A direct observation of the grain shape accommodation during liquid phase sintering of Mo“Ni alloy

Scripta METALLURGICA et MATERIALIA

Vol. 24, pp. 927-930, 1990 Printed in the U.S.A.

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A DIRECT OBSERVATION OF THE GRAIN SHAPE ACCO~IODATION DURING LIQUID PHASE SINTERING OF Mo-Ni ALLOY Dong-Duk Lee', Suk-Joong L.Kang, and Duk N. Yoon Department of Mater. Sci. & Eng., Korea Advanced Institute of Science & Technology P.O.Box 131 Cheongryang, Seoul, Korea * Now at the Fine Ceramics Lab. of Korea Institute of Science & Technology (Received January 23, 1990) (Revised March 6, 1990) Introduction In the liquid phase sintering of an alloy with a low liquid content, a necessary condition for densification is that the grains become somewhat polyhedral, because the interstitial space between the grains cannot be completely filled by the liquid if the grains retain their equilibrium shape (which is spherical if the grain-liquid interfacial energy is isotropic). In Kingery's model(l) two grains were assumed to be separated by a film of liquid and dissolution to occur at the contact region due to the capillary pressure produced by the liquid menisci. This mechanism was, however, never demonstrated to be important during liquid phase sintering by a direct observation. Furthermore, because the liquid phase was assumed to be present only around the contact region, the possibility of material transport to or from other regions of the grain surface was ignored. Even from the early stages of the liquid phase sintering of such alloys as W-Ni-Fe, fully dense regions develop consisting of many grains surrounded by the liquid matrix, with the pores meandering throughout the specimen(Z). The grain coarsening and shape change thus occur while the grains are completely surrounded by the liquid and in contact with their neighboring grains at only a limited fraction of their surface area. But even in this case the capillary pressure due to tile liquid menisci at the surface and the pores produces a compressive stress at the contact areas, where dissolution can occur as in Kingery's model. But it is also possible that material transport between the grains surfaces in contact with the bulk liquid matrix is more important in the grain shape change and coarsening. The purpose of this study is to make a direct observation of the shape change of the grains completely surrounded by a liquid matrix and under a capillary pressure. Earlier, Yoon and Huppmann(3) made such an attempt, but their initial condition was somewhat artificial because large spherical W particles were used. In this study initially spherical grains are produced by first sintering fine Mo powder mixed with a large amount of fine Ni powder. This piece is then put under a capillary pressure by bringing it into contact with a Mo-Ni fine powder compact with a low Ni content and only partially sintered to retain some porosity. In this way the grain shape change in the first part could be observed while the liquid was continuously drained from it to the other piece in contact. The Mo-Ni alloy was selected as a model system, because it was previously observed(4) that after cyclic sintering treatments and strong etching the shape of the individual grains at different stages of sintering could be displayed. Furthermore, the liquid phase sintered Mo-Ni exhibits a structure that is typical of those alloys with isotropic solid-liquid interracial

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Experimental Procedure The specimens were prepared from Mo powder of about 3~m size and 99.9Z purity and Ni powder of about 5~m size and 99.5Z purity. For the first part of the specimen (part A) the Mo and Ni powders were mixed at an 85 to 15 ratio by weight and compacted to cylindrical pellets of 10 mm in diameter and about 5 mm in height at 40 MPa pressure. The second part (part B) was prepared by mixing the No and Ni powders at a 98 to Z ratio by weight and compacting into cylindrical pellets of Z2 mm in diameter and about 10 mm in height at 150 ~ a . Each part was separately presintered at 1450 °C in a tube furnace under a hydrogen atmosphere. The pellets were slowly (at a heating rate of about 10 °C/min) pushed into the center of the preset furnace and pulled out immediately. (Since the pellets were at the sintering temperature for a very short time, this sintering treatment will be referred to as "0" time sintering). The presintered part A was cut into two equal pieces along its axis and its cut surface was polished. A flat surface of the presintered part B was ground and polished until its weight was 30 times that of the polished part A. The part A was then placed on B with the polished surfaces in contact and the whole assembly was sintered again at the same temperature (1450°C). This sintering treatment was carried out in cycles; after sintering for a certain period, the specimen was pulled out of the furnace at a cooling rate of aboht 40 °C/min and pushed into it again at about the same heating rate to be sintered further. After the sintering treatment A was broken off from B, sectioned into two pieces along the radial direction, and polished for microstrnctural examination. Results and Discussion The part A was found to be fully densified after the "0" time presintering t r e a t ~ n t as shown in the microstructure of Fig.l. The grains were nearly spherical because of the large liquid volume fraction which was determined to be 33Z by the point counting method. The part B with estimated liquid volume fraction of only 3~ had a 5Z porosity after the presintering treatment. When both parts were sintered again in contact with each other for "0" time, the grains in A moved closer to each other, forming a nearly close packed structure with a sudden decrease of the liquid volume fraction from 33Z to Z3Z as shown in Fig.g. Upon further sintering, the grains in A became anhedral as shown in Figs.3 and 4 with gradual reduction of the liquid volume fraction. It was shownearlier(5) that the a~unt of the liquid phase in this type of liquid phase sintered systems determined the grain shape and the curvature of the liquid menisci at the specimen surface and pores. A change of the liquid content will immediately change the menisci curvature and hence the capillary pressure, which, in turn, will induce a more gradual change of the grain shape. In the presintered part A, the surface menisci are expected to be nearly flat with an infinite radius of curvature, since the spherical grains are floating in a large amount of liquid. In the presintered part B, the menisci radii will be small, producing a relatively large capillary pressure because of the low liquid content. When the two parts were brought into contact during the sintering for "0" time, some liquid flowed from A to B because of the capillary pressure, filling some pore volumes in B, until the grains in A moved closer to each other to a nearly close packed structure. After this sudden liquid flow, the liquid pressure in both parts and hence the menisci curvature would be uniform. Hence, in A there would be a net force due to the capillary pressure to change the spherical grains to an anhedral shape and reduce the liquid volume fraction in that part. The cyclic sinte~ng treatment used in this study was essential for observing the shape change of the individual grains. In previous studies the same technique was used to make direct observations of the Ostwald ripening(4,6) and grain growth around pores in liquid phase sintered Mo-Ni alloys(7). During cooling after each sintering cycle and heating to the next, thin layers of No-Ni alloy of reduced Ni content (because of the retrograde solidus(8)) are apparnntly deposited on the surface of the growing grains and show up as ghost boundaries after strong etching. These etch boundaries thus show the grain surface after each sintering cycle. After sintering for Z h, the liquid volume fraction in A was reduced to about 18Z as shown in Fig.g, and the grains became

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anhedral as shown in Fig.3. The etched boundaries in some of the large grains show the grain surfaces a f t e r the presintering treatment. ~:e two grains at tim center of Fig.3 were nearly spherical and in contact with each other after the presintering treatment, and during the sintering treatment for Z h in contact with B, the intergranular contact area increased, producing contact flattened or shape accommodated grains of anhedral shape. C!early, tMs grain shape change occurred by preferential growth at the grain s,affaces in contact with the bulk liquid a~,'ay from the contact regions. This observation is contrary to Ningery's assumptioni 1) that preferm!tial dissolution occurred at the contact region. A similar growth pattern is shown in a grain at the center of FigA. The inner etch boundary show~ the spherical grain shape after the presintering treatment and the grains became anbedral by ~referential growth to ce~ain directions during the 2 h - 3 h sintering cycle with a reduction o f the liquid volume fraction. Both preferential growth as observed here and preferential dissolution as proposed by gingery are driven by capillary pressure and will produce the same shape accommodated to the neighboring grains. Their kinetics, however, are different. As has been pointed by Yoon and Huppmann(3). and Kang et a l . ( 9 ) , when tile grain coarsening rate is relatively high as in this alloy, tile preferential growth is the faster process and i t s kinetic rate is expected to be related to tile Ostwald ripening process. A complete theoretical description of such a preferential growth process under a capillary pressure i s , however, yet to be obtained. Conclusions

The experimental evidences for Kingery's model of contact flattening have been obtained from comparisons of the predicted densification rate equation to the observed density changes(lO). However, the division of the observed densification curves into three stages, corresponding to Kingery's liquid phase sintering model, has often been arbitrary. Furthermore, tile green compact volumes were used as the i n i t i a l volumes for the second stage of solution and reprecipitation i , some analyses, which s t i l l showed apparent a~eement with the predicted rate law(ll,lZ). As has been pointed out by P r i l l et al.(13), the i n i t i a l volume corresponding to each stage should have been used instead in such m~alyses. From the early stages of liquid phase sintering, groups of grains usually become completely surrounded by the liquid phase and rapid grain coarsening occurs. The grain shape accommodation is thus expected to occur by preferential growth as observed directly in this study, gingery's dissolution process can occur in the very early stage when the contact areas are very small and hence under a very high compressive stress, but such a change is not expected to contribute significantly to the densification. References

1. W.D. Kingery, J. Appl. Phys., 30. 301(1959). 2. J.K. Park, S.-J.L. gang, K.Y. Eun, and D.N. Yoon, ~letali. Trans., ,20A, 837(1989). 3. D.N. Yoon and W.J. Hnppmann, Acta ,.'letall., 27, 693(1979~. 4. S.S. Kim and D.N. Yoon, Acta Metall., 31, 1151f1983}. 5. H.H. Park, S.-J.L. gang, and D.N. Yoon, Meta!l. Trans.. 17A, 325(19861. 6. S.S. Kim and D.N. Yoon, Acta Metalt., 33, 251(1985). 7. S.-J.L. Kang and P. Azou, Powd. ~ e t a l l . , 28, 90(1985). 8. S.-J.L. Rang, Y.D. Song, W.A.Kaysser, and H. Hofmann, Z. ~letallk., 75, 86(1984). 9. S.-J.L. Kang, W.A. Kaysser, G. Petzow, and D.N. Yoon, Acta Metall., 33, 1919(1985). 10. W.D. Kingery and N.D. Narasi~an, J. Appl. Phys., 30, 307(19591. 11. P.J. Jorgensen and R.W. Bartlett, J. Appl. Phys., 44, 2876(1973). 12. W.D. gingery, E. Niki, and M.D. NarasiNnn, J. Am. Ceram. Soc., 44, 29(1961). 13. A.L. P r i l l , H.W. Hayden, and J.H. Brophy, Trans. AINE, Z33, 960(1965).

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=.~3 ~. om~ *--I before sintering q

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Sintering time (h) Fig.l. 85Mo-15Ni (wt%) alloy (part A) presintered for "0" time at 1450°C.

Fig.2. The observed liquid volume fraction change in A during sintering in contact with B at 1450 °C.

Fig.3. The part A sintered for 2 h in contact with B at 1450 °C.

Fig.4. The part A sintered for 2 h and 3 h in contact with B at 1450 °C.

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