\input texp.tex  \parskip=3mm
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\def\b{$\beta$}
\def\bp{$\beta'$}
\def\gp{$\gamma'$}
\def\um{$\mu$m}
\def\degg{$^\circ$C}
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\leftline{\large Communication}
\centerline{Refinement and Characterization of a Thermodynamically Stable}
\centerline{\b[Ni(Al,Ti)]--\bp[Ni$_2$AlTi]--\gp[Ni$_3$(Al,Ti)] Metal--Metal
Composite}
\bigskip
\centerline{J. D. Robson\footnote{$^{\dag}$}{\ninerm Now of the
 Manchester Materials Science Centre, Grosvenor Street, Manchester, M1 7HS.}
 and H. K. D. H. Bhadeshia}
\medskip
\parindent=0pt

\centerline{University of Cambridge}
\centerline{Department of Materials Science and Metallurgy}
\centerline{Pembroke Street, Cambridge CB2 3QZ, U.K.}
\bigskip

\x The intermetallic compounds which are under consideration for use in aerospace
applications
often suffer from either poor ambient temperature ductility or insufficient elevated
temperature strength.
One promising route for obtaining the desired properties is to combine various
intermetallic phases with
complementary properties. Several workers have achieved finely dispersed multiphase
intermetallic materials by directional solidification of ternary or quaternary
alloys [1--4]. A different production route was used by Hsuing and Bhadeshia to
create a multiphase Ni--Al--Ti alloy [5]. Three single phase powders were co--
extruded to give a three phase \gp + \b + \bp\ metal--metal composite (MeMeC). The
aim of the present work was to refine the structure of such a material and
investigate in more detail its mechanical properties. In particular, the variation of %%@
mechanical properties of the MeMeC material with temperature are reported here for %%@
the first time. 

Three powders were manufactured, each designed to consist of one of the three
constituent phases.
Following Hsuing and Bhadeshia, the composition of each powder was chosen to
correspond to the appropriate corner of
the equilibrium tie triangle containing the three phases.

\midinsert \thicksize=1pt \thinsize=0.8pt \tablewidth=6
truein
\begintable
{Phase} & Ni & Al & Ti \cr
\gp & 72.7 (70.8) & 12.9 (13.5) & 14.4 (15.7) \nr
\b & 56.5 (56.1) & 35.9 (36.3) & 7.6 (7.6) \nr
\bp & 53.2 (52.7) & 25.1 (25.8) & 21.7 (21.5)
\endtable
\tabtit {\tablaa : }{Compositions (in atomic\% ) of the three corners of the
900\degg\ \gp, \b, \bp\ tie
triangle [6]. The actual compositions of the three alloy powders are shown in
brackets. }
\endinsert

The \b\ and \bp\ powders were prepared by mechanically crushing cast ingots which
proved more economical and less problematic than the gas atomisation route used by
Hsuing and Bhadeshia. \gp\ having a higher ductility is not easily pulverised in
this way and therefore the \gp\ powder was produced using argon gas atomisation. The
mean size of the \b\ and \bp\ powder particles was 50$\,\mu$m, the mean size of the
\gp\ particles was 100$\,\mu$m . The powders used here are considerably finer than
those used by Hsuing and Bhadeshia which had a minimum size of 150\um . The powders
were analysed using X--ray diffraction which revealed that each case small
quantities of secondary phases were also present (e.g. \gp\ was present in both the
\b\ and \bp\ powders).

In previous studies [5], two variants, MeMeC~B (50\gp--25\b--25\bp wt\%) and MeMeC~C
(70\gp--15\b--15\bp wt\%) were manufactured. Preliminary tests showed that
in both materials some of the strength could be usefully sacrificed to obtain
increased ductility. In an
attempt to realise this, the quantity of the more ductile \gp\ phase was increased
to
80$\,$wt\%, with the \b\ and \bp\ fractions reduced to 10$\,$wt\% each. For
consistency with the previous work
this variant is identified as MeMeC~D. The three powders were therefore mixed in
these proportions and co-extruded. A set of trials was conducted which found the %%@
optimum extrusion conditions to be a preheat to 1170\degg\ for 70 minutes followed by %%@
extrusion with a reduction ratio of 16:1 at a ram speed of 25$\,$mm{$\,$s$^{-1}$} %%@
using double E-glass and fibre glass lubricant. The
microstructure of as--extruded MeMeC~D is shown in \fagg\ and it can be seen that
the desired intimate dispersion of the three constituent phases has been achieved.
Detailed microscopy revealed the presence of secondary phases in the nominally
single phase regions. In particular \bp\ precipitates were observed in the \gp\
regions. Heat treatments were performed in the temperature range 700\degg\ to
1000\degg\ for times up to 1000~h followed by microscopy. The results were
consistent with previous work [5] confirming the as--extruded structure has a high
degree of stability.

\midinsert
\vskip 2.5truein
\tabtit{\figg\ : }{Optical micrograph showing the structure of as--extruded
MeMeC~D.}
\endinsert

Compression tests were performed using a 50$\,$kN servohydraulic testing machine on
rectangular blocks 2.5$\,$mm $\times$
2.5$\,$mm $\times$ 6.0$\,$mm cut from the extruded bar in either a longitudinal or
transverse orientation.  Interrupted tests were also performed where samples were
compressed to 90\% of the failure strain and then examined. In these samples cracks
were observed, the majority of which lay along the interfaces between the nominally
single phase regions. In an attempt to strengthen the interfaces several samples
were heated at 900\degg\ for 24~h. It was found that this pre-treatment did not make
any significant difference to the results obtained on compression testing. An
optical micrograph showing a typical crack in a sample from an interrupted test is
given in \fagg\ (this sample had been heat treated prior to testing). The crack has
propagated along an interface between the \b\ and \gp\ phases. It is interesting to
note that the crack leaves the interface at one point and instead seems to take a
path following a line of closely spaced \bp\ precipitates in the \gp\ matrix. When
this line stops the crack reverts to propagation along the interface. In this
example the crack can be seen to reach the end of the \b\ region and can then only
continue by passing through the \gp . When this happens it can be seen that the
crack path becomes much more tortuous, jumping between \bp\ precipitates. These
observations indicate that the presence of brittle \bp\ precipitates in the \gp\
matrix may be detrimental to toughness by providing an easier crack path through
this phase.

\midinsert
\vskip 2.5truein
\tabtit{\figg\ : }{Optical micrograph showing a crack observed after an interrupted
room temperature compression test. The compression axis is horizontal.}
\endinsert

\x Elevated temperature compression tests were then performed at temperatures up to
800\degg\ on both MeMeC~C and D. The 0.2\% compressive proof stress and compressive
plastic strain at failure are shown for MeMeC~C (70\% \gp ) and MeMeC~D (80\% \gp )
in \fagg . It can be seen that a constant proof stress is maintained up to
approximately 500\degg\ above which the strength falls. The fall in proof stress
above 500\degg\ is accompanied by a significant progressive increase in the plastic
strain to failure. Above
650\degg\ failure had not occurred after 30\% strain, by which time the sample had
barrelled significantly. It is also interesting to note that despite the high \gp\
content of MeMeC~D (80$\,$wt\%) the
characteristic peak in strength of \gp\ is not detected in the MeMeC material.
Comparing the two variants, little difference is observed between the
strains to failure and 0.2\% compressive proof stress in MeMeC C and D despite
microstructural and constitutional differences. The proof stress of MeMeC~D appears
to fall more rapidy than that of MeMeC~C between 600 and 800\degg . This may be a
result of the higher level of \gp\ in MeMeC~D, since above the peak in \gp\ strength
(at around 700\degg\ [7]) the strength of this phase falls more rapidly than that of
\b\ and \bp\ [8,9] (at least until approximately 950\degg).

\midinsert
\vskip 3truein
\special{psfile='papic/mechprop.eps' hoffset=0 voffset=0 hscale=53 vscale=53}
\tabtit{\figg\ : }{(a) Variation of 0.2\% proof stress and (b) mean plastic strain
to failure for MeMeC~C and D.}
\endinsert

In summary, it has been demonstrated that a three phase intermetallic MeMeC material
has the potential to offer a combination of high strength along with considerably
greater compressive failure strain than monolithic \b\ or \bp\ intermetallics [8,9].
This high strength is maintained at an approximately constant level up to around
600\degg , above which temperature the strength falls, more sharply in the MeMeC
variant which has a greater fraction of \gp . Studies have shown that the interfaces
between nominally single phase regions provide lines of weakness along which cracks
propagate during compressive loading. This is in contrast to the strong interfaces
observed in a directionally solidified \b\ + ($\gamma$ + \gp ) alloy [1] which also
has a well defined Kurdjumov-Sachs orientation relationship between the brittle and
ductile phases. The strong interfaces and well defined orientation relationship were
identified as important in facilitating slip transfer and enhancing ductility in
this alloy. Strengthening the interfaces in the MeMeC material and promoting slip
transfer might therefore be expected to improve its mechanical properties.
Preliminary attempts to strengthen the interfaces by a 24~h heat treatment at
900\degg\ have been unsuccessful. This suggests that either a more severe heat
treatment is required or another strategy must be employed.

{\parindent=10pt \narrower
\def\ref#1#2#3#4#5#6{\item{#1. } #2: {#3} { #4}
{(#5) #6} }

\input bibed2.txt

\vfill\eject

\midinsert
\tabtit {Table 1 : }{Compositions (in atomic\% ) of the three corners of the
900\degg\ \gp, \b, \bp\ tie
triangle [6]. The actual compositions of the three alloy powders are shown in
brackets. }
\endinsert
\bigskip
\midinsert
\tabtit {Table 2 : }{The optimum extrusion conditions.}
\endinsert
\bigskip
\midinsert
\tabtit{Figure 1 : }{Optical micrograph showing the structure of as--extruded
MeMeC~D}
\endinsert
\bigskip
\midinsert
\tabtit{Figure 2 : }{Optical micrograph showing a crack observed after an interupted
room temperature compression test. The compression axis is horizontal.}
\endinsert
\bigskip
\midinsert
\tabtit{Figure 3 : }{(a) Variation of 0.2\% proof stress and (b) mean plastic strain
to failure for MeMeC~C and D.}
\endinsert

\vfill\eject\bye

