ISSN 0031918X, The Physics of Metals and Metallography, 2011, Vol. 112, No. 3, pp. 283–289. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.A. Skudnov, S.V. Kharitonov, L.A. Oshurina, A.A. Khlybov, R.A. Blyakevichus, 2011, published in Fizika Metallov i Metallovedenie, 2011, Vol. 112, No. 3, pp. 301–307.
STRENGTH AND PLASTICITY
Structure and Phase Transformations in an Elinvar Alloy after Various Regimes of Heat Treatment V. A. Skudnov, S. V. Kharitonov, L. A. Oshurina, A. A. Khlybov, and R. A. Blyakevichus Nizhni Novgorod State Technical University (NGTU), ul. Minina 24, Nizhni Novgorod, 603950 Russia Received June 13, 2010; in final form, February 22, 2011
Abstract—The microstructure, phase composition, and mechanical properties of the alloy 44NKhTYu have been studied using metallography, Xray diffraction, and acoustic testing. The main peculiarities of the for mation of intermetallic phases and their effect on the mechanical properties of the alloy have been deter mined. Keywords: elinvar alloy 44NKhTYu, heattreatment regimes, microstructure, Xray diffraction analysis, acoustic testing DOI: 10.1134/S0031918X11030276
INTRODUCTION In the precision instrument engineering, materials are used whose properties should remain stable in the process of exploitation. In particular, to ensure the sta ble work of elastic elements materials are employed for their production which should have high strength and a temperaturestable modulus of normal elasticity in the temperature range from –60 to +60°С, such as elinvars [1]. The use of such materials as precision alloys for elasticsensitive elements makes it possible to substantially reduce the temperaturerelated errors of measured quantities. The alloy 44NKhTYu refers to the class of elinvars, which are used for the production of elastic elements. One specific feature of elinvar alloys is their meta stable state after some regimes of heat treatment com bining quenching and aging. This means that their val ues of the elasticity modulus can depend on their structural state, being controlled by heattreatment conditions [2]. This work is aimed at the investigation of the microstructure and phase composition of the alloy 44NKhTYu and at their acoustic testing after multi step heat treatment.
For the investigation, we used cylindrical samples with a diameter d = 10 mm and a height h = 8 mm after various regimes of heat treatment, which are presented in Table 2. The microstructure was studied using micrographs obtained in an MIM8 optical microscope and a Canon digital camera under a magnification of ×160. The Xray diffraction investigation was performed using a DRON2 diffractometer (U = 25 kV and I = 25 mA). The acoustic tests were performed using an Astron multifunctional spectral acoustic system in a pulse regime with the registration of the time of propagation and time of decay of elastic pulses that pass through the medium to be controlled. The data acquisition and processing were performed using a notebooktype computer entering into the composition of the Astron system. The processing was performed on a real time basis. Times of the propagation (delay) of longitudinal and shear (transverse) waves were measured. The accuracy of the determination of time parameters does not exceed 10–9 s. The thickness of the samples was calculated to an accuracy of 0.01 mm. The velocity of propagation of elastic waves was determined from the results of measurements of the time of delay and thick ness of the samples at a frequency of 8.5 MHz. The transverse waves were used to measure the delay in two mutually perpendicular directions. The measurements of acoustic characteristics were performed at least at
EXPERIMENTAL The chemical composition of the alloy is given in Table 1.
Table 1. Chemical composition of the 44NKhTYu alloy (%; Fe for balance) Grade
Ni
Cr
Ti
Al
44NKhTYu
43.5–45.5
5.0–5.6
2.2–2.7
0.4–0.8
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Table 2. Regimes of heat treatment of samples of the 44NKhTYu alloy Regime
Heattreatment regime
Regime 1
Water quenching from 940–960°C after holding in castiron chips for 30 min
Regime 1.2
Water quenching from 850–900°C after holding in castiron chips for 30 min
Regime 1.3
Water quenching from 950–1000°C after holding in castiron chips for 30 min
Regime 2
Water quenching from 940–960°C after holding in castiron chips for 30 min. Aging at 680–700°C with holding in castiron chips for 3–3.4 h, cooling to 150°C, further cooling to room temperature in air. Sta bilizing aging at 340–360°C in vacuum for 6–8 h, cooling to 150°C, further cooling in air
Regime 2.2
Quenching + aging at 680–700°C with holding in castiron chips for 3–3.5 h, cooling to 150°C, further cooling in air. Stabilizing aging at 340–360°C in vacuum for 2–3 h, cooling to 150°C, further cooling in air
Regime 2.3
Quenching + aging at 680–700°C with holding in castiron chips for 3–3.5 h, cooling to 150°C, further cooling in air. Stabilizing aging at 340–360°C in vacuum for 4–4.5 h, cooling to 150°C, further cooling in air
Regime 3
Water quenching from 940–960°C after holding in castiron chips for 30 min. Aging at 680–700°C with holding in castiron chips for 3–3.5 h, cooling to 150°C in air. Stabilizing aging at 340–360°C in vacuum for 6–8 h, stabilizing aging at 160–170°C, cooling to 150°C, further cooling in air
Regime 3.2
Quenching + stabilizing aging at 340–360°C in vacuum for 6–8 h, cooling to 150°C, further cooling in air. Stabilizing aging at 160–170°C for 2–4 h, cooling to 150°C, further cooling in air
Regime 3.3
Quenching + aging at 680–700°C with holding in castiron chips for 22.5 h, cooling to 150°C in air. Sta bilizing aging at 340–360°C in vacuum for 4–6 h, cooling to 150°C, further cooling in air
ten points of each sample. The data obtained were averaged and the values of the time delay were deter mined for each type of the employed elastic waves after different regimes of heat treatment given in Table 2. The velocity Ci (i = 1, 2, 3) of the elastic waves upon the propagation through the sample was determined by the formula
Ci = 2L t i ,
(1)
where L is the thickness of the sample, m. Since the measurements were performed in the regime of emis sion and detection, the path in formula (1) was assumed to be equal to 2L. The detection and emission of elastic waves were implemented using the same transducer. The following designations were used in the work: indices 1 and 2 correspond to transverse waves whose vectors of polarization are oriented in mutually per pendicular directions. For a standard fivefold speci mens prepared according to Russian standard GOST 149784, the index 1 corresponds to the direction along the sample; index 2, to the perpendicular direc tion. Index 3 refers to a longitudinal wave. According to these designations, t1,2 is the delay (time of propaga tion) of transverse waves, s; and t3 is the delay of longi tudinal waves, s.
The error of measurements of the velocities was 0.08% at the sample thickness of 10 mm. The main contribution to the error comes from the error in the measurement of the sample thickness. We used an MKTstype micrometer with a digital readout indica tor whose maximum error is 0.004 mm. For the longi tudinal wave the time of propagation in a sample of thickness 10 mm is approximately 3400 × 10–9 s. For a transverse wave, the time of propagation in the same sample is ~6100 × 10–9 s. The error in the propagation of ultrasound was a few thousandth of a percent. An analysis of the data obtained indicates that the change in the velocity after heat treatments was more than 1%, which exceeds the spread in data related to the mea surement errors [3]. RESULTS AND DISCUSSION The change in the temperature of quenching of the alloy from 850 to 950°С did not exert a substantial effect on the microstructure, whereas the increase in the quenching temperature to 1000°С led to some grain coarsening. The microstructures are shown in Figs. 1a–1c. The Xray diffraction results show that in the pro cess of heat treatment of the 44NKhTYu alloy there occurs a change in the mechanism of phase transfor
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mations depending on the regimes of quenching and subsequent aging. In particular, there is observed a change in the content of the γ' phase with an fcc lattice upon aging by the continuous mechanism and in the content of the η phase Ni3Ti with an hcp lattice in the case of aging by discontinuous mechanism (Table 3). As the quenching temperature increases, the pro cess of discontinuous transformation with the forma tion of the η phase is activated simultaneously with an increase in the amount of the γ' phase that is formed during the continuous transformation. An increase in the time of holding at 680–700°С to more than 2 h decreases the fraction of discontinuous transforma tion, which unfavorably affects the magnetostriction characteristics of the alloy. The time of holding upon the stabilizing aging at 340–360°С only weakly affects the change in the amount of the intermetallic phase (the fraction of discontinuous transformation remains unaltered), but leads to a 5% increase of the fraction of the continuous transformation. The amount of the γ' phase in the regime 3.2 decreases by 10%, and in the regimes 3 and 3.3 is equal. The time of the stabilizing aging at 160–170°С virtually does not change the amount of the γ' phase., but the fraction of the continu ous transformation increases by 10% in the regime 3.2 and the fraction of discontinuous transformation decreases by 10% as compared to the regimes 3 and 3.3. In the course of the investigations, we studied changes in the stresses of the 1st and 2nd kind depend ing on the regimes of heat treatment (Tables 4, 5). The tensile body stresses upon quenching by the regime 1.3 increase as compared to regime 1.2. An increase in the time of holding at 650–700°С leads to a growth of compressive stresses. Upon the treatment by regime 1, the magnitude of microstresses and the dimensions of blocks remain unaltered. Treatment by regime 2 with an increase in the holding time leads to an increase in the size of blocks and to a decrease in microstresses. The regimes 2.1 and 2.3 change these characteristics equally. Treatment by regime 3 with increasing holding time also causes an increase in the size of blocks and a decrease in micros tresses. To study the effect of heat treatment on the processes occurring in the 44NKhTYu steel, we investigated the velocity of propagation of elastic waves in samples. The results of acoustic tests are given in Table 6. Based on the results of acoustic investigations, we determined the elasticity moduli of the 44NKhTYu alloy. The elasticity moduli were determined using the relationships that are well known in the theory of elas ticity:
(
ρ × Ct × 3 × Cl − 4 × Ct 2
E=
G = ρ × Ct2,
2 Cl
ν=
2
−
2 Ct
2
),
Cl2 − 2 × Ct2 , 2 2 2 × Cl − 3 × Ct
(2) (3)
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(a)
(b)
(c) Fig. 1. Structure of samples of the 44NKhTYu alloy after heat treatment (magn. ×160): (a) regime 1: water quenching from 940–960°С after holding in castiron chips for 30 min; (b) regime 2: water quenching from 940–960°С after holding in castiron chips for 30 min, aging at 680–700°С with holding in castiron chips for 3–3.5 h, cooling to 150°С, further cooling to room temperature in air; stabilizing aging at 340–360°С in vacuum for 6–8 h, cooling to 150°С, further cooling in air; and (c) regime 3: water quenching from 940–960°С after holding in castiron chips for 30 min, aging at 680–700°С with holding in castiron chips for 3–3.5 h, cooling to 150°С in air; stabilizing aging at 340–360°С in vacuum for 6–8 h, stabilizing aging at 160–170°С, cooling to 150°С, further cooling in air Vol. 112
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Table 3. Dependence of the fractions of the discontinuous and continuous transformations and of the amounts of in termetallic phases on the regime of heat treatment Heattreat Amount of γ Amount of fcc γ' Amount of hcp η phase phase ment regime phase, % Regime 1
80
10
10
Regime 1.2
70
15
15
Regime 1.3
80
20
10
Regime 2
70
15
15
Regime 2.2
60
25
15
Regime 2.3
70
15
15
Regime 3
60
20
20
Regime 3.2
50
25
20
Regime 3.3
60
20
20
Regime 4
50
20
30
Regime 4.2
50
30
20
Regime 4.3
50
20
30
where G, E, and ν are the shear modulus, Young’s modulus, and the Poisson ratio, respectively; and Ct and Cl are the velocities of the transverse and longitu dinal waves, respectively. The increase in the elasticity modulus observed during aging can be explained by the precipitation of a strengthening phase, in this case, of the intermetallic compound Ni3Ti, which has a higher elasticity modu lus (Figs. 2, 3). The values of the velocities of propagation of elastic waves obtained after heat treatment show that the low est velocity of wave propagation is observed after quenching by regime 1 (Fig. 4). The aging processes lead to an increase in the velocity of propagation of longitudinal and transverse waves and to a decrease in the decay time of the waves. For the problems of practical operative control of the quality of heat treatment of the alloy, the following dimensionless acoustic diagnostic parameter can be used:
D=
ct , cl
(4)
where ct is the velocity of the transverse wave, and cl is the velocity of the longitudinal wave.
Table 4. Dependence of the magnitude of body (1st kind) macrostresses on the regime of heat treatment Heattreatment regime
2θ, deg
Δθ, deg
Magnitude of macrostresses σ
0
–
Regime 1
65
Regime 1.2
65.1
+0.1
+0.782 × 10–3 E/μ
Regime 1.3
65.2
+0.2
+1.5 × 10–3 E/μ
Regime 2
64.9
–0.1
–1.6 × 10–3 E/μ
Regime 2.2
64.6
–0.5
–4.8 × 10–3 E/μ
Regime 2.3
64.9
–0.3
–2.45 × 10–3 E/μ
Regime 3
64.75
+0.25
+1.99 × 10–3 E/μ
Regime 3.2
64.35
–0.25
–1.99 × 10–3 E/μ
Regime 3.3
64.65
–0.25
–1.99 × 10–3 E/μ
Regime 4
64.75
+0.25
+1.99 × 10–3 E/μ
Regime 4.2
63.85
–0.5
–3.9 × 10–3 E/μ
Regime 4.3
63.15
–0.5
–3.9 × 10–3 E/μ
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STRUCTURE AND PHASE TRANSFORMATIONS IN AN ELINVAR ALLOY Table 5. Dependence of the size of blocks and magnitude of textural (2nd kind) microstresses on the regime of heat treatment Magnitude of textural Heattreatment Block size D, μm (2nd kind) micros regime tresses Δa/a Regime 1
0.25
2.3 × 10–4
Regime 1.2
0.25
2.3 × 10–4
Regime 1.3
0.25
2.3 × 10–4
Regime 2
0.20
2.5 × 10–4
Regime 2.2
0.15
2.9 × 10–4
Regime 2.3
0.18
2.7 × 10–4
Regime 3
0.20
2.5 × 10
Regime 3.2
0.15
2.9 × 10–4
Regime 3.3
0.20
2.5 × 10–4
Regime 4
0.20
2.5 × 10–4
Regime 4.2
0.15
3.0 × 10–4
Regime 4.3
0.20
2.7 × 10–4
287
The effect of heat treatment on the diagnostic cri terion D is illustrated in Fig. 5. The variation of the parameter D correlates with the change in the fraction of the discontinuous trans formation in this alloy after various heattreatment regimes. The stabilization of the diagnostic criterion in the regime 4 is related to the equalization of the frac tions of the discontinuous and continuous transforma tions. The start of the decomposition upon aging occurs when using regime 2.2, which ensures a signif icant fraction of the continuous transformation and corresponds to a decrease in the magnitude of the diagnostic criterion. The minimum value of the diag nostic criterion D corresponds to the optimum regime of heat treatment (regime 3.2, Fig. 5). An analysis of the results of the investigations shows a high sensitivity of the method of acoustic testing to the determination of changes in the parameters of the elinvar alloy and its microstructure. The process of the accumulation of structural damages is determined by a number of factors related to both the technology of making steel and its heat treatment and to the condi tions of its exploitation. The effect of the entire set of technological and service factors upon the estimation of the structural state of an alloy can be taken into account by determining the velocities of elastic waves.
–4
Table 6. Results of acoustic measurements Sample
H, mm
Vl, m/s
Vt, m/s
Density, kg/m3
Shear modulus
Young’s modulus
Diagnostic parameter
Asdelivered
14.500
5398
3055
8040
75037521000
1.89752E + 11
0.766939
1
7.930
5315
2979
8020
71173016820
1.80919E + 11
0.784156
1.2
7.953
5325
2973
8020
70886606580
1.80557E + 11
0.79112
1.3
7.951
5320
2973
8020
70886606580
1.80469E +11
0.789438
2
7.933
5390
3036
8040
74107059840
1.87883E +11
0.775362
2.2
7.941
5400
3051
8040
74841152040
1.89429E +11
0.769912
2.3
7.923
5410
3050
8040
74792100000
1.89529E +11
0.77377
3
7.939
5382
3024
8050
73613836800
1.8688E + 11
0.779762
3.2
7.940
5365
3040
8050
74394880000
1.88002E + 11
0.764803
3.3
7.921
5404
3030
8050
73906245000
1.8783E + 11
0.783498
4
7.936
5398
3055
8040
75037521000
1.89752E + 11
0.766939
4.2
7.941
5390
3050
8040
74792100000
1.89148E + 11
0.767213
4.3
7.954
5390
3050
8040
74792100000
1.89148E + 11
0.767213
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SKUDNOV et al. Young’s modulus, MPa 1.92E + 11 1.90E + 11 1.88E + 11 1.86E + 11 1.84E + 11 1.82E + 11 1.80E + 11 1.78E + 11 1.76E + 11 1.74E + 11 1 1.2
1.3
2
2.2
2.3
3
3.2
3.3
4
4.2
4.3
Fig. 2. Dependence of Young’s modulus on the regime of heat treatment.
Young’s modulus, MPa 7.6E + 10 7.5E + 10 7.4E + 10 7.3E + 10 7.2E + 10 7.1E + 10 7.0E + 10 6.9E + 10 6.8E + 10 1 1.2 1.3
2
2.2
2.3
3
3.2
3.3
4
4.2
4.3
Fig. 3. Dependence of the shear modulus on the regime of heat treatment.
Vt, m/s 3080 3060 3040 3020 3000 2980 2960 2940 2920 1
1.2
1.3
2
2.2
2.3
3
3.2
3.3
4
4.2
4.3
Fig. 4. Dependence of the velocity of shear wave on the regime of heat treatment (Vt is the velocity of propagation of the transverse wave). THE PHYSICS OF METALS AND METALLOGRAPHY
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D 0.80 0.79 0.78 0.77 0.76 0.75 1
1.2
1.3
2
2.2
2.3
3
3.2
3.3
4
4.2
4.3
Fig. 5. Dependence of the dimensionless acoustic diagnostic parameter D on the regime of heat treatment.
CONCLUSIONS (1) The results of Xray diffraction and acoustic investigations show that in the process of aging there occur changes in the relative amounts of the γ' and η phases, which affects the elasticity modulus of the alloy 44NKhTYu. (2) Upon the development of the discontinuous reaction of aging, a jumplike change in the composi tion of the matrix causes the strongest change in the elinvar properties of the 44NKhTYu alloy. At the same time, the continuous decomposition, which involves the entire volume of grains and occurs with a smooth change in the composition of the matrix solid solution, provides the best effect of precipitation hardening. An optimum combination of strength and thermoelastic properties of elinvars is achieved by the predominant development of the continuous precipitation and the restriction of the fraction of discontinuous decompo sition. The realization of such transformation kinetics is achieved by using a special heat treatment. (3) The performed complex investigations of the structure of the 44NKhTYu alloy show that the opti mum regime of heat treatment is regime 3.2 (quench ing + stabilizing aging at 340–360°С in a vacuum for 6–8 h, cooling to 150°С in vacuum, and subsequent cooling to room temperature in air). To quantitatively estimate the elinvar properties, a special parameter γ can be used. The optimum variant is if γ tends to zero,
THE PHYSICS OF METALS AND METALLOGRAPHY
i.e., the effect of temperature on the variation of the elasticity modulus should be eliminated (elinvar effect). Therefore, we should tend to such a heat treat ment in which the relationship between the fractions of the discontinuous and continuous transformations is optimum. Of all the aboveconsidered regimes of heat treatment of this steel, the optimum regime is the regime 3.2. (4) The stabilization at a temperature of 160– 170°С affects the elimination of internal stresses after heat treatment and forms in the structure of the 44NKhTYu alloy extremely small regions inside grains (nanolevel) from which later in the process of aging new intermetallic particles are developed with ele ments of a superstructure. REFERENCES 1. Precision Alloys: A Handbook, Ed. by B. V. Molotilov (Metallurgiya, Moscow, 1974), Vol. 2 [in Russian]. 2. V. Baraz and V. Strizhak, “Elinvar Alloys: Peculiarities of Structure and Properties. Part 1,” Nats. Metall., No. 4, 96–98 (2003). 3. A. A. Khlybov and V. A. Skudnov, “Estimation of Struc tural Changes in Structural Metalic Materials by Acoustic Methods for Guarantee of Exploitation Secu rity of Technical Objects,” in Tr. Nizhegorod. Gos. Tekh. Univ., No. 1 (80), 200–209 (2008).
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