Hugoniot of a Reactive Metal Powder Mixture

C.H. Braithwaite, D.J. Chapman, P.D. Church, P.J. Gould, and D.M. Williamson

1 Introduction

The shock behaviour of reactive metal powders is an area of significant current re-search. These materials can be compressed under quasi-static loading to form a solidpowder compacts that can hold their form. This is possible due to the permanentplastic deformation of at least one of the metal species involved. The compressedmaterials can then be used as structural components of various larger systems. Ifthe correct choice of powdered materials is made then under suitable loading con-ditions, there is the potential for an exothermic intermetallic reaction. This abilityto have components which are inert under the vast majority of loading conditions,but which can be induced to give out energy under specific conditions is very usefulfor example in the design of shell cases [1]. While the reactivity of these materi-als is critical to the design of systems utilising this technology, an understandingof the material properties in the unreacted state is also important. This is both be-cause knowing the stress levels reached can be key to understanding the reactivebehaviour, and because the materials are often intended to be used structurally. Thispaper examines the measurement of one aspect, the Hugoniot, of the mechanicalperformance of a specific powder compact. The powder compact was a 75% TMDnickel aluminium mixture in a 1:1 stoichiometric ratio.

2 Materials

The nickel and the aluminium powders used in this series of experiments were stockmaterials supplied by Alfa Aesar. The aluminium was described as spherical 10-14

C.H. Braithwaite · D.J. Chapman · P.D. Church · P.J. Gould · D.M. WilliamsonSMF Group, Cavendish Laboratory, J.J. Thomson Av., Cambridge, CB3 0HE · Institute ofShock Physics, Royal School of Mines, Imperial College, London, SW7 2AZ · QinetiQ, FortHalstead, Sevenoaks, Kent · QinetiQ, Bristol, BS16 1FJ

154 C.H. Braithwaite et al.

micron average particle size and the nickel was described as spherical 5-15 micronaverage particle size. Verification of particle size was determined by experimentscarried out at the Cambridge University Geography Department using a MalvernMatersizer 2000. The results are shown in figure 1, with the aluminium results onthe left and the nickel on the right.

Fig. 1 Particle size analysis for the aluminium (left) and nickel (right) powders used in thisinvestigation

Figure 1 shows that the aluminium does fall into the bounds described by thesupplier in terms of particle size. In the right of figure 1, which refers to the nickelit can be seen that the distribution is bimodal. This should not however be takenas evidence that the supplied information is incorrect, it is merely indicative of theclumping that tended to occur in the nickel powder.

The powders for the experiments were mixed in a 1:1 stoichiometric ratio. Thepowders were weighed out to an accuracy of ± 0.001 g and subsequently agitatedfor at least 12 hours in a mixing machine that used both rotary motion and tumblingas mixing methods. The mixed powders were pressed to the required density (75%of the theoretical maximum density) using a steel piston and dye in a hydraulicpress. Note that the percentage of the TMD that is referred to is 5.16 g cm−3 forthe 1:1 molar mixture. There is likely to be an oxide layer present on the surface ofthe particles, although this will not contribute significantly to the predicted TMD, itmay however, influence the reactivity of the material.

A number of SEM images were obtained of the raw powders to examine the mor-phology. An example of these for both the aluminium (left) and the nickel (right) canbe seen in figure 2. The figure shows that while the powders are broadly spherical,there are some instances of elongated grains. In addition to the raw powders, SEMimages were also produced which show a section through one of the pressed powdercompacts. This is shown in figure 3, and it is clear that the aluminium (which is thedarker phase) has deformed around the nickel. There is significant voiding and thegrain boundaries are, in the main, still visible.

Hugoniot of a Reactive Metal Powder Mixture 155

Fig. 2 SEM images of the raw aluminium (left) and nickel (right) powders used in thisinvestigation

Fig. 3 SEM image showing a pressed powder compact at 75% TMD

3 Experimental Methods

The experiments in this paper were all performed on the plate impact facility at theCavendish Laboratory, Cambridge. The facility consists of a single stage light gasgun ([3]) using either air or helium as a propellant (to a pressure of up to 350 Bar)and is capable of velocities of up to 1100 m s−1. The barrel has a length of 5 m anda bore of 50 mm. Velocity is measured (± 0.5 %) by a series of sequentially shortedpins. At the end of the barrel an alignment system allows for samples to be alignedto an accuracy better than 1 mrad, although the resulting impact tilt (which arisesfrom a combination of effects) is likely to be slightly higher, in the region of 2 mrad.

In material stress was measured using piezoresistive manganin gauges which arecommercially available from Micromeasurements. This type of gauge is used ex-tensively in the shock physics community. In order to embed the gauges withinthe target, they are encapsulated in a low viscosity two part epoxy resin. Gauges

156 C.H. Braithwaite et al.

Fig. 4 A schematic of the equation of state experimental set-up

are insulated by sheets of 25 μm Teflon and the samples are pressed to ensure anabsence of air bubbles. Overall, a gauge package has a typical thickness of 200 μm.In operation, the gauges need to be connected to a constant voltage power supply(which is pulsed to avoid gauge damage). This power supply is set up such thatthe gauge forms one leg of a Wheatstone bridge circuit. The bridge is balanced togive zero output, and subsequently the voltage output is calibrated by connectingthe bridge to a succession of resistances in series. In order to calculate the stressfrom the voltage-time trace, it is necessary to use a calibration, such as the one dueto Rosenberg [4].

A diagram of the main experimental design for the experiments is shown in figure4. The data from the experiments give a measure of the shock velocity (via transittime between the two gauges) and a measure of stress at the interface between thematerials (impedance matching can be used to find other parameters). Four differenttypes of experiment were performed. Three are based on the diagram in figure 4 anddiffer in the materials used for the fliers and the anvils (copper flier with aluminiumanvils, copper flier with copper anvils and aluminium flier with aluminium anvils).The other experimental type was a reverse impact with a Ni/Al flier impacting a2mm copper plate (where the free surface velocity was monitored with a VISARsystem).

One point that should be made in regards to the copper flier/aluminium anvilexperiments is that owing to the interaction of the waves in the experiment, twopoints on the Hugoniot can potentially be determined as there is a release wave thatis reflected back towards the gauge from the flier/target interface (this release beinginitially generated at the Al and Ni/Al interface).

Hugoniot of a Reactive Metal Powder Mixture 157

4 Results

An example of the experimental data produced in this investigation is shown infigure 5. The shock velocity measurement can be made simply from examiningthe transit time which is clear in the figure. Additionally the release state in the

Fig. 5 A typical gauge trace from the investigation, showing a copper impactor impacting acopper anvil.

Fig. 6 Two representaions of the Hugoniot of the 75% pressed powder compacts, in σ −up

space (left) and Us −up space (right).

158 C.H. Braithwaite et al.

front anvil, which corresponds to the Hugoniot state of the powder compact, can bedetermined from the long plateau shown after the initial rise in the front gauge. Thematerial Hugoniot is shown below in both σ − up space (left) and Us − up space(right) in figure 6.The Ni/Al samples had a density of 75.6% ± 0.06 of the theoreti-cal maximum density. The errors in the shock speed have been determined from theuncertainties in sample thickness and time of arrival on the gauge traces. The errorsin the particle velocities and the stresses incorporate an error associated with thecalibration of the gauges, the uncertainty in the Hugoniots of the well-characterisedmaterials and the difficulties in determining the stress level from gauges traces(hence the proportionally larger error in the points determined from the transientstates).

The data for the Us−up space Hugoniot can be well a linear fit with values of co

and s of 0.26 km s−1 and 3.43 respectively.

5 Conclusion

The Hugoniot of a well characterised metal powder compact has been determinedthrough plate impact experiments. If fitted with a linear Hugoniot in Us − up spacethen values of co and s of 0.26 km s−1 and 3.43 respectively can be determined.

References

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2. Porter, D., Gould, P.J.: Predictive nonlinear constitutive relations in polymers through losshistory. International Journal of Solids and Structures 46, 1981–1989 (2009)

3. Bourne, N., Rosenberg, Z., Johnson, D., Field, J., Timbs, A., Flaxman, R.: Design andconstruction of the uk plate impact facility. Meas. Sci. Technol. 6, 1462 (1995)

4. Rosenberg, Z., Yaviz, D., Partom, Y.: Calibration of foil-like manganin gauges in planarshock wave experiments. J. Appl. Phys. 51, 3702–3705 (1980)