Mechanical properties of boron-doped directionally-solidified Ni3Al containing carbon, magnesium, calcium and rare earth elements

Materials Science and Engineering, A153 (1992) 364-369

364

Mechanical properties of boron-doped directionally-solidified Ni3A1 containing carbon, magnesium, calcium and rare earth elements Yun Zhang and Dongliang Lin Department of Materials Science and Engineering, ShanghaiJiao Tong University, Shanghai 200030 (China)

Abstract The temperature dependence of the yield stress in boron-doped directionally-solidified Ni3AI with the additions of microalloying elements such as carbon, magnesium, calcium and rare earth elements was systematically investigated by tensile tests in the temperature range 123-1223 K. The effects of the alloy elements on the temperature dependence of the yield stress were evaluated. The measured activation energy of a thermally activated process which results in the anomalous mechanical behavior in Ni3AI decreases for each element addition. The influences of element addition on yield stress near liquid nitrogen temperature are discussed as a measure of the degree of solid solution strengthening. It is found that the increment of the yield stress, Aay/AC, could not simply be correlated with the lattice strain change, Ae/Ac, per atomic portion of the element additions. The mechanisms of the anomalous mechanical behavior in the Ni3A1 as affected by the element additions are also discussed.

1. Introduction The mechanical properties of NiaAI are strongly affected by the addition of tertiary alloy elements. However, the majority of previous investigations have concentrated on the macroalloying effects [1-5], and only a few works have reported recently on the microalloying effects [6-8]. Therefore, extensive work is needed to understand the mechanical behavior of Ni3AI with microalloying element additions. To design ~ Ni3AI alloys for practical uses, boron, which has a strong beneficial effect on the ductility of Ni3A1 at ambient temperature [9, 10], should be taken as a base alloy element. It has also to be noted that the alloy stoichiometry of Ni3A1 can change the behavior of the ductility improvement by any boron dopant [10]. Thus, as a base material the nickel-rich off-stoichiometric Ni76A124 doped with boron would be a suitable choice in the present work. Furthermore, the yield stress in boron-doped directionally-solidified (DS) Ni3A1 with the addition of carbon, magnesium, calcium and rare earth elements in a solid solution state has been investigated by tensile tests. It is known that the effects of solid solution strengthening in Ni3Al alloy can be divided into two parts, i.e. the strengthening at lower temperatures by solid solution hardening and that at higher temperatures by a thermally activated process [11]. In the present investigation the solid solution strengthening effect was evaluated by measuring the yield stress at a temperature near that of liquid nitrogen and the magnitude of the positive temperature depen0921-5093/92/$5.00

dence of the yield stress was evaluated by the activation energy. The mechanisms of solid solution strengthening and the anomalous mechanical behavior in Ni3AI affected by the element additions are also discussed.

2. Experimental details The alloy elements added to boron-doped Ni3A1 were magnesium and calcium (group 2A), carbon (group 4B), and the rare earth elements yttrium, lanthanum and cerium (group 3A). For these microalloy additions, the alloys were designed based on the formula xB0.02Mx (1) where M is the additive element and x is its weight percentage value. The amount of element addition was within its solubility limit for each additive element as X-ray diffraction and transmission electron microscopy (TEM) results showed that the alloyed Ni3AI 'maintained the single phase state. Using the starting materials, which were electrolytic nickel (99.9 wt.% purity) and aluminum (99.9 wt.% purity), and other high purity materials as available, the alloys were prepared by induction melting and directional solidifying in argon gas. It should be noted that the boron was added by an intermediate alloy Ni-B with a boron content of 18.57 wt.%, and both lanthanum and cerium were added by a La-Ce master alloy with a lanthanum content of 30.10 wt.%. The resulting DS Ni3A1 alloy [Ni76A124199.98

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© 1992--Elsevier Sequoia. All rights reserved

Y. Zhang, D. Lin

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plates ( 170 mm x 70 mm × 15 mm) were homogenized at 1373 K in air for 2 days, followed by furnace cooling. The widths of columnar grains were approximately 500-1000 /xm for all the alloys. The discrepancies between the designed and analyzed chemical compositions of the prepared alloys are very small (relative error within +_0.5 wt.%). Therefore, the nominal compositions are used in the following analyses unless otherwise stated. The round bar tensile specimens (4.1 mm diameter × 28.6 mm long) were machined with the tensile axis parallel to the axis of the columnar structure. The tensile tests were carried out using a Shimadzu-type machine at a nominal strain rate of 2.9 × 10 4 s - ' over a temperature range from 123 to 1223 K. Tests at 123 K were performed by cooling samples with liquid nitrogen. The yield stress was measured at 0.2% plastic strain.

3.1. Temperature dependence of yield stress The temperature dependence of yield stress in boron-doped DS Ni3AI alloy with the different alloy additions was determined. Results show that the yield stress of the DS Ni3AI alloys increases progressively with temperature to a peak value at an elevated temperature, and beyond this peak it decreases with temperature. The yield stress vs. temperature curves for the alloys are shown in Figs. 1-5. From these figures, it is generally found that the yield stress level at temperatures below the peak increases with increasing concentration of alloy elements for every alloy system, and this increment is more significant with the addition of the rare earth elements. However, at temperatures far below the peak, the addition of small amounts of mag-

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nesium, calcium, yttrium and L a - C e reduces the yield stress of the boron-doped Ni3AI alloy. However, the strengthening effect of the additive elements at sufficiently high temperatures beyond the peak weakens or

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disappears. In particular, at 1223 K the yield stress of boron-doped Ni3Al with the additive elements is lower than that of boron-doped Ni3Al without them. The peak temperature for the alloy with carbon additions is found to be identical to that of boron-doped Ni3Al, i.e. 973 K, but for other element additions the peak is about 1073 K, higher than that of boron-doped NiaAI. It has been proposed that the yield stress vs. temperature curve in the L 12-type alloys can be regarded as a sum of two temperature dependent terms [11]. The stress component, Oath, has the ordinary negative temperature dependence of the stress arising from the shear modulus change with temperature, while Oth has the positive temperature dependence of the stress caused by the Kear-Wilsdorf mechanism [12]. Thus, the observed yield stress, Oy, is expressed as

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where o0 is the yield stress at 0 K, U is the activation energy for the thermally activated process, R is the gas constant, T is the absolute temperature, and A and B are constants. By applying this analysis to each yield stress vs. temperature curve, a quantitative evaluation of the effects of each alloy element on the yield stress of boron-doped DS Ni3AI can be made for the two parameters, i.e. Oathand Oth, separately. 3.2. Effect of the alloy elements on the positive temperature dependence of the yieM stress According to eqns. (2)-(4), Arrhenius type curves for each alloy were plotted for ln[oy - o0( 1 - BT)] with reciprocal test temperature, l/T, in which a welldefined linear relation is obtained. A plot for carbondoped alloys is shown in Fig. 6 as an example. It is noted that o0 is the yield stress at 123 K, and B is

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Fig. 7. Effect of alloying elements on the activation energy of boron-doped DS Ni3AI. related to the shear modulus change with temperature and can be supposed to be 0.0003 for Ni3A1 [13]. The effect of each alloy element on the activation energy for the thermally activated process which causes the positive temperature dependence of the yield stress for the Ni3A1 is shown in Fig. 7 as a function of the concentration of alloy elements. It is demonstrated that every alloy element in boron-doped Ni3AI reduces the value of the activation energy and the slope of the U vs. additive element content curves becomes steeper in the sequence of C, Mg < Ca < Y < L a - C e for

Y. Zhang, D. Lin

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the addition of these elements to the boron-doped Ni3AI. 3.3. Effect o f the alloy elements on solid solution strengthening

In the present work, the effect of each alloy element on the solid solution strengthening was evaluated with the yield stress at 123 K, where the contribution of oth is negligible. The yield stress obtained at 123 K as a function of the alloy element concentration in the boron-doped Ni3AI is shown in Fig. 8. In the case of alloying with carbon, an almost linear correlation between the yield stress and the alloy element content is found, which was also observed in previous work [8]. However, when alloying with magnesium, calcium, yttrium and La-Ce, the effects on yield stress differ with the increase of the element content. Compared with additions of higher amounts of magnesium, calcium, yttrium and La-Ce, the strengthening capacity is weak for small additions of yttrium and La-Ce, and even

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negative, i.e. the yield stress is reduced for small additions of magnesium and calcium. It is evident from Fig. 8 that the strengthening capacity becomes stronger in the sequence Mg < C < Ca < Y < La-Ce.

4. Discussion 4.1. Positive temperature dependence

The positive temperature dependence of the yield stress in many L 12 intermetallics has been explained by the cross-slip of screw dislocations from {111} to {100} planes [ 12, 14]. As the cross-slip is driven by the difference of the antiphase boundary (APB) energies on the two planes, a relative change in the fault energies could result in a change in the anomalous yield stress behavior of the L12 intermetallics. Thus, with regard to the effect of alloy elements on the positive temperature dependence of the yield stress in Ni3A1, the effect of alloy elements on the crystal structure and, in particular, on the energy of the planar faults on the two planes must be taken into account. Previous work [15, 16] showed that the APB energy on the (hkl) plane of L12type structures can be expressed as APB 7,h~t)

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Fig. 8. Variations of the yield stress at 123 K with the additive element content in boron-doped Ni3AI.

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where a is the lattice constant, V is the ordering energy and S is the degree of the long-range ordering. In an unpublished work [17], the parameters a and S of boron-doped Ni3AI with additions of carbon, magnesium, calcium, yttrium and L a - C e were determined by X-ray powder diffraction analysis. If it is supposed that the other parameters in eqn. (5) are not altered by alloying with these elements, the effects of the alloy additions on the APB energy on (111) plane can be evaluated by the ratios of S2/a 2 of the boron-doped Ni3AI with and without additive elements. These ratios of S2/a 2 are listed in Table 1. The results show that the

TABLE 1. The effect of the additive elements of the APB energy on ( 111 ) plane: the ratios of S2/a 2 of the boron-doped Ni3AI with element addition to that without element additions Alloys (at.%)

Ratios of S2/a 2

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Ratios of S 2/a 2

368

Y.Zhang, D. Lin / B-doped Ni~Al with C, Mg, Ca and rare earth elements

APB energies on (111) increase when the amount of carbon, calcium, yttrium and La-Ce additions is higher, while they actually remain constant when the amount of the element additions is lower. In the case of adding magnesium to boron-doped Ni3AI , the APB energies on ( 111 ) also do not change very much even if the amount of the additions is low or high. Therefore, from the variation of the ratios of S:/a 2 with element addition, it is predicted that the cross-slip process would be promoted, and that the activation energy decreases for boron-doped Ni3AI with higher amounts of carbon, calcium, yttrium and La-Ce additions, while the activation energy would not be changed for the alloys with lower amounts of the above element additions and with the addition of magnesium. Apparently the latter prediction is not applicable to the experimental results. Therefore, we suggest that the segregation of these additive elements to the planar fault and/or the dislocations, especially in the case of Ni3AI with lower amounts of addition, may be another important factor affecting the cross-slip process. Further work is needed in this respect.

tions respectively. For the addition of the different alloy elements in Fig. 9 the experimental points are not rationalized by a single correlation, i.e. a single straight line. Based on the data a straight line with a slope of about 0.64G can be obtained for the addition of magnesium, calcium, yttrium and La-Ce to boron-doped Ni3AI. Compared with the strengthening capacity of magnesium, calcium, yttrium and La-Ce, carbon stands out as a strong strengthener in boron-doped Ni3AI with the slope equal to about 1.16G. From unpublished work [17], it is known that the carbon atom occupies the octahedral interstitial sites, and magnesium, calcium, yttrium, lanthanum and cerium substitute for aluminum atoms at cubic corners in Ni3AI. Thus, the above results imply that the mechanisms of solid solution strengthening in Ni3AI differ for the different substitutional types of atoms, i.e. interstitial and substitutional types. Another unknown mechanism corresponding to the extra strengthening arising from the addition of carbon should be studied further.

4.2. Solid solution strengthening As for the capacity of the solid solution strengthening of the additive elements in the boron-doped DS Ni3AI, the atomic size effect in the elastic interaction would be a main factor. Figure 9 shows the correlation between the Aoy/Ac and A e / A c for the additions of magnesium, and for the higher amounts of magnesium, calcium, yttrium and La-Ce. In Fig. 9, Aay/Ac, which is determined at 123 K, is the yield stress increment per atomic fraction of the alloy element normalized by the shear modulus G ( G = 6.5 x 104 Mpa) and A e / A c is the lattice strain increment per atomic fraction of the alloy element, where the At = ( a - a o ) / a o. In the present work, a and a 0 are the lattice parameters of the boron-doped Ni3A1 with and without the element addi-

5. Conclusions

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The mechanical properties of the yield stress in boron-doped DS Ni3AI alloyed with carbon, magnesium, calcium and rare earth elements were systematically investigated by tensile tests. Emphasis was placed on the temperature dependence, solid solution strengthening and the species of the alloy element. We conclude the following. ( 1 ) The yield stresses are generally found to increase with increasing alloy element content at temperatures below the peak, while this strengthening effect at temperatures above the peak weakens or disappears. (2) The addition of each alloy element decreases the value of the activation energy for the thermally activated process in the boron-doped D S Ni3A1. With increasing alloy content, the activation energy decreases. (3) The yield stress at 123 K, taken as a measure of the degree of the solid solution strengthening, varies with different additive elements. In the case of alloying with carbon, the yield stress increases linearly with carbon content. However, when alloying with magnesium, calcium, yttrium and La-Ce, the yield stress increases significantly when the additions are higher, while it increases moderately (for yttrium and La-Ce) and even decreases (for magnesium and calcium) when the additions are lower. It is also observed that solid solution strengthening arising from these additive elements cannot be interpreted only by the atomic size effect on the elastic interaction between solute atoms and dislocations.

Y. Zhang, D. Lin

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B-doped Ni~AI with C, Mg, Ca and rare earth elements

Acknowledgment This research was s p o n s o r e d by the A d v a n c e d Materials C o m m i t t e e of China.

National

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