CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
INFLUENCE OF THE STRUCTURAL LEVELS ON THE ELASTICPLASTIC HARDENING OF METALS UNDER SUBMICROSECOND
SHOCK LOADING
Youry Sud'enkov
St. Petersburg State University, St. Petersburg, Russia
Abstract. As results of experimental investigation of elastic-plastic deformation of the metals (Cu,Ni,Fe,
aluminium alloy D16) with various grain size (20 -f- 200 (im) the violation of the Hall-Petch relation at
pulse duration of 80 ns was found. The increment of the Hugoniot's and microhardness under increment of
the grain size was observed. The reason of the such material behavior under shock loading was determined
by measurement of elastic waves velocities along two axes before loading and after it. The main
mechanism of plastic flow is rotational for specimen with grain of 20 jim and twinning for the coarsegrained specimen. In this way plastic flow is determined by various deformation structural scales,
depending on correlation of characteristic scales, were ~ 10 (im for the fine-grained specimen and
apparently ~ 10 nm for the coarse-grained specimen. Apparently the appearance of the rotational modes of
plastic flow is stipulated by the twinning process under high grain boundaries density in the fine grained
specimen. This determines the transition from one structural level of deformation to another one with an
essentially larger scale.
INTRODUCTION
radiation on thin absorbing layer in acoustic contact
with the sample on one side and an optical window
on the other. This arrangement permitted excitation
of pressure pulses up to 2 GPa. A laser apparatus
with A, = 1.06 jam and T = 25-10"9s was used. To
provide the flatness of shock wave, a diffusing
screen was placed in front of the irradiated zone.
The pressure pulses were measured by a quartz
sensor operating in the current generator mode. A
series of control measurements was made by laser
interferometer. The inaccuracy in pressure
measurements was smaller than 5% and the time
resolution was 3-10'3 sec.
The variation of initial structure was achieved by
vacuum annealing of the samples. The temperature
of annealing was from 50 to 1100°C. The samples
was held at this temperature for one hour, then they
were cooled down slowly (together with oven).
Studies of elastic-plastic process in metals under
short loading (10~7 s) have been very few in number
and fragmentary , and the influence of structure on
these processes has hardly been studied at all.
Subject of this work is a experimental studying of
plastic flow in metals under impact loading. The
objective was studying of affect of initial structure
of material on the mechanism of plastic flow. For
that we made following experiment. Different
metals (iron, nickel, copper, aluminum alloy D16)
was tested by sub-microsecond pulse loading. The
material parameters were measured and calculated.
OBJECTS AND EXPERIMENTAL METHODS
The shock loading was done by a plane wave of
duration 8-10~ 8 sec, created by the action of laser
627
These features, apparently, are due to changing
of the deformation mechanism that give the
government contribution to the metal mechanical
properties after deformation.
It is known [1,2] that the decrease of temperature
and the increase of deformation rate promote
twinning and cause the suppression of gliding. The
critical value of twinning stress is independent on
temperature. Twinning becomes easier essentially as
grain size increases.
Taking into account these factors, we can say, that
the main mechanism of plastic flow in large-grain
samples at loading shorter than 10~7 sec is twinning.
The critical stress of twins defines the <TH value and
the twins density defines the value of AH after
loading. It is confirmed by results of [3], where the
governing role in hardening of Cu - 8.7Ge alloy
under loading with duration in -7-10"8 sec was noted.
The decrease of a// and AH with decreasing of
grain size, when the twinning process is hampered,
is apparently due to the contribution of the intergrain
mechanism of plastic flow. Its intensity increases as
the volume density of grain boundaries increases.
The intergrain mechanism includes both intergrain
slipping and rotational processes [4, 5]. This
mechanism of plastic flow practically does not effect
on the metal hardening, what is observed in
experiment.
To determine the contribution of rotational
mechanism, we made the measurements of
longitudinal sound velocity in samples with different
grain sizes. The velocities in directions along and
across the direction of disturbance wave propagation
Grain size D, microhardness H, static stress — strain
diagram were measured.
RESULTS AND DISCUSSION
We tested the samples with different thickness from
two to ten millimeters. In that way we the estimated
the damping of elastic precursor (Fig. 1), and, hence
the value of the Hugoniot's elastic limit a H . Besides,
we found the velocities of elastic and plastic waves.
By these data we constructed the dynamic stress strain diagrams. We measured also microhardness H
before and after the loading, so we obtained the
hardening AH.
The changing of microhardness and Hugoniot's
limit reflects the contribution of microstructure,
generated by deformation, to the metals properties.
In quasi-static testing the changing of these
parameters is determined mainly by the resistance to
dislocation guide. So, the relation between grain
size D and Hugoniot's limits is described by Hall Fetch type relation:
a H =a 0 +K.D- 1/2
Here a0 is the strength of the matrix, K is coefficient
of proportionality.
This relation is fulfilled for dynamic testing also,
but for duration longer than ~10~6 sec [2]. At high
rate loading the situation changes.
On Figs. 2,3 the dependences of Hugoniot's elastic
limits and the hardening obtained in our experiments
are shown. The relation (1) does not held for either
of the plots. We have maximum of aH and AH at
maximal values of grain size.
GH,MPa
240
Shock
Stationary
I - initial structure
• - annealing at 250°C
o-annealing at 900°C
200-i
1-Fe
2-Ni
3-Al alloy
4-Cu
160
120800
2
4
6
FIGURE 1. Damping of elastic precursor
0,3^
8
.
X,rnrn
FIGURE 2. Dependence of Hugoniot's limits on grain size
628
12-
40
0,0
0,1
80
120
160
0,2
FIGURE 3. Dependence the hardening on grain size.
twinning process under high grain boundaries
density in fine grained specimens.
were measured. The inaccuracy of measurement was
smaller than 0.1%.
These data together with values of sound velocity
in crystal in direction of maximal anisotropy [6],
enabled us to estimate the changing of
crystallographic orientation of metal structure An.
(An - is the number of grains that changed their
crystallographic orientation). The dependence An
from D is shown on Fig. 4. An increases
substantially if grain size is less then the spatial
dimension of the shock loading front (about 40 jim
in our case).
D<c r i f
At this grain size the rotational mechanism of
plastic flow becomes the deciding mechanism in the
relaxation of pulse stress. It is confirmed by
practically the complete absence of shock hardening
in samples when D < 40 (im.
In conclusion we note, that in our opinion
observed features of the response of materials on
sub-microsecond loading are conditioned:
• first, by "freezing out" of the gliding
mechanism and by prevalent role of twinning
for plastic deformation, if the grain size is large
REFERENCES
1. Trefilov V.I., Moiseev V.F., Pechkovskii U.L.
et al, Strain Hardening and the Failure of
Poly crystalline Metals, (in Russian), Naukova
Dumka, Kiev, 1989
2. M.A. Meyers, L.E. Murr (eds.) Shock waves
and high-ate-strain phenomena in metals:
concepts and applications, Plenum Press, New
York,1981
3. LaRouche S., Mikkola D.E.,. Scripta Mtf.,12,
p.543-547, 1978
4. Meshcheryakov Yu.L, Atroshenko S.A. et al.
Preprint no. 57(in Russian), Leningrad Branch
of Institute for Mechanical Engineering USSR,
p. 45 , 1990
5. Disclinations and Rotational Deformation of
Solids. Proc. Phys. Tech. Inst. AS USSR
(1990).
6. Frantsevich I.N., Voronov F.F., Bakuta S.A.,
Elastic Constants and Moduli of Metals and
Nonmetals(in Russian), Naukova Dumka,
Kiev,1982
(D>C{-Tf)
•
second, by increasing of the efficiency of the
intergrain mechanism of plastic flow with the
main contribution of rotaries, if the grain size
is comparable with or less then spatial
dimension of shock loading front (D < c,-Tf).
• apparently the appearance of the rotational
modes of plastic flow is stipulated by the
629