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(1)Change in Volume on Ordering of Iron-Cobalt Superlattice AIIoys By. Takasi TAKEZAWA* and Hidetaka IWANANI"" (Received June 1, 1981). Synopsis. L. The thermal expansions of FeCo alloys in the bcc range have been measured. The volume of the 30i-v70 at% Co alloys decrease at the order-disorder transition temperature Tc on ordering, and the expansivity of these alloys decrease as the long-range order parameter increases. The mean expansivities of the ordered 25"v70 at% Co alloys are less than those of the disordered. ones in the temperature range from 1000 to 2000C. The volume due to the ordering at room temperature decreases in 27. 5.-v33. 3 at% Co and increases. in 40･-v70 at% Co.. The volume increase due to the ordering at room temperature of the 40Av70 at% Co alloys is mainly attributed the expansivity in the ordered state.. gl. Introduction. The following has made clear by the neutron diffraction experiment about FeCo superlattice alloys done by Lyashenko et al.(i) That is, there exist CsCl type (B2 type) superlattice alloys in 25tv75 at% Co alloys which belong to alphaphase (bcc range), and their order-disorder phase transitions is the second order. Ellis et al.(2) measured the lattice constants of FeCo alloys at room tempera-. ture with various heat treatments. As the result of that they observed the volume increase due to the ordering in 40tNy60 at% Co alloys and this differs from the usual cases. Yokoyama(3) measured thermal expansion of 1:1 atomic ratio FeCo alloys and as the result of that the lattice constants decrease during. ordering at order-disorder transition temperature, 730eC, however after' the quenching from various temperatures the lattice constants increase at room temperature range. Asano et al.(4)(5) measured the lattice constants of alpha-phase. FeCo alloys at room temperature with various heat treatments using fine particle samples, and they reported that the volume due to the ordering " Depertment of Technical Arts, Faculty of Education, Yokohama National University, Yokohama.. *" Depertment of Mechanical Engineering, Nagano Prefectural Minowa Technical. High School, Nagano. ･･ '･.

(2) 46 T. TAKEzAwA and H. IwANAMi increase in 40r'v60 at% Co alloys, whereas decrease in 30N40 at% Co alloys, either of the alloys is in bcc range.. Generally, it has been known that the substitutional type alloys decrease their volumes due to the ordering. In this paper, the increase in the volume due to the ordering of FeCo alloys was studied with the thermal expansion. measurement.. g2. Experimental Procedures. The alloys were prepared from iron and cobalt of 99.99% purity. The melting condition was as same as the previous paper(6). The dimention of specimen was 5mm in diameter and 50 mm in length, and on one side a hole whose dimention. was 2.8mm in diameter and 15mm in length was made to put a thermocuple. The specimens had the constitution of nine alloys covering the range 25 to 70. '. at% cobalt. Thermal expansion was measured in the vacuum furnace at pressure of 2rv5×10H5mmHg in the heating or cooling process and the rate of heating or cooling was O.70C/min by means of P.I.D. thermocontroller. Platinumplatinum containing 13 at% rhodium thermocouple was used to measure the temperature. Differential transformer whose output was 7.7mV against the speci-. men's expansion of O.1mm was used on the measurement. The method of differential expansion measurement was deviced by authers(7), and under this method the linear expansion coethcient was obtained.. g3. Experimental Results and Discussions. (1) Dzlzft?rential exPansion curves.. Figure 1 shows ditiferential expansion curves of 25rtv75 at% Co alloys (let us. write these "d.e. curves" for short). The order-disorder transition temperatures. (let us write this "T," for short) of each composition alloys are shown with arrows on the d.e. curves respectively. The increase of the slope of the d.e. curves indicate expansion, on the contrary the decrease indicate shrinking. The sharp inflection at the T, point was found on each d,e. curves of 30tN/60 at% Co alloys. These changes show shrinking due to the ordering. Figure 2 shows linear expansien curves obtained from the d, e. curves in Fig.. 1. The change in slope on the d.e. curves in 70 at% Co alloys is not clear, however the change in the linear expansion curve is obviously. The linear expansion coefiicient of 50 at% Co alloys decreases gradually as the temperature decreaseS from higher part to T,, it increases steeply at T,. This increase is due to the volume shrinking during ordering. Below T,, the linear expan'sion coefficient continues decreasing,. the value is smaller than that of disordered state. below 7100C. This tendency was recognized on 30rv60 at% Co alloys. The 1'inear expansion coefficient curve of 70 at% Co alloys did not show theincrease. at Tc, but it showed an increase as same as 30rv60 at% Co alloys Wheri the. 'j.

(3) Change in Volume on Ordering of Iron-Cobalt. 14. Wii 30. 25qte/oCo. = m c a q x tu. .9. t. `. il. l;. 41.9. ll. t. 50. lt 60 l 7d. ,. 3oo 4oo'soo 6od 7oo soo goo Temperature (ec) Fig. 1. Differential expansion curves of FeCo alloys at a cooling rate of O.70Cl. mm. '' '. ;. 1= 10 ･tr. 9. v 12 q. :--=-+. -. `. 41.9. l. ,. 5o. 't. . 12 g. 'm-'. alO. di. ,. 'u 12. o ￠. .E 1O. L. .. + iJ 33.3. 12. A12 u. 30. -1 O. ￠. 'a-. = i'k. .-x. c 9. t. 40. I. 6 ･=. 25 atOIo Co. 12. i . 33.3 I. 47. ". IM. u. 60. 70. 12. 10. 4oo 50e 6oo 7oo 8'OO Temperature <ec). Fig.2. Linear expansivity curves of. FeCo alloys.. '. length of sample of 70 at% Co alloys was 70mm. The curve of 25 at% 'Co alloys decreased gradually, it did not show an increase even when the length of the sample was 70mm. The degree of long-range order of 25 at% Co alloys was about O.3 from the result of the neutron diffraction, The shrinking in this degree of order due to the ordering is very small, while the volume expansion depend on ,the temperature is large, so that it could not recognize the change of liner expansion coeMcient.. (2) The linear expansion coevOicient 0f the ordered and disordered states. The linear expansion coeflicient of the ordered and disorded states in Fig. 2. cannot be compared because of the difference of T, of each alloys and the influenceoftemperature. Figure3 shows mean linear expansivities between 200o and 3000C at a cooling rate of O,50C/min of the ordered and disordered states of FeCo alloys, in this temperature range the exchange of atomic position is difficult. The heat treatment of ordering" on each alloys was done suMciently considering the heat treatment time to requlre ' the thermal equilibriUm of FeCo alloys in previous paper(8)(9). For the heat treatment of disodering,'each alloys was kept at 9000C for O.5 hr and then at T,+700C for 3hr, they were quenched.

(4) T.. TAKEzAwA and H. IwANAMi. 48. 12. 6. ,g. 11. b. ut. ; 10. .. q. OL. order(B). )-. a E 9. ･･. =.2 2. 9. 8. g. /. di 1.5 si 1.0. :t t"'-'--<. {- O.5. o. (A)-( B). - -- -t-. s. tr. v.. o. l g= -. 1.0 e?. NN. 2e 3o 4o so 6o 7o. U 05 ge. og"a. l. Co<at `76). Fig. 3. Mean linear expansivities between 2000 and 100eC at a cooling rate of O.50C/min of the ordered and disordered states of FeCo alloys.. 1. in to salt water at 100C. The linear expansivity curves of the ordered state (open circle) and the disordered state (solid circle) in Fig. 3 show concave lines, and both of the center. of the curves are about 50 at% Co alloys. The linear expansivity '' curveofthe ordered state (B) shows smaller value than that of the disordered state (A), therefore the decrease of linear expansivity due to ordering can be recognized.. The phenomenon that the linear expansivity of ordered state is smaller than that of disordered state agrees with most order-disorder alloys. This can be understood considering that in ordered state nuclear bindings are solid for the thermo-dynamical stability. This agrees too with that metals which have higher melting point, that is, metals which have solid nuclear bindings show small linear expansivities.. The curve (A)-(B) shows convex whose center is'about 50 at% Co alloys. Long-range order parameter, S, obtained by neutron diffraction experiment is showed as a broken line in the same figure. These two lines coincide with their tendencies, order parameter is almost proportional to the defference between '. order and disorder linear expansivity,. '. '. (3) Total exPansion curves of ordered and disordered states. ･ From the results of (1) and (2), the followings have made clear. That is, 30tNv70 at% Co alloys show shrinking due to the ordering, the mean linear expansivities of these alloys between 2000 and 100eC of the ordered state show smaller values than these of the disordered state. When the lattice parameters of Fe-Co alloys are measured at room temperature ran' ge, the volume increase of 40nu60 at% Co alloys during ordering(2)""(5) is due to the contribution of decrease of expansivity during ordering from the.

(5) Change in Volume on Ordering of lron-Cobalt 49 results of the above mentioned.. To confirm this presumption experimentally, the shrinking curves of the ordered state which cooled slowly from high t}emperature ought to be compared to the shrinking curves of the disordered state which quenched from high temperature to room temperature. It is diflicult to measure exactly the shrinking curves of the disordered state which quenched because of the dithculty of measuring exact relation between the volume shrinking of the sample and the temperature for the rapidness of cooling rate. Kaya and et al.(iO) showed about Ni3Fe alloys that the expansion curves of the disordered state at room temperature are on the extended curves of the disordered state at high temperature. Therefore total expansion curves in heating process of the disordered states. '. which quenched from high temperature were measured, then smooth lines were drawn from high temperature part of the curves to low temperature part. These curves can be considered as the curves of the disordered statQs in whole temperature range, these were compared with the curves in cooling process of the ordered state.. Figure 4 shows the total expansion curves at heating and cooling rates of O.70C/min from the disordered state in room temperature range to the temperature higher than T, of FeCo alloys. T,(`)(5) which known already are shown with arrows on each curves. At the temperature range higher than T, of disordered state, the curves in heating processes agree with that of the cooling processes in each composition alloys.. Clooling curve of 50 at% Co alloys shows shrinking in order, a-->b-c.d-" e-f. In this curve, a line between the high temperature and T,, i.e., ab shows shrinking curve of the disordered state, This curve shows remarkable shrinking at b (T,) due to the ordering. Gradient of this curve becomes gentle as the temperature gradually decrease to room temperature, f, because expansivity becomes small as the temperature decrease. This tendency of cooling curve can be recognized in 30rtv70 at% Co al!oys, too. These curves are cooling curves of ordering, so let us term these as ordering curves.. Heating curve of 50 at% Co alloys shows expansion in order, i.h->e->g--> d->c->b-->a. Between room temperature, i and about 4000C, h, this curve is nearly linear. From the result of the measurement of specific heat,(ii) h corre-. sponds to the temperature of the beginning of ordering. Internal change is hardly caused below 4000C, the curve, ih is a usual expansion curve of the disordered state. It is obvious from the measurement of the specific heat(ii) that. above 4000C ordering becomes remarkable. Consequently, the curve shows remarkable expansion because the degree of order becomes nearly equilibrium at that temperature. This curve is crossed with the ordering curve at e, reaches. to g, closes by the ordered curve gradually and then coinsides with it. This corresponds to "the change at 5500C of Fe-Co alloys(8)(9)", ordering connot advance in equilibrium state as for the speedy rate of the heating, so the ordered. state at low temperature are brought in high temperature. The same tendency.

(6) T. TAKEzAwA and H. IwANAMi. 50. 25ateleC. Ttll30ll33. l. g. I;Lallbh h 40. iJ. :n ... fxL. 7Z,. AE E. out. ;`"'. 11. ". v. g. c. .9. d. mc U. ax. tu. dv -o -. f. i. o. Z 7 Z 7 Z 7. ttt. a. t. l50 Za. .,li. Zi"ibl. c. d. :. g e 77 711. t. h. l. h 70. t. l 7. 2oo 4oo 6eo soo Temperature (eC ). Fig. 4. Total expansion curves at heating and cooling rates of' O.70C/min. from the disordered states of FeCo alloys.. can be recognized for the heating curves of 40 and 41,9 at% Co alloys too. Above 550eC, d, the curve in heating process is agreed with that in cooling process, they become disordered state above T,. The parts of disordered state. r. ,1. in this curve in heating process are curve, ih in low temperature and curve, ba in high temperature above T,. The curve, abdhi linking ih to ba smoothly with a broken line can be regarded as a curve measured in disordered state in whole temperature range. The curves in heating process regarded as disordered state in whole temperature range were obtained by linking low temperature part to high temperature part of disordered state in 30rtv70 at% Co alloys, too. These are termed as disordering curves. The ordering curve, bcdef of 50 at% Co alloys was compared with the disordering curve, bdhi. These two curves crossed at about 5500C, d, the ordering. curve, bcd is under the disordereing curve, bd between T, and 550eC, below 5500C the ordering curve, def is upper the disordering curve, dhi. This signifies. r.

(7) Change in Volume on Ordering ,of Iron-Cobalt 51 that when the sample in the disordered state becomes the ordered state, it shows. expansion below 550eC and shrinking above 5500C. The volume of the sample in the ordered state at room temperature shows expansion of O.06% than that of disordered state. This expansion was recognized in 40rtv70 at%'Co alloys,. too. -. When the ordering curve, bcd of 33.3 at% Co alloys was compared with the. disordering curve, b]lg, the ordering curve, bcd is always under the disordering. curve, bie, they does not cross each other as not for 40rv70 at% Co alloys. The volume of the sample in ordered state at room temperature shows shrinking O.f .sO'O,3 ,[?o'g:,O.,t,kikfeo,402'7V.s70a.adt%3oCaOt;liiOcYoS'.iiToyhsi?. tsoh,fi"ki"g at room temperature. The heating curve of 25 at% Co alloys agreed with the cooling curve in 'tt. whole temperature range, influences by ordering cannot be recognized.. Table 1 shows change in volume on ordering at room temperature obtained by comparing the volume of ordered state with disordered state. The change g. in volume on ordering decrease in 27.5rv33.3 at% Co alloys and increase in 40rv 70 at% Co alloys. The change in volume on ordering obtained from lattice constant by Asano et al.(`)(5) (when the measurement was done for different composition alloys, the value was obtained from the composition curves of lattice constant vs Co) was shown in the table too. Compared these obtained results, the tendency of change in volume due to the ordering showed good agreement. The values of the ratio for each composition differ slightly, this reason is for the difference of the method of measure-nent and heat treatment of the samples for the most part. '. Table 1. Change in volume on ordering of FeCo alloys 4t room temperature.. Icoat%l2si27.si3o 33.3 4oI4i.gisol6o 7o ii wws. -,.,:iiIo,igiiI:igg[I---[-tgliil'o'oi. lgiizRg･.il lgi2: +o･og. g. t'. 'B":. obtained from the' lattice constant(`)(5).. From the results above mentioned, the ordering curve in cooling process contains two factors which are the volume shrinking and the decrease of expansivities due to the ordering. Thus, the change in volume on ordering at room temperature shows increase or decrease due to the contribution of volume shrinking or decrease of expansivity on ordering below T,. That is, the reason why the volume of 40tv70 at% Co alloys in the ordered state increases than that of disordered state is due to that the contribution of decrease of expansivity on ordering is larger than that of the volume shrinking below Tc..

(8) T. TAKEzAwA and H. IwANAMi. 52. g4. Conclusions. The change in volume due to ordering of Fe-Co alloys in the alpha-phase was studied by measuring thermal expansions, and the followings were obtained.. (1) The 30rv70 at% Co alloys show shrinking due to the ordering at the order-disorder transition temperature T, and their expansivities decrease during ordering,. (2) The mean linear expansion coefficients in the 25rv70 at% Co alloys of ordered state are smaller than those of disordered state.. (3) The volume on ordering at room temperature decreases in 27.5rv33.3 at% Co alloys and increases in the 40rv70 at% Co alloys. (4) The 40rv70 at% Co alloys show expansion in volume due to ordering at room temperature, this is due to that the contribution of decrease of expansivities. is larger than that of the volume shrinking below the order-disorder transition temperature Tc.. e References [1] B.G. LyAsHcHENKo, D.F. LiTviN and Yu.B. ABov: Sov. Phws. Cryst., 6 (1962) 443. [2] W.C. ELLis and E.S. GREiNER: Trans. ASM, 29 (1941) 415. [3] T. YoKoyAMA: J. Japan Inst. Metals, 21 (1957) 321. [4] H. AsANo, Y. BANDo, N. NAKANisHi and S. KAcHi: J. Japan Inst. Metals, 30 (1966) 684.. [5] H. AsANo, Y. BANDo, N. NAKANisHi and S. KAcHi: Trans. JIM, 8 (1967) 180. [6] T. YoKoyAMA and T. TAKEzAwA: J. Japan Inst. Metals, 35 (1971) 860. [7] T. YoKoyAMA and T. TAKEzAwA: J. Japan Inst. Metals, 38 (1974) 472. [8] T. YoKoyAMA and T. TAKEzAwA: J. Japan lnst, Metals, 33 (1969) 541. [9] T. YoKoyAMA, T. TAKEzAwA and Y. HiGAsivDA: Trans. JIM, 12 (1971) 30. [10] S. KAyA, H. SATo and Y. KuMAsAKA: J. Japan Inst. Metals, 6 (1942) 188. [11] T. YoKoyAMA: J. Japan Inst, Metals, 20 (1956) 647.. n.

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