Scripta METALLURGICA Vol. 2~, pp. 2065-2070, 1990 Pergamon Press plc et ~%TERIALIA Printed in the U.S.A.

ON THE VALIDATION OF A STRE~GTHENLNG CONCEPT

V. Provertzano. N.P Louat*, M.A. Imam and K. Sadananda Materials Science and Technology Division

Naval Research Laboratory, Washington, IX? 20375-5000

(Received May 21, 1990) (Revised August 20, 1990)

Introduction

The strength of a material is defined as its resistance to deformation. On an atomic basis, resistance to deformation in a cryst',dline material corresponds to its resistance to dislocation motion. Dislocation motion, in turn, is resisted by intemal stresses. Therefore, in order to achieve maximum strength, the internal stresses in the material should meet the following requirements: a) act in unison, b) be as large as possible, c) not be easily avoidable, and d) not be thermally surmountable at significant rates.

Mechanisms used to strengthen conventional alloys include hardening by solid solutions, precipitation and dispersion of particles and strain hardening. However, none of these mechanisms meet all the requirements listed and thus do not maximize strength. Importantly, conventional strengthening mechanisms lose their effectiveness at high temperature due to thermally activated processes such as creep. For example, even the best currently available nickel based superalloys strengthened by conventional schemes lose nearly all of their strength at temperatures between seven to eight tenths of the melting point of the nickel matrix. Consequently, these considerations lead to the conclusion that in order to obtain dramatic increases in strength an alternate approach is needed.

Louat formulated a strengthening theory(l ) in which the four conditions specified above could be met in two phase materials in which the minor phase forms the matrix. Additionally. thermal stability is insured if the two phases are immiscible. It follows from Louat's theory that material with superior strength can be obtained by embedding a high volume fraction (greater than 50%) of ultrafme panicles in a ductile matrix. In contrast to conventional materials, these panicle reufforced materials are predicted to retain much of their strength even when the matrix melts. In addition, the theory predicts that the strength of such materials increases with decreasing panicle size. Specifically, at temperatures below the melting point of the matrix and over a wide range of particle size the

strength, o, should follow a Hall-Perch relation of the form,

k (1) o = oo + ~-~

where o o is the flow stress of the matrix, k is related to the shear moduli of the panicles and the matrix and d is the average particle size. Further, the theory predicts that for the case where the particles are so closely packed as to form a rigid skeleton, the expected minimum strength when the matrix melts is given by:

2ycosO (2) O - r '

where y is the surface energy, 0 the contact angle between the particle and the matrix, and r the effective radius of the interstices between the particles; r is proportional to d, the average particle size. Implicitly, Eqs. (1) and (2) contain some basic requirements for high strength. These are: (1) that there is good bonding between the panicles and the matrix; (2) the particles should be very small. In addition to these two requirements it is also important, for stability against Ostwald ripening, that the particles and the matrix should be immiscible. FinaLly, for adequate ductility considerations, the panicles should be deformable.

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2066 STRENGTHENING CONCEPT Vol. 24 , No. ii

An initial experimental confirmation of this theory was first obtained on Fe-Hg system(2). For this system, a yield strength of 32.4 MPa was obtained at 90% of the melting temperature of mercury. Following this work, the research was extended to include materials which are technically more relevant(3). The materials included in that study were various carbide particles embedded in a copper matrix. For these material combinations, micron and submicron size commercially available carbide powders were used. Consistent with the theory, the room temperature strength of the consolidated specimens followed the Hall-Petch relation given in Eq.(l). Moreover, a yield strength of 2413 MPa was obtained for 0.51aSh size TiC particles in a copper matrix. This strength value is more than 30 tunes that of annealed copper.

The results presented in this paper refer to the case of iron or copper particles embedded in a lead matrL, c These particle-matrix combinations were chosen because: (a) both iron and copper are immiscible in lead; (b) the particles are deformable at low temperature; (c) a lead based material can be considered to be in the high temperature regime even at room temperature. In sunmmry, therefore, both material combinations are appropriate model materials to test the strengthening concept further.

Experimental Details

The particulates used in this study were copper powder with an average particle size of 2.71am and iron powders

with four different particle sizes in the range of: 2.8-25gm. The purity of the five powders was 99.9%. The following procedure was used to prepare the composite compacts. First, powder of either iron or copper was placed inside a hollow lead cylinder and back-flushed in a hydrogen atmosphere, afterwards the cylinder was capped with lead. The capped cylinder was then wrapped with a tantalum foil. The wrapped cylinder was canned using a stainless steel bag, evacuated for several hours and then given two successive treawnents in a hot isostatic pressing (HIP) unit: one for 2 h at 400°C (above the melting temperature of lead) followed by another 3 h at 300°C (below the melting temperature of lead); the isostatic pressure for the two hipping treatments was 207MPa. The tantalum wrapping helps prevent interaction between the stainless steel bag and the lead during the hipping treatments, whereas the flushing with hydrogen lowers the oxygen content of the powders. It is assumed that during the higher temperature hipping the lead melted and coated the powder particles by liquid infiltration, while during the lower temperature hipping the material was consolidated and the internal porosity, including that produced by the freezing of lead, was eliminated.

The consolidated specimens were sectioned with a diamond saw and polished by standard metallogr-aphic techniques. The yield strength of the polished sections was determined by impression tests using a cylindrically- shaped indenter of lmm in diameter. The yield strength measurements were conducted from room temperature to 250°C. Complete details of the experimental apparatus used for the impression test have been given elsewhere(4).

The microstructural features of the hipped materials were examined by scanning electron microscopy (SEM) using a Hitachi Model S-800 high resolution field emission microscope equipped with a Princeton Gamma-Tech energy dispersive X-ray system for composition analysis. The energy dispersive X-ray system has also a windowless detector for light element analysis.

Results and Discussion

The significant microstructural features of two different size iron particles in a lead matrix are illustrated by the SEM micrographs presented in Fig. 1. The resulting microstructure for the two powders is uniform with most of the iron particles surrounded by the lead matrix. Image analysis conducted on the two specimens yielded an average

particle size of 2 .8~n for the freer powder and an average size of 151am for the other; the particle volume fraction was close to 50% for the two types of specimen.

The mechanical data obtained in the lead-iron composite samples are summarized by the plot presented in Fig. 2. The room temperature yield strength for the four different size iron particles is plotted against the inverse square root of the average particle diameter. The data clearly show that the strength of the lead-iron composites increases as the particle size decreases and that it follows the Hall-Petch relation as predicted by the theory. Moreover, when extrapolated to very large particle sizes the resulting strength is close to the room temperature strength of the lead

matrix. Finally, for particles having an average size of 2.8~n, the strength of the composite material is 186 MPa. This value is about fifteen times larger than the strength of lead and about three times the strength predicted by a simple nile of mixtures.

Vol. 24, No. Ii STRENGTHENING CONCEPT 2067

Figure 3 is a plot of the yield strength of lead-infdtrated copper powder with an average particle size of 2.7gin, as a function of the homologous temperature, where the melting point of lead was used as the reference temperature. In the same plot the yield strength as a function of the homologous temperature of dispersion and precipitation strengthened Mar-M-200 nickel based superaUoy is shown for comparison. The reference temperature for this latter plot is the melting temperature of the nickel matrix. In Fig. 3 the two sets of data clearly show that the ~peralloy loses about 90% of its room temperature strength at about eight tenths of the melting temperature of nickel, mostly

because of the dissolution of the "/- precipitates. By contrast, the lead-copper composite sample loses only about 35% of its room temperature strength at about 90% the melting temperature point of lead.

The mechanical restdts presented in Fig. 3 are pivotal to the experimental validation of the strengthening theory proposed by Louat. The data shows that, whereas the best alloys strengthened by conventional mechanisms lose nearly all of their strength at temperatures between seven to eight tenths of the melting point of the matrLx, the materials studied here retain most of their strength even when the temperature is close to the melting point of the matrix.

References

I. N.P. Louat, Acta Met., Vol. 33, pp 59-69 (1985). 2. N.P. Louat and M.A. Imam, Scripta Met., Vol., pp 721-726 (1989). 3. V. Provenzano, N.P. Louat, K. Sadananda, M.S. Imam, C.J, Skowronek, J. Calvert and B.B. Rath, Syrup.

on Surface Modification Technologies II, Ed. T.S. Sudarshan and D.G. Bhat, pp 313-321 (1989). 4. H.Y. Yu, M.A. hnam and B.B. Rath, J. Mat. Sci., Vol. 20, pp 636-642 (1985).

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2070 STRENGTHENING CONCEPT Vol. 24, No. ii

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