionised’’ for Ti3Ni4. The fact that the matrix composition of these two samples is slightly lower than the original (the TMeA temperature is slightly higher than the original value before ageing) implies that there may still exist some stable precipitates (TiNi3) at this temperature. 4.3. Mechanism of the effects of cyclic ageing This study is a continuation of the early works by Horikawa et al. [11] and Zhang et al. [13]. There have been discussions of the mechanism of this reversible movement of transformation temperatures with cyclic ageing, being either the reversion of unstable Ni-rich GP zones [11] or dissolution of metastable Ti3Ni4. In our opinion, the two hypotheses have no real contradiction in the principle of reversible change of the matrix Ni content, differing only on the aspect of which Ni-rich ‘‘phase’’ is formed and dissolved at different temperatures. This clarification is fundamentally necessary because Ni-rich near equiatomic NiTi alloys are known to develop several precipitates [1,3]. For each of these precipitates 1000
753 K
T3
320
950
TM→A 833 K
Temperature (K)
Transformation Temperature (K)
673 K
280 TA→M 240
900 850
T2
TiNi3
Ti2Ni3
T1
800 750
200 50.0
700 50.2
50.4
50.6
50.8
Ti3Ni4
51.0
Ni content (at ) Fig. 6. Ni-content dependences of TAeM and TMeA temperatures. Data expressed in solid symbols are from this study. Data expressed in crossed open symbols are from Ref. [23]. Data expressed in open symbols are from Ref. [24].
650 50.0
50.2
50.4
50.6
50.8
51.0
Ni Content (at ) Fig. 7. Solvus of Ti3Ni4 in B2eNiTi as estimated from the transformation temperature measurements.
398
F. Jiang et al. / Intermetallics 16 (2008) 394e398
(and GP zones), similar alternating cycles of formation and dissolution may occur. In this regard, the alternating variations of the transformation temperatures and sequences with alternating ageing treatment are intrinsic property of the alloys due to the varying solvuses of the precipitates. 5. Conclusions 1. Cyclic ageing at alternating temperatures within the range of 673e833 K causes reversible cyclic variations of transformation sequences and temperatures of Tie50.8 at.% Ni alloy. TAeM and TMeA temperatures increase with lowering ageing temperature and decrease with increasing ageing temperature reversibly. The R-phase transformation appears with lowering ageing temperature and disappears with increasing ageing temperature reversibly. 2. This reversible ageing effect is attributed to equilibrium between the B2eNiTi matrix and Ni-rich precipitates. Higher Ni content in the B2eNiTi matrix results in lower temperatures for the martensitic transformations in NiTi and vice versa. The presence of Ti3Ni4 is attributed to the cause of the appearance of the R-phase transformation. 3. The solvus of Ti3Ni4 in B2eNiTi is estimated based on transformation temperature measurement. The solvus of Ti3Ni4 in B2eNiTi for the Tie50.8 at.% Ni alloy is estimated to be at 833 K.
Acknowledgement The authors acknowledge the financial support for this study from the Outstanding Youth Fund of Heilongjiang
Province (JC200608) and the supply of the NiTi alloys by Jiangyin Fasten-PLT Materials Science Co., Ltd. References [1] Nishida M, Wayman CM, Honma T. Metall Trans A 1986;17: 1505e15. [2] Nishida M, Wayman CM. Metall Trans A 1987;18:785e99. [3] Wasilewski RJ, Butler SR, Hanlon JE, Worden D. Metall Trans A 1971;2:229. [4] Taylor A, Floyd RW. Acta Crystallogr 1950;209:1269. [5] Hara T, Ohba T, Otsuka K, Nishida M. Mater Trans JIM 1997;38: 277. [6] Nishida M, Wayman CM, Kainuma R, Honma T. Scripta Metall 1986;20:899. [7] Saburi T, Nenno S, Fukuda T. J. Less-Common Met. 1986;125:127. [8] Nishida M, Wayman CM. Metallography 1988;21:255. [9] Miyazaki S, Wayman CM. Acta Metall 1988;36:181. [10] Nishida M, Honma T. Scripta Metall 1984;18:1293. [11] Carroll MC, Somsen Ch, Eggeler G. Scripta Mater 2004;50:187. [12] Okamoto Y, Hamanaka H, Miura F, Tamura H, Horikawa H. Scripta Metall 1988;22:517. [13] Zhang J, Ren X, Otsuka K, Asai M. Scripta Mater 1999;14:1109. [14] Zhang J. Ph.D, thesis. University of Tsukuba; 2000. [15] Favier D, Liu Y, Orgeas L, Sandel A, Debove L, Comte-Gaz P. Mater Sci Eng A 2006;429:130. [16] Liu Y, Kim JI, Miyazaki S. Philos Mag A 2004;84:2083. [17] Miyazaki S, Wayman CM. Metall Trans A 1988;36:181. [18] Liu Y, Chen X, McComick PG. J Mater Sci 1997;32:5979. [19] Favier D, Liu Y, McCormick PG. Scripta Mater 1993;28:669. [20] Tirry W, Schryvers D. Acta Mater 2005;53:1041. [21] Liu Y, Yang Hong. Mater Sci Eng A 1999;260:240. [22] Fukuda T, Saburi T, Doi K, Nenno S. Mater Trans JIM 1992;33: 271. [23] Liu Y, Favier D. Acta Mater 2000;48:3489. [24] Chiang LJ, Li CH, Hsu YF, Wang WH. J Alloys Compd 2007 [published online].
Intermetallics 16 (2008) 394e398 www.elsevier.com/locate/intermet
Cyclic ageing of Tie50.8 at.% Ni alloy Fei Jiang a, Li Li a, Yufeng Zheng a,b, Hong Yang c, Yinong Liu a,c,* b
a Centre for Biomedical Materials and Engineering, Harbin Engineering University, Harbin 150001, PR China Department of Advanced Materials and Nanotechnology, College of Engineering, Peking University, Beijing 10087, PR China c School of Mechanical Engineering, The University of Western Australia, Crawley, WA 6009, Australia
Received 5 October 2007; received in revised form 24 November 2007; accepted 27 November 2007 Available online 14 January 2008
Abstract This study investigated the effects of cyclic ageing on the transformation behaviour of Tie50.8 at.% Ni by means of differential scanning calorimetric analysis. It is found that cyclic ageing between two different temperatures causes reversible changes of the transformation behaviour and alternating increases and decreases of transformation temperatures. This is attributed to the temperature dependence of the phase equilibria between the B2 matrix and Ni-rich precipitates. Based on the experimental evidences, the solvus of the precipitates (Ti3Ni4) in B2eNiTi is estimated. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: B. Martensitic transformations; B. Shape-memory effects; C. Heat treatment; F. Calorimetry
1. Introduction Near-equiatomic NiTi alloys are attractive functional shape memory alloys for a wide spectrum of innovative applications, largely owing to their novel properties of shape memory effect and pseudoelasticity. It is well known that the transformation behaviour and thermomechanical properties of NiTi alloys with excess Ni to the equiatomic stoichiometry are sensitive to ageing treatment due to the formation of Ni-rich precipitates [1,2]. Three types of Ni-rich precipitates have been observed, including TiNi3, Ti2Ni3 and Ti3Ni4 [1e7]. TiNi3 [4] and Ti2Ni3 [2,5] are incoherent in B2eNiTi, thus ineffective to the transformation behaviour of the B2 matrix. In comparison, Ti3Ni4 is more influential to the transformation behaviour and thermomechanical properties of NiTi, including the R-phase transformation [8,9], all-round shape memory effect [10] and multiple-stage transformation behaviour [11]. Ti3Ni4 has a hexagonal structure and is coherent with the B2-matrix [6,7].
* Corresponding author. School of Mechanical Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Tel.: þ61 8 6488 3132; fax: þ61 8 6488 1024. E-mail address: [email protected] (Y. Liu). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.11.012
Precipitation is a thermally reversible process. In 1988, Horikawa et al. [12] conducted a study of ‘‘short-time’’ cyclic anneals between 713 K and 783 K of a Tie51 at.% Ni alloy. They observed an interesting up-and-down movement of the transformation temperatures with anneal cycles. Zhang et al. [13] later conducted a similar study with long-time ageing and observed the same reversible change of transformation temperatures with repeated ageing cycles. They explained the phenomenon in terms of phase equilibrium between the NiTi matrix and the metalstable Ti3Ni4 precipitates at different ageing temperatures. Whereas the concept of the equilibrium between B2-matrix and Ti3Ni4 has been reasonably well established, actual data on the position of the solvus [14] and cyclic variation of the transformation behaviour [13] have been scarce in the literature. This study investigated the effect of cyclic ageing on the transformation behaviour of a Tie50.8 at.% Ni alloy. Based on the transformation temperatures determined, the solvus between Ti3Ni4 and the B2eTiNi matrix is estimated. 2. Experimental procedure A commercial Tie50.8 at.% Ni alloy wire of 1 mm in diameter was used in this study. Samples were solution treated
F. Jiang et al. / Intermetallics 16 (2008) 394e398
in vacuum at 1273 K for 3.6 ks. Two solution treated samples were further heat treated following different ageing routines. Sample A was cyclically aged at 673 K for 126 ks and 753 K for 45 ks. Sample B was cyclically aged at 753 K for 45 ks and 833 K for 10.6 ks. The ageing treatment durations were chosen to be long enough for the respective ageing temperatures to ensure full precipitation. After each ageing treatment, the samples were slightly chemically etched to remove surface oxide layer using an aqueous solution of 40 vol.% HNO3 and 10 vol.% HF. The transformation behaviour of the samples was characterized using a PerkineElmer Diamond Differential Scanning Calorimeter (DSC) with a heating and cooling rate of 20 K/min. 3. Results Fig. 1 shows DSC measurements of the transformation behaviour of the Tie50.8 at.% Ni alloy over three ageing cycles between 673 K and 753 K. Curve (a) was measured under the solution treated condition. The sample underwent a singlestage A 4 M transformation, at 240 K and 276 K on cooling and on heating, respectively. Curve (b) was measured after ageing at 673 K for 126 ks. The sample developed two-stage transformations both on cooling and on heating, which are identified to be A 4 R (1e10 ) and A 4 M (2e20 ) transformations. It is necessary to clarify that these are two independent transformation streams. The occurrence of such separate transformation streams is indicative to microstructural inhomogeneity of the matrix [11]. The A 4 M transformations occurred at significantly increased temperatures compared to the transformations in the solution treated sample, indicating depletion of Ni content in the B2 matrix due to formation of Ni-rich precipitates. Curve (c) was measured after a second ageing treatment at 753 K, 80 K above the initial ageing temperature, for a further 45 ks. The sample exhibited a transformation sequence of
TAeM ¼
DH AeM DH DH ReM þ TAeR TReM
where DH are the transformation enthalpies, which can be determined experimentally from the DSC curves, and TAeR and TReM are peak temperatures of the respective transformations. The TAeM calculated using Eq. (1) is also shown in the figure, as the dashed curve. It is seen that TAeR, TReM, TAeM and TMeA all show similar zig-zag patterns of increase and decrease in accord with the ageing temperatures. It is also noticed that there were slight increases of TMeA and TReM (and TAeM) with increasing ageing cycles for both ageing
2'
1' 2
Transformation Temperature (K)
(exothermic)
1
(c)=(b)+753Kx45ks 3' (d)=(c)+673Kx126ks 3 (e)=(d)+753Kx45ks
TM-A 340
320
TA-R
300
TR-M
TA-M
280 673 K 260
(f)=(e)+673Kx210ks
753 K 240 0.0
220
240
ð1Þ
AeR
(endothermic)
(b)=(a)+673Kx126ks
Heat Flow
A / R / M on cooling and M / A on heating. The critical temperatures of the transformations are obviously lower than those after the first ageing treatment. This is attributed to increase in the Ni content of the matrix caused by the dissolution of Ni-rich precipitates at higher temperatures [13]. Curve (d) was measured after a further ageing treatment at the same temperature as for curve (b), at 673 K for 126 ks. The sample exhibited a similar transformation sequence of A / R / M on cooling and M / A on heating as curve (c) but at similar temperatures as (b). Continued ageing cycles practically repeated the transformation behaviours shown in curves (c) and (d). Fig. 2 shows the transformation peak temperatures as a function of the number of ageing cycles between 673 K and 753 K. The data points appearing at ‘‘half’’ cycles represent ageing at 673 K and those at ‘‘full’’ cycles represent ageing at 753 K. The two initial data at ‘‘0’’ cycle represent the as-solution treated condition. The occurrence of the A / R / M transformation on cooling suppressed the A / M transformation, thus direct measurement of the transformation is impossible. However, the temperature of the prohibited A / M transformation may be estimated based on thermodynamic principles as [15,16]:
360
(a) solution treated
200
395
260
280
300
320
340
360
380
0.5
1.0
1.5
2.0
2.5
3.0
Cycle Number
Ageing Temperature (K) Fig. 1. Transformation behaviour of Tie50.8 at.% Ni alloy after cyclic ageing at 673 K and 753 K.
Fig. 2. Effect of cyclic ageing at 673 K and 753 K on the transformation temperatures of Tie50.8 at.% Ni alloy. The dashed curve indicates TAeM calculated using Eq. (1).
F. Jiang et al. / Intermetallics 16 (2008) 394e398
temperatures. This is attributed to the incompleteness of the ageing process at the temperatures up to the ageing times. In contrast to the martensitic transformations, TAeR showed little variation with ageing cycle, implying much weaker composition and precipitation dependences of TAeR compared to the temperatures of the martensitic transformations [17]. Fig. 3 shows DSC measurements of the transformation behaviour of the alloy over two ageing cycles between 753 K and 833 K. After ageing at 753 K, the sample showed a two-stage A / R / M transformation on cooling and a single-stage M / A transformation on heating, typical of fully aged condition (curve (b)). When the ageing temperature was raised to 833 K (curve (c)), the transformation behaviour was restored to becoming similar to that of the solution treated sample (curve (a)), i.e. a single-stage A / M transformation, implying a solutionised condition. It is also clear that repeating the alternative ageing process simply restored the transformation behaviour to those shown in curves (b) and (c) alternatively. Fig. 4 shows the transformation temperatures as a function of the number of ageing cycles between 753 K and 833 K. The data points appearing at ‘‘half’’ cycles represent ageing at 753 K and those at ‘‘full’’ cycles represent ageing at 833 K. The sample showed no R-phase transformation after ageing at 833 K and in this case the TAeM values were directly determined from the DSC measurements. When aged at 753 K, the sample exhibited a two-stage A / R / M transformation on cooling. In this case TAeM is calculated using Eq. (1). It is seen that the transformation temperatures of this sample exhibited similar alternating increases and decreases with cyclic ageing to the sample shown in Fig. 2. Fig. 5 shows the dependences of TAeM and TMeA on ageing temperature. The data shown are collected from the last ageing cycle from each sample, because the ageing process is regarded the most complete in these cycles (refer to Fig. 1). The data for ageing temperature of 753 K are from the second sample (shown in Fig. 3). TAeM values shown for ageing
340
Transformation Temperature (K)
396
320 833 K 300
280 TM-A 260
753 K TA-M
240
220 0.0
0.5
1.0
1.5
2.0
2.5
Cycle Number Fig. 4. Effect of cyclic ageing at 753 K and 833 K on the transformation temperatures of Tie50.8 at.% Ni alloy.
temperatures of 673 K and 753 K are calculated values from TAeR and TReM using Eq. (1). The data at above 900 K are from two additional samples, whose transformation behaviours are shown in the inset. It is seen that the ageing temperature may be divided into two distinctive regions, as indicated in the figure. In region I, TAeM and TMeA decreased with increasing ageing temperature to 833 K. For higher ageing temperatures in region II, TAeM and TMeA remained unchanged, implying that the ageing temperature has reached above the solubility limit. This is consistent with the behaviour of single-stage A 4 M transformation observed, as shown in the inset. 4. Discussion 4.1. Effects of matrix composition and internal stresses Ageing of Ni-rich B2eNiTi alloys causes complex variations to the transformation behaviour, as evident in this study
(a) solution treated
Heat Flow
(c)=(b)+833Kx10.6ks
(d)=(c)+753Kx45ks
(e)=(d)+833Kx10.6ks
200
220
240
260
280
300
320
340
360
380
Ageing Temperature (K) Fig. 3. Transformation behaviour of Tie50.8 at.% Ni alloy after cyclic ageing at 753 K and 833 K.
903K 1.8ks
Heat Flow
Transformation Temperature (K)
340 (b)=(a)+753Kx45ks
TM-A
320
933K 1.8ks
TA-M
300
200
250
300
350
Temperature (K) 280 I II
260
240 650
700
750
800
850
solution
900
950
Ageing Temperature (K) Fig. 5. Ageing temperature dependence of TAeM and TMeA temperatures of Tie50.8 at.% Ni alloy.
F. Jiang et al. / Intermetallics 16 (2008) 394e398
as well as being well documented in the literature [18,19]. These effects have been traditionally attributed to variations in the chemical composition of the B2 matrix and the influences of internal stresses associated with precipitates, in particular coherent Ti3Ni4 [20]. It is our view that claims of the effect of internal stresses on transformation temperatures are incorrect. The common perception of the effect of internal stresses on transformation temperatures is based on the wellestablished understanding of the effect of applied constant stresses on transformation temperatures, as expressed in the ClausiuseClapeyron relation [21]. The analogy of internal stress to applied stress is inappropriate. In the case of a constant applied stress, the stress remains effective throughout the process of the transformation. In the case of local internal stresses, which are contained in the form of elastic strains, the stresses are relaxed immediately with the nucleation of martensite, due to the fact that the first martensite variant is formed ‘into’ the stress field, thus are ineffective to the bulk of the process of the transformation. In this regard, we consider that the transformation temperatures are mainly affected by the variations of the matrix composition and that the effect of Ti3Ni4 is mainly to facilitate the nucleation of the R-phase [22]. Fig. 6 shows the Ni-content dependences of TAeM and TMeA using data from this study as well as from the literature [23,24], as under solution treated condition. The slopes are determined to be 106 K/(at.% Ni) and 101 K/(at.% Ni) for TAeM and TMeA, respectively. Based on these dependences, Ni content of the matrix after ageing may be estimated using the transformation temperatures experimentally measured. Using the TMeA values shown in Fig. 5, which are less affected by the occurrence of the A / R transformation (as compared to TAeM), the Ni contents of the matrix after ageing at the three different temperatures are estimated to be 50.11 at.%, 50.34 at.% and 50.76 at.%, as indicated in Fig. 6.
360
397
4.2. Solubility of Ni in B2eNiTi Using the Ni content determined in Fig. 6, the solvus of Ti3Ni4 in B2eNiTi is estimated as shown in Fig. 7. For the Tie50.8 at.% Ni alloy, the dissolution temperature for Ti3Ni4 is estimated to be 833 K, which is consistent with that reported by Zhang [13]. The data points of the samples heat treated at 903 K and 933 K clearly indicate that they are above the solvus of Ti3Ni4, i.e. correspond to the ‘‘solutionised’’ condition. Also shown in the figure are the tentative positions of the solvuses of Ti2Ni3 and TiNi3 in B2eNiTi (the solvus of TiNi3 is defined in the equilibrium NieTi phase diagram). This implies that for a given alloy composition, different solution treatment temperatures exist for the three different precipitates, as indicated by T1, T2 and T3. In this regard, the condition of the two samples heat treated at 903 and 933 K is more appropriately regarded as ‘‘solutionised’’ for Ti3Ni4. The fact that the matrix composition of these two samples is slightly lower than the original (the TMeA temperature is slightly higher than the original value before ageing) implies that there may still exist some stable precipitates (TiNi3) at this temperature. 4.3. Mechanism of the effects of cyclic ageing This study is a continuation of the early works by Horikawa et al. [11] and Zhang et al. [13]. There have been discussions of the mechanism of this reversible movement of transformation temperatures with cyclic ageing, being either the reversion of unstable Ni-rich GP zones [11] or dissolution of metastable Ti3Ni4. In our opinion, the two hypotheses have no real contradiction in the principle of reversible change of the matrix Ni content, differing only on the aspect of which Ni-rich ‘‘phase’’ is formed and dissolved at different temperatures. This clarification is fundamentally necessary because Ni-rich near equiatomic NiTi alloys are known to develop several precipitates [1,3]. For each of these precipitates 1000
753 K
T3
320
950
TM→A 833 K
Temperature (K)
Transformation Temperature (K)
673 K
280 TA→M 240
900 850
T2
TiNi3
Ti2Ni3
T1
800 750
200 50.0
700 50.2
50.4
50.6
50.8
Ti3Ni4
51.0
Ni content (at ) Fig. 6. Ni-content dependences of TAeM and TMeA temperatures. Data expressed in solid symbols are from this study. Data expressed in crossed open symbols are from Ref. [23]. Data expressed in open symbols are from Ref. [24].
650 50.0
50.2
50.4
50.6
50.8
51.0
Ni Content (at ) Fig. 7. Solvus of Ti3Ni4 in B2eNiTi as estimated from the transformation temperature measurements.
398
F. Jiang et al. / Intermetallics 16 (2008) 394e398
(and GP zones), similar alternating cycles of formation and dissolution may occur. In this regard, the alternating variations of the transformation temperatures and sequences with alternating ageing treatment are intrinsic property of the alloys due to the varying solvuses of the precipitates. 5. Conclusions 1. Cyclic ageing at alternating temperatures within the range of 673e833 K causes reversible cyclic variations of transformation sequences and temperatures of Tie50.8 at.% Ni alloy. TAeM and TMeA temperatures increase with lowering ageing temperature and decrease with increasing ageing temperature reversibly. The R-phase transformation appears with lowering ageing temperature and disappears with increasing ageing temperature reversibly. 2. This reversible ageing effect is attributed to equilibrium between the B2eNiTi matrix and Ni-rich precipitates. Higher Ni content in the B2eNiTi matrix results in lower temperatures for the martensitic transformations in NiTi and vice versa. The presence of Ti3Ni4 is attributed to the cause of the appearance of the R-phase transformation. 3. The solvus of Ti3Ni4 in B2eNiTi is estimated based on transformation temperature measurement. The solvus of Ti3Ni4 in B2eNiTi for the Tie50.8 at.% Ni alloy is estimated to be at 833 K.
Acknowledgement The authors acknowledge the financial support for this study from the Outstanding Youth Fund of Heilongjiang
Province (JC200608) and the supply of the NiTi alloys by Jiangyin Fasten-PLT Materials Science Co., Ltd. References [1] Nishida M, Wayman CM, Honma T. Metall Trans A 1986;17: 1505e15. [2] Nishida M, Wayman CM. Metall Trans A 1987;18:785e99. [3] Wasilewski RJ, Butler SR, Hanlon JE, Worden D. Metall Trans A 1971;2:229. [4] Taylor A, Floyd RW. Acta Crystallogr 1950;209:1269. [5] Hara T, Ohba T, Otsuka K, Nishida M. Mater Trans JIM 1997;38: 277. [6] Nishida M, Wayman CM, Kainuma R, Honma T. Scripta Metall 1986;20:899. [7] Saburi T, Nenno S, Fukuda T. J. Less-Common Met. 1986;125:127. [8] Nishida M, Wayman CM. Metallography 1988;21:255. [9] Miyazaki S, Wayman CM. Acta Metall 1988;36:181. [10] Nishida M, Honma T. Scripta Metall 1984;18:1293. [11] Carroll MC, Somsen Ch, Eggeler G. Scripta Mater 2004;50:187. [12] Okamoto Y, Hamanaka H, Miura F, Tamura H, Horikawa H. Scripta Metall 1988;22:517. [13] Zhang J, Ren X, Otsuka K, Asai M. Scripta Mater 1999;14:1109. [14] Zhang J. Ph.D, thesis. University of Tsukuba; 2000. [15] Favier D, Liu Y, Orgeas L, Sandel A, Debove L, Comte-Gaz P. Mater Sci Eng A 2006;429:130. [16] Liu Y, Kim JI, Miyazaki S. Philos Mag A 2004;84:2083. [17] Miyazaki S, Wayman CM. Metall Trans A 1988;36:181. [18] Liu Y, Chen X, McComick PG. J Mater Sci 1997;32:5979. [19] Favier D, Liu Y, McCormick PG. Scripta Mater 1993;28:669. [20] Tirry W, Schryvers D. Acta Mater 2005;53:1041. [21] Liu Y, Yang Hong. Mater Sci Eng A 1999;260:240. [22] Fukuda T, Saburi T, Doi K, Nenno S. Mater Trans JIM 1992;33: 271. [23] Liu Y, Favier D. Acta Mater 2000;48:3489. [24] Chiang LJ, Li CH, Hsu YF, Wang WH. J Alloys Compd 2007 [published online].