Preyield Plastic Deformation in Copper Polycrystals

TL;DR: In this article, the dislocation loops generated by the first few active sources on both primary and secondary slip systems generally traversed the entire cross section of the grain and the external surface was found to be a preferential site for dislocation multiplication even when grown in segments.

Abstract: Preyield dislocation motion and multiplication were studied in large‐grained polycrystalline OFHC copper by the etch‐pit technique. Specimens were loaded in compression and dislocation arrangements were observed in both the stressed and unstressed conditions. The dislocation loops generated by the first few active sources on both primary and secondary slip systems generally traversed the entire cross section of the grain. The observations were consistent with the idea that jog density is an important factor in determining the mobility of a dislocation segment. The external surface was found to be a preferential site for dislocation multiplication even when grown in segments, where they cut the surface, were immobile, and stress concentrations were carefully avoided. It is suggested that the rapid increase in length per unit volume of moving dislocation line that is associated with macroscopic yield requires motion of dislocation segments of high jog density.

Summary

INTRODUCTION

• It is well known that high sensitivity strain measurements or dislocation.
• D1s lbt'at ion motion and multiplication . . 8 JIluch before'yielding.
• Copper ,single crystals with eXtremely low dislocation density.
• In the present investigation an attempt has been m~de to get some additional ~information on these aspects by studying, using the etch pit technique, dislocation behavior in the very early deformation stages of large grained copper polycrystals.

II. EXPERH1E1'TAL PROCEDURE

• Since dislocations in copper can be revealed by etching cmly vThen a 10'lT index ple.ne is parallel to the surface of observation special experimental techniques ;-rere needed to prepare specimens that had one or more surface grair1s in an etchable orientation.
• Since most reliable etchants have been devebped only for planes of the, [ill} type in copper,9 experiments .. {ere ai::ls:i at obtaining a specimen in ''i'lhich at least one, grain ';.:ould have a [Ill} plane pa:::-allel to the surface.
• \-;hen interior grains .lere to be exarrined after" defol'l!l2,ti0.
• Yi the removal of the necess~rily large amcl1.:nts of IT'.2.teyial . . -3- UCRL-18524 from the surface without introd~cing any mechanical damage was accdmplished 11 in a modified polishing apparatus using a solution of 1: 1 nitric acid followed by the chemical polishing solution . , ..

III. RESULTS AND DISCUSSION

• The rectystallized grains had a relatively high perfection with dislocation densities always less than 104jcm2 and frequently of the order of 10 3 /cm 2 Dislocations were quite uniformly distributed and there were usually no sub-boundaries.
• When a new surface is exposed by exten,sive polishing just before application of a load, as has been done in previous experiments, some of the most mobile dislocations with lo~ jog densities will also intersect the surface at small angles so as to be able to shorten their length by conservative motion.
• When a stress is app~ied some of these should start to mthe authors even at the smallest st:resses but multiplication should not take place.
• Atentati ve mechanism that can explain this observation is as follows.
• The tripole will be unstable unless the number of jogs on the initial segment was very small.

Full text

Lawrence Berkeley National Laboratory
Recent Work
Title
PRE-YIELD PLASTIC DEFORMATION IN COPPER POLYCRYSTALS
Permalink
https://escholarship.org/uc/item/67x0w0zd
Authors
Vellaikal, Gopinathan
Washburn, Jack.
Publication Date
1968-11-01
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PRE-YIELD PLASTIC DEFORMATION
IN COPPER
I?(}t;YC~YSTALS
Gopinathan Vellaikal and Jack Washburn
November 1968
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PRE-YIELD
PLASTIC
DEFORMATION
IN
COPPER
POLYCRYSTALS
Gopinethan
Vellaikal
'and
~ack
Washburn
Inorganic
,Materials
Research
Division,
Lawrence
Radiatioll
Laboratory,
Department
of
Mineral
Technology,
College
01'
Engineering,
oL,the
iUniversity
of
California,
Berkeley,
California
ABSTRACT'
Pre-yield
dislocation
motion
and
multiplication
were
studied
in
large
gr,~aned
polycrystalline
copper
by
the
etch
pit
technique.
Specimens
were
loade'd
in,
compression
and
dislocation
arrangements
were
observed
in
both
'~
,
the
stressed
and
unstressed
conditions.
The
dislocation
lpops
generated
by
the
first
few
active
sources
on
both
primary
and
secondary
slip
systems
'generally
t,raversed
th~':'entire
cross
section
of
the
grain.
The
observa-
tions
were
consistent
with
the
idea
that
,jog
density
is
an
important
factor
in
determining'
the
mobility
of
~
dislocation
segment.
The
external
surface
was
found
to
be
a
preferential
site
for
dislocation
multiplication
even
when grown
in
segments,
where
they
cut
the
surface,
were
inunobile
,
'and'stress
concentrations
were
carefully
avoided.
It
is
,suggested
that
the
rapid
in,crease
in
length
per
unit
volume
of
mov.;i.ng
dislocation
line
that
is
associated
with
macroscopic
yield
requires
motion
of
dislcc
ation
segments
of
high
jog
density.