Shock consolidation of a rapidly solidified steel powder

wol-6160/84 s3.00+0.00 Copyright Q 1984 Pergamon Press Ltd

,&~a maall. Vol. 32, No. 8,

pp. 1235-1241, 1984 Printed in Great Britain. Al8 tights reserved

SHOCK CONSOLIDATION OF A RAPIDLY SOLIDIFIED STEEL POWDER P. EASIRAJ,

T. VREELAND Jr, R. 8. SCHWARZt and T. J. AHRENS

California Institute of Technology, Pasadena, CA 91125, U.S.A. (Received 29 June 1983; in revised form 4 Janwry 1984) Abstract-Rapidly solidified AISI 9310 steel powders were consolidated by shock waves produced from the impact of high velocity flyers. Dependence of the microhardness and the ultimate tensile strength-of the compacts on the initial shock pressure (from 3.6 to 17.9 GPa) and the maximum shock pressure (from 6 to 37GPa) was measured foran initial powder density 0.6 of the bulk density and a shock duration of 2-3 ps. Photomicrographs and SEM fractographs were used to study the interparticie bonding in the compacts. Results show that for initial shock pressures below 4GPa, the compacts have negligible strength, However, above this threshold the strength of the compact rises rapidly until a maximum value of I.3 rf: 0.1 GPa is reached for an initial shock pressure of 12.4 GPa. The strength then remains constant before decreasing at the highest initial shock pressure. In marked contrast, with increasing shock pressure, the diamond pyramid hardness increases very gradually from a value of about 340 for the powder to about 500 at the highest shock pressure. The maximum strength obtained correlates reasonably well with the strength-expected from microhardness measurements. R&mn&Nous avons consolide des poudres d’acier AISI 9310 ~pide-solidity, par des ondes de choc produites par l’impact de balanciers B tris grande vitesse. Nous avons mesum les variations de la microdureti et de la resistance ii la rupture des poudres en fonction de la pression de choc initiate (de 3,6 ii 17.9 GPa) et de la pression de choc maximale (de 6 li 37 GPa), pour une densite initiale de la poudre de 0,6 (par rapport au m&al massif) et une dur& du choc de 2-3~s. Nous avons utilist les photomi~o~aphi~ et les fractographies du MEB pour etudier les liaisons entre particufes dans la poudre. Nos r&hats montrent que pour des pressions de choc initiales in!&ieures a 4 GPa, Ies poudres ont une resistance negligeable. Cependant, audessus de ce seuil, la resistance de la poudre augmente rapidement jusqu’& atteindre une valeur maximale de 1,3 f 0,l GPa pour une pression de choc initiale de 12,4 GPa. La resistance reste constante avant de d&croitre pour les pressions de choc initiales les plus fortes. Au contraire, lorsqu’on augmente Ia pression de choc, la durett avec des pyramides de diamant au~ente tres progressivement dune valeur d’environ 340 pour.la poudre a environ 500 avec la pression de choc la plus Clew%. La resistance maximale obtenue est en accord raisonnable avec la resistance attendue a partir des mesures de microdurete. Z~~~~~Ra~h erstarrte Pulver aus Stahl AISI 9310 wurden mit ~h~kwelien durch Einschlag von Hochgeschwindigkeitsgeschossen verdichtet. Die Abhitngigkeit von Mikrohgrtc und Zugfestigkeit der Proben von Anfangsdruck des Schockes (zwischen 3,6 und 17,9GPa) und maximalem Schockdruck (xwischen 6 und 37 GPa) wurde fur die anf”angliche Pulverdichte von 0,6 der Dichte des Vollmaterials und fiir eine Schockdauer von 2-3~s gemessen. Mit optischen Aufnahmen und fraktografischen Unte~uchun~n im ~tereiektronenmikroskop wurde die Bindung xwischen den Teilchen der Proben untersucht. Die Ergebnisse xeigen, daB die Proben fiir einen Anfangsschockdruck kIeiner als 4GPa vemachhissigbare Festigkeit aufweisen. Oberhalb dieses Wertes jedoch steigt die Festigkeit rasch an bis N einem Maximalwert von I,3 f 0.1 GPa, der mit einem Anfangsschockdruck von 12,4GPa erhalten wird. Dartiber bleibt die Festigkeit konstant und sinkt bei dem ho&ten Anfangsschockdruck wieder ab. In deuthchem Kontrast dazu nimmt die Diaman~~~e sehr allm~lich von ungetXhr 340 beim Pulver a,uf etwa SO0 beim hiiehsten Schockdruck A). Die erhattene maximale Festigkeit korreliert hinreichend gut rnir der aus den Mikrohzirteversuchen erwarteten Festigkeit.

1. ~RODU~ION Shock wave consolidation

of powders is a promising technique for obtaining strong bulk materials. It is possible to obtain consotidated materials with this method which retain nonequilibrium PrOpertieS of the initial powder. Furthermore, extensive chemical interaCtiOn b&WCCn powder mixtures may be avoided I% tOn leave from MST Division. Argonne National Laboratory, Argonne, IL 60439, U.S.A.

Qualitatively, consolidation by shock waves is achieved via preferential short-duration deposition of energy at particle surfaces through various mechanisms such as heavy plastic deformation and friction bet ween particles [l-3]. This deposition occurs during t he sh oc k rise time which can be as short as tens of nanoseconds. The selectiveness of the heating allows for the possibility ofobt$ning w&bon&d compacts without subjecting the bulk to high homologo~ temperature for long durations, as required in conventional powder metallurgical processes. Although consolidation without local melting of particles may

1235

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KASIRAJ

et al.:

SHOCK CONSOLIDATION

take place [4,5], it appears that energy deposition which causes interparticle melting is required before very strong compacts are produced [6]. Two general methods have been used to affect the shock consolidation of powders. In the explosive compact method, consolidation is achieved by detonating explosives attached to the powder in various geometries [7-g]. The other method involves the production of shock waves by high-velocity impact of projectiles [2]. The latter method is much more attractive in terms of studying the shock consolidation process since the shock history of the powder can be more easily controlled and defined. Although shock compaction has been studied since the 1950’s [IO], there is a scarcity of quantitative investigations on the relevant parameters which govern the consolidation process. Previously, powders of lead, iron, aluminum, steel, and metallic glasses have been compacted by flyer plate impact [I, 6,1 I, 121.In these studies the powder was subjected to relatively low shock pressures (less than 2 GPa) and the steel compacts produced had relatively low strength in relation to their hardness. The shock consolidation process at higher pressures was not explored. Raybould [6] and Morris [13] studied the amount of melting produced in consolidations of tool steel powders by metallographic and TEM observations of phase changes near particle boundaries. Morris mea: sured the amount of melt as a function of shock pressure up to 8 GPa. The strength of the compacts was not measured. Several quantitative models to explain the shock consolidation have been advanced. Raybould [l] introduced a shock rise-time criteria which requires that the shock-energy deposition rate at the surface of the particles be higher than the heat conduction rate into the interior of the particles, so as to reach the melting temperature at the particle surfaces. Staver [3] argued that the compressive shock duration must be long enough to insure that the melted regions have solidified and strengthened before release occurs. In a following paper [14] we develop a quantitative treatment of this condition. For a more detailed and comprehensive discussion on the physics of shock consolidation, the reader is referred to the above mentioned papers. In the present study we have shock compacted rapidly solidified powders of AISI 9310 steel. The metallography, ultimate tensile strength (UTS), and microhardness of the compacts were studied as a function of initial shock pressures from 3.6 to 17.9 GPa. One of the goals of the present investigation was to determine the shock conditions required to produce very strong compacts. The initial shock pressure is an important parameter in the consolidation process since most of the energy deposition occurs during the passage of the initial shock front, which produces a large irreversible volume change. Other parameters which may affect the final characteristics of the compact are the initial powder

OF STEEL POWDER

density and particle size, shock duration, release shock waves, dynamic plastic behavior, thermal properties of the material, and oxides on the particle surfaces. In the present study, only the dependence of the consolidation process on the initial shock pressure was explored. All other independent parameters were held constant although some preliminary investigations on particle size and initial density dependence are presented. 2. EXPERIMENTAL. Rapidly solidified powders (RSP) of AISI 9310 alloy which passed through 200 mesh (74pm), but was retained in 325 mesh (44 pm) screens, were used for most of the consolidation work reported here. These iron-based powders were provided by United Technology Research Center and had 3.2 Ni, 1.39 Cr, 0.65 Mn, 0.24 MO, and 0.10 C % by weight [l5]. The powders were produced by a centrifugal atomization process and were mostly spherical, as shown in Fig. 1. Optical and X-ray metallography studies [I51 revealed that individual particles have little variation in microstructure and composition. The major constituent phase was bee with a lattice constant of 2.86 A. A small amount of fee structure, with a lattice constant of 3.58 A, was also detected. The diamond pyramid hardness (DPH) of the individual particles was 344 f 34. The powders were stored at ambient conditions and no attempt was made to remove surface oxides prior to shock consolidation. In order to shock compact powder samples of well-defined initial density and shape, cylindrical powder charges were statically compressed inside the shock recovery assemblies into discs. Typically these discs were 15.2 mm in diameter and 1.3 mm in thickness, weighed 1 g, and had densities of 4.79+0.10g/cm3 (61% of the bulk alloy density of 7.90 g/cm3). Figure 2 shows a schematic cross-section

Fig. 1. SEM micrograph of AISI 9310 rapidly solidified powder particles (-200 + 325 mesh).

KASIRAJ er al.:

SHOCK CONSOLIDATION

AX IAL SPALL PLATE Fig. 2.

Cross-section of the target recovery assembly used in the present shock compaction experiments.

of a shock recovery assembly. The powder is contained inside a steel washer and sandwiched between a 76pm thick copper cover and a steel base. It was found that this arrangement facilitates the recovery and easy removal of compacted samples. Conventional fly-away momentum traps were used to absorb much of shock energy after its passage through the powder. A 20 mm propellant gun was used to launch discshaped flyer plates, 14.8 mm in diameter, to speeds of 0.89 to 1.72 rnrn/ps. Most flyers were made from 304 stainless steel. Flyers made of 2024 Al and of Lexan were also used to obtain lower shock pressures. The flyer velocity before impact was measured to 2% accuracy by timing the interruption of two laser beams set 57.8 mm apart in the path of the projectile. Prior to shock consolidation, the gun and recovery assemblies were evacuated to 20 mtorr. The initial shock pressure through the porous sample, and the subsequent maximum shock pressure state achieved via multiple reflection from the supporting base, were calculated by the impedence match method, as described in Appendix A. In order to correlate the mechanical properties of the compacts with the planar shock pressures calculated in Appendix A, it is desired that the consolidation be as homogenous and one-dimensional as possible. To achieve this, typical aspect ratios (diameter/thickness) of 12 were used. Unavoidable edge effects become important in the central region of the compact when waves arrive from these edges before complete consolidation has occurred. The compacts produced were large enough so that several small tensile test specimens could be made from the central region of the recovered discs. The duration of the compressive shock state is proportional to the flyer thickness (Appendix A). Most of our flyers were 5.1 mm thick, giving a flyer-to-sample thickness ratio of 4. The typical shocked state in the central portion of the samples lasted from 2.7 ps (at the front surface) to 2.1 ps (at the back surface). Furthermore, the maximum shock presure attained was the same throughout this central portion (see Appendix A). A.M.

32/8-G

OF STEEL POWDER

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The recovered samples were sectioned for density measurements, metallographic examination, DPH measurements, and the preparation of tensile test samples. The density of the compact was determined from weight measurements in air and toluene. The sections for tensile test samples were mechanically polished to a thickness of about l/4 mm using wet SIC paper and diamond paste through 1 pm grit. Dog-bone shaped tensile samples, with a gauge section 2mm long and 0.7 mm wide were spark machined from the polished sections. These were pulled to failure in an Instron Universal Testing Machine at a strain rate of approximately 4 x 10W3/sutilizing a tensile system designed to minimize sample bending. Because shock compaction deforms the powder particles anisotropically (see Fig. 1 in Ref. [14]), the UTS may also be anisotropic. However, in the present study the tensile axes were parallel to the impact face. The fracture surfaces of the tensile specimens were then examined by scanning electron microscopy. 3. RESULTS Table 1 is a summary of the data from our present investigation. The shock pressures and velocity were calculated as explained in Appendix A. The particle size quoted in column 2 corresponds to the mesh sizes used to separate the powder. However, as seen in Fig. 1, the powder also contains particles smaller than the lower value quoted. These are attached to the larger particles, presumably during the centrifugal atomization process. With the exception of shots 719,725, and 729, the powders had a nominal particle size between 44 and 74pm. This size range was used to study the dependence of mechanical properties of the compacts on initial shock pressure. Shots 719, 725, and 729 are preliminary experiments to investigate particle size and geometry dependence. The final densities of samples 719, 733, 734, and 754, were 7.84 + 0.15, 7.84 + 0.08, 7.68 + 0.09, 7.86 f 0.28 g/cm’, respectively. Therefore, within the uncertainty of the density measurements, these samples reached the bulk alloy density of 7.90g/cm3. Because these compacts were consolidated at the lowest shock pressures, for all shock conditions used in the present study we achieved full density. This conclusion was further collaborated by the metallographic observations which revealed void-free surfaces on all of our compacts. Photomicrographs of etched sections of consolidated samples, cut parallel to the impact face, are shown in Fig. 3. In our consolidations of AISI 9310 RSP, the regions which presumably melted are not delineated by the etchant. Thus we were not able to estimate the amount of melt from the photomicrographs as had been previously done in studies of tool steel powders [6, 131. However, the figure provides what we interpret as indirect evidence of interparticle melting. Figure 3 (a) shows a sample consolidated by an initial shock pressure of 5.9 GPa

KASIRAJ

1238

ef al.:

SHOCK CONSOLIDATION

OF STEEL POWDER

Table 1. Shock consolidation data for AISI 9310 powders. Data in columns 5. 6 and 7 are calculated

Distention Particle Shot

number 713 719 725’ 727 728 729 730 732 733b 734 754*

size

Olm) 44-74 74300 74-300 44-74 4474 <36 44-74 44-74 44-14 44-74 44-74

-PlDlid PillW

@VP

9

1.64f 0.03 1.72 1.72 1.64

0.96 f 0.02 0.96 1.29 I.21

I.64 I.64

I .37 I.19 1.60 1.72 I .08 1.22 1.29

:z I:64 1.64 I.41

Initial

Maximum

pfCSS.lMC WW

pressure (GW

(mmh

7.0 f 0.7 7.0 11.2

19&-2 19 13

1.8kO.2 2.0 2.5

10.1 12.4 9.8 16.0 17.9 3.9 10.2 3.6

25 29 24 34 37 13 25 6

:::

Flyer velocity

Shock velocity

2.2 2.8 2.9 1.7 2.2 IS

s)

DPH 442f13 445f24 433f 25

UTS W’s) 0.76f 0.09 0.61f 0.09 I .02

510*20 S02f 16 475f II 482 f IS 500*11 413 *26 44lklO 393 f 50

I.18 kO.02 1.30*0.14 I.02 1.29 0.81 0.34 1.12 e

‘Shot 725 used a 2.5 mm thick flyer. Shot 754 used a 18.4mm thick flyer. All other shots used 5.1 mm thick flyers. “Shot 733 used a 2024 Al flyer. Shot 754 used a Lexon flyer. All other shots used 304 stainless steel flyers. Uot measured due to insufficient cohesiveness in recovered compact.

where the boundaries between individual particles are easily discernible by the preferential etching. In the 12.4 GPa consolidation shown in Fig. 3 (b), most of the particle boundaries are difficult to resolve. Figure 3 (c) shows the sample consolidated at the highest initial pressure of 17.9 GPa. Here no particle boundaries are resolved. We attribute the preferential etching which reveals particle boundaries to oxides and other impurities on the particle surfaces. The disappearance of this preferential etching indicates that the oxides are more thoroughly broken and mixed with increasing pressure and shock energy input. Breakup of the oxide films on the particle surfaces appears to be a requisite for strong interparticle bonding. The typical hardness of the wrought 93 10 steel [16] equals that of the RSP powder which has a DPH of 340. The consolidated samples are even harder than the initial powder. Figure 4 shows the dependence of DPH on the maximum shock pressure. The mean of at least 10 DPH measurements on each sample is plotted, and the bars denote the standard deviation in the measurements. The hardness increases with maximum shock pressure up to 20GPa, where it reaches a saturation value of 500. Saturation hardening has also been reported by Raybould [6] in 304 stainless steel powders, but for pressure less than 2 GPa. Without detailed knowledge of the hardening

mechanisms, one cannot say how much of the resultant hardening is influenced by the shockpressure/time-history of the powder. In order to obtain hardness values which minimize surface hardening effects from the mechanical polishing, 500 g loads were used whenever possible. However, when small indentation sizes were required, the hardness was determined with 100 g loads. This load was used to measure the DPH of the powder itself, and that of sample 754, where the 500g load was observed to push-out particles from the poorly bonded compact. In cases where both 500 and 100 g loads were used, the former hardness readings were lower by about 10%. The 500 g indentations produce diagonals of ap proximately 43 pm and penetration depths of 6 pm. Since the particle sizes are in the 44-74 pm range, our measurements average out possible differences in hardness between particle interiors and boundaries, which have been noted by other investigators [6,13]. Nevertheless, our measurements give the average bulk hardness which allows us to estimate the intrinsic strength of the shocked material using the simple linear relationship between hardness and UTS observed in wrought steel [17]. The UTS of the compacts vs the initial shock pressure is shown in Fig. 5. In contrast to the DPH

(b)

Fig. 3. Photomicrographs

of compacted samples with initial shock pressures of: (a) 5.9 GPa, (b) 12.4 GPa, and (c) 17.9 GPa. Etchant = 5% nital, room temperature.

KASIRAJ et al.:

SHOCK CONSOLIDATION

c

AISI

700

0

0

4

6

12

MAXIMUM

16

20

SHOCK

24

28

PRESSURE

32 IGPa)

Fig. 4. The dependence of the diamond pyramid hardness on the maximum shock pressure experieliced by the compact.

values, also shown in this figure, the UTS has a strong dependence on initial shock pressure. Below 4GPa the compacts have negligible strength. With increasing shock pressure the compacts become progressively stmnger, until a maximum UTS of 1.3 GPa is reached at a pressure of 12 GPa. The maximum strength is larger than the 1.2 GPa strength of wrought 9310 [16]. The strength decreases at our highest consolidation pressure. Possible explanations of the strength decrease will be discussed later in the

Fig. 6. Low magnification

la1

2

9310 ~owdar

25

1

4

INITIAL

36

1239

OF STEEL POWDER

6

8

SHOCK

10

12

PRESSURE

14

16

16

20

t GPol

Fig. 5. Dependence of the diamond pyramid hardness and ultimate tensile strength on the initial shock pressure for shock consolidations of AISI 9310 powders with initial distension of 1Xi4and shock duration of 2-3 p s. The dashed line represents equation (17) from Ref. [14]. paper. The dashed line plotted in Fig. 5 represents equation (17) in Ref. [14]. SEM scans of the fractured surfaces of three tensile test specimens are shown in Fig. 6. This series of

scans show that, as the shock pressure is increased, the nature of the fracture changes from predominantly interparticle to intraparticle. At the lowest pressure [Fig. 6 (a)] the fracture surface shows mostly protrusions which particle-like disappear progressively at higher pressures [Fig. 6 (b) and (c)l.

SEM fractographs of compacted samples with initial shock pressures of: (a) 5.9 GPa, (b) 12.4 GPa, and (c) 17.9 GPa.

lb1

ICI

Fig. 7. Normal SEM views of fractured surfaces from compacted samples with initial shock pressures of: (a) 5.9 GPa, (b) 12.4 GPa, and (c) 17.9 GPa.

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KASIRAJ et a/.:

SHOCK CONSOLIDATION OF STEEL POWDER

At the highest shock pressure the fracture is much smoother although some particle boundaries are still discernible. Figure 6 (c) also shows a crack, oriented nearly parallel to the tensile axis, which we believe existed before the tensile test. Normal views of the same three fractured surfaces at higher magnification show that the area exhibiting ductile fracture increases with increasing shock pressure (see Fig. 7). 4. DISCUSSION

To a good approximation the total energy per unit mass deposited by a shock wave of amplitude P, traveling through a porous sample is P( Vi - k’,)/2, where V, and V, are the initial and tinal specific volumes of the compact, respectively [12]. Since in our experiments the initial shock produces full densifi~tion of the powder (see Appendix A), most of this energy is deposited by the initial shock and little energy is added by subsequent shocks [14]. Because the strength of the compact is determined mainly by the interparticle welding, which in turn depends on the amount of energy input by the shock, a high degree of correlation should exist between UTS and initial shock pressure for a constant initial powder density. Figure 5 shows that the DPH and UTS of the compacts have a different dependence on initial shock pressure. The relatively small change in microhardness indicates that the intrinsic strength of the RSP particles does not increase si~ifi~ntly with increasing shock pressure. Whether thii is due to a saturation in dislocation density or in microstructural refinement of the compact may require TEM work, and is beyond the scope of present study. Contrary to the weak dependence of the DPH on shock pressure, the UTS of the compacts increases drastically with pressure. This increase correlates qualitatively with the fractographi~ observations and indicates that the completeness of the bonding between particles increases with pressure. In Fig. 5 the ordinate scales for the DPH and UTS measurements were adjusted so as to correspond to the linear relationship found in steels 1171.The fact that the UTS values are lower than the corresponding DPH strength is in agreement with the fractographs, indicating that the UTS achieved correlates well with the corresponding hardness value only when the interparticle bonding is complete. The maximum compact strength was within SO-90% of the strength of the shock hardened powder material expected from the standard relationship between DPH and UTS. The decrease in strength of the compact at our highest shock pressure of 17.9GPa was due to the existence of a crack in a plane nearly parallef to the tensile axis [see Fig. 6 (c)l. The failed surface, otherwise, indicated very good interparticle bonding. If the crack did not exist, one would expect from Fig. 5 a UTS of 1.3 GPa. Because unavoidable rarefraction waves are present in practical shock recovery pro_

cesses, recovery problems, such as cracking, have been predicted [3, 14) at high pressures when the compressive shock duration is less than the time required for all molten regions to solidify and cool slightly. However, further experiments are required to determine whether the cracks in our 17.9 GPa test were caused by an insufficient shock duration or by rarefraction waves exceeding the strength of the consolidated sample. Since photomicrographs and SEM observations of fracture surfaces indicate that the bonding is more thorough in the 17.9 GPa sample (which produced our strongest tensile specimen) we expect the strength of our recovered samples may be further increased to 1.6 GPa by increasing the shock duration at the higher pressure consolidations and/or reducing the magnitude of tensile waves. 5. SUMMARY AND CONCLUSIONS Rapidly solidified 93 10 powders have been consolidated with shock waves having initial shock pressures from 3.6 to 17.9 GPa and maximum shock pressures from 6 to 37 GPa, respectively. The powders had initial densities between 0.58 and 0.76 of that of the solid. The main results of the study are: (a) The density of the compacts is equal to that’of the solid (within the experimental error of 1 to 3%). (b) The microhardness of the compacts increases graduaily with shock pressure. (c) The ultimate tensile strength, UTS, of the compact is negligible below initial shock pressures of 4GPa. For increasing shock pressures above this threshold the strength rises rapidly, reaches a broad m~imum at the initial shock pressure of 12.4GPa, and decreases (for shock durations of 2-3 11s). The dependence of UTS on initial shock pressure is explained in a companion paper [14]. (d) The maximum obtained strength of I.3 f 0.1 GPa correlates reasonably well with microhardness and is greater than the UTS of wrought 9310. (e) The microhardness tests alone are insufficient to determine the effectiveness of interparticle bonding in compacts. Nonetheless, when combined with UTS measurements they can be used to determine if complete bonding has been achieved, since the compact cannot be stronger than the particle strength deduced from the hardness. Acknowledgements-Research supported by United Technologies Research Center and Defense Advanced Projects Agency through the U.S. Army Materials and Mechanics Research Center. R. B. Schwan is partially supported by the U.S. Department of Energy. Contribution NO. 3919. We are grateful to Dan Kostka and Mary Jane Bartholomew for their contributions to the experimental work. REFERENCES I. D. Raybould, Inj. J. Powder MetalI. Powder Tech. 16, 9 (1980). 2. D. Raybould. D. G. Morris and G. A. Cooper, J. Mater. Sci. 14, 2523 (1979). 3. A. M. Staver, in Proc. Int. Co& Shuck Wares and High-Strain-Rate Phenomena in Metals. (Edited by M.

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SHOCK CONSOLIDATION

A. Meyers and L. E. Murr), p. 865. Plenum, New York (1981). 4. C. L. Hoenig and C. Yost, Bull. Am. Ceramic Sot. 60, 1175 (1981). 5. L. E. Murr, S. M. Tuominen. A. W. Hare and S. H. Wang, Mater. Sci. Engng 57, 107 (1983). 6. D. Raybould, J. Mafer. Sci. 16, 589 (1981). 7. D. S. Witkowsky and H. Otto, Proc. 4th Inr. Con/. on High Energy Rate Fabrication, Vail, CO p. 931 (1973). 8. M. A. Meyers, B. B. Gupta and L. E. Murr, J. Metals 33, 21 (1981). 9. R. A. Pruemmen and A. Ziegler, Powder Metall. Int. 9, 11 (1977). 10. E. W. LaRocca and J. Pearson, Rev. Sci. Inrlrum. 29, 848 (1958). 11. D. Raybould, in Proc. 15th Int. Mach. Tool Design and Res. Con/. (Edited by S. A. Tobias and F. KOengsberger), p. 627. MacMillan, New York (1975). 12. D. G. Morris, Metal Sci. 14, 216 (1980). 13. D. G. Morris, Metal Sci. 15, 116 (1981). 14. R. B. Schwarz, P. Kasiraj, T. Vreeland Jr and T. J. Aluens, Acra metall. 32, 1243 (1984). 15. F. D. Lemkey. private communication. 16. Metals Properties Handbook, 1st edn, p. 287. Am. Sot. Metals, McGraw-Hill, New York (1954). 17. Metals Handbook, 8th edn, Vol. 1, p. 1234. Am. Sot. Metas, Met&s Park, OH (1961). 18. B. M. Butcher and C. H. Kames, J. appl. Phys. 40.2967

OF STEEL POWDER

30

c

25

-

20

-

; 1

75 -

E F

lo05

1241

-

Fig. A2. Time-position histogram followed during the shock compaction in a typical experiment. The segmented lines denote the boundaries of the flyer, powder, and base. The encircled numbers denote pressure states which are also shown in Fig. A3.

(1969).

19. G. A. Siions and H. H. Legner, J. appl. Phys. 53,943 (1982).

20. J. M. Walsh, M. H. Rice, R. G. McQueen and F. L. Yarger, Phyr. Rev. 108, 196 (1957). 21. R. Kinslow, High Velociry Impact Phenomena, p. 524. Academic Press, New York (1970). 2’. To be published. See also R. B. Schwara, P. Kasiraj, T. V&and Jr and T. J. Ahrens, Bull. Am. Phys. Sot. 28, 460 (1983).

23. R. G. McQueen, S. P. Marsh, J. W. Taylor, J. N. Fritz and W. J. Carter, in High Velocity Impact Phenomena (edited by R. Kinslow), p. 293. Academic Press, New York (1970). APPENDIX

A

Shock-wave data for iron powder (181 was fit with the analytical expression derived by Simons and Legner [ 191for the Hugoniots of porous materials, as shown in Fig. Al. This analytical expression was then used to describe the properties of our 9310 steel powder. The use of the Hugoniot for iron to describe that of iron-based alloys (e.g. 9310 ahoy) is supported by (a) the observation that the Hugoniots of solid iron and stainless steel are very similar in the pressure ranges of our experiment, as shown in Fig. Al, and

164

0

02

04

06

06

10

12

14

16

J 16

20

VP (km/s)

Fig. 1A. Shock Hugoniots for pure Fe (Ref. [ZO]). 304 stainless steel (Ref. (21]), and porous iron (points from Ref. (I 81; curves from Ref. [19]) used in calculations.

Fig. A3. States of pressure versus particle velocity encountered during the shock compaction in the typical experiment. (b) direct Hugoniot measurement made on porous AISI 9310 samples [22]. The evolution of the shock pressure during the compaction of the powder was calculated by the impedancematching method [23]. In this method, the continuity boundary conditions of pressure and particle velocity at the impact face are used, together with the known shock Hugoniots of the flyer material and target sample and the measured projectile velocity, to determine the initial shocked state. In turn, the velocity of the initial shock wave is calculated from the known initial density of the powder, the powder Hugoniot, and the initial shock pressure using conservation laws for the propagation of a steady shock front [23]. Figures A2 and A3 show the time vs position and pressure versus particle velocity during the ideal shock compaction of an iron-based powder of distention 1.64 held in contact with a steel base and hit at time zero by a steel flyer at a speed of 1.2mm/ps. The flyer thickness is here approximately five times the thickness of the unshocked powder (Fig. A2). The Hugoniot of the compacted powder, used to calculate states 2, 3, and 0, has been assumed equal to that of steel so that, after the initial compaction, the flyer-powder and the powder-base interfaces can be treated as having the same shock impedance. This assumption involves two approximations: (a) the compact has zero porosity and (b) the heating effects on the Hugoniot of the compact are neglected. The first assumption is valid for an initial shock pressure above 2.6 GPa [18]. The error introduced by the second assumption cannot be calculated accurately because the temperature in the compacted powder is very inhomogeneous.