A. Shubin, K. Shunyaev, A. Dolmatov
Institute of Metallurgy, Ural Branch of RAS, Ekaterinburg
The alloys of rare and disseminated elements with non-usual
properties are the subject of considerable attention for many years.
Earlier [1] we studied mechanical and rheological properties of some
gallium pastes and hardened samples. These alloys demonstrate unique
properties (e.g. hardening at room temperature). This fact shows a good
perspective for the alloys mentioned to be used as lead-free solders,
dental filling materials etc. The aim of the present work is to investigate
the microstructure and compression strength of Cu-Ga-In-Sn alloys
produced by the mixing of liquid (Ga alloy) and solid (Cu alloy powder)
components. For the alloy group mentioned we studied the influence of
Bi-powder addition on the hardened alloy properties.
Taking into account the specific details of the diffusion-hardened
alloys (DHA) producing we considered separately the liquid component
(Ga-In or Ga-Sn alloys) and the solid one (copper or copper-tin alloy
powders mixed with the additions of Bi metallic powder).
Initially we performed the EDX analysis of the liquid eutectics GaIn and also Ga-Sn which were further used to produce the metallic DHA
pastes. According to our data obtained using the SEM-EDX system, the
indium content in the eutectic alloy is 21.32 wt. %. This value is almost
equal to real eutectic composition in the accepted Ga-In phase diagram
(21.4 wt. % In, Ga – the rest). According to the Ga-Sn phase diagram,
the eutectic alloy with melting point 20.5 C meets the following
composition – Sn: 13.5 wt.%, Ga – the rest. The composition found by
our EDX measurements corresponds to 13.35 wt. % of Sn (Ga – the
rest). The analysis results obtained show high accuracy of our SEMEDX complex and allow us to be sure in quantitative results of the
micro-analysis of other gallium alloy samples.
Further the series of DHA samples were prepared. The samples
were produced by the intensive mechanical mixing of the powders of
Cu-Sn alloy (5 wt. % Sn, Cu – the rest) and bismuth, and also the liquid
phase – Ga-In eutectic of the composition described above. The metallic
pastes prepared were used to form the cylindrical samples for the
metallographic analysis and also EDX and other investigations. As a rule
the samples contained 33 wt. % of the liquid component (eutectic) and
67 wt. % of the powders mix. This powders mix included Cu-Sn and Bi
powders for different Bi content (from 100 to 0 per cent with the step of
about 10 wt. %). In such a way we prepared the samples of 11 different
compositions. All the powders had compact (non-dendritic) particles
shape and were prepared by gas-phase spraying or mechanical grinding.
The particles size corresponded to the fraction (- 40 m).
The SEM-EDX investigation was performed for all the samples
series. We studied both non-polished hardened alloys surface and the
prepared metallographic sections (Fig.1).
Fig.1. a – non-polished surface of the hardened alloy; b – polished surface of
the metallographic section (back-scattering electrons detector).
The surface layer of the samples (made as the spheres of the
diameter 5-7 mm) consists mainly of the following phases:
1. Copper-gallium intermetallic compound (IMC) CuGa2. This
phase can easily be identified by EDX detector. It often forms specific
crystals that have almost stoichiometric composition (gray regions, Fig.
2. InBi intermetallic compound. Its composition also is close to the
stoichiometric (light regions, Fig. 1a).
3. Two-phase region InBi-Ga. It includes up to 20 at. % of Ga and
small quantity of the dissolved copper (light regions, Fig. 1a).
The samples surface includes well-marked residual spherical
particles of the initial powder that are coated by the interaction products.
The study of the polished samples shown the presence of the
following phases:
1. The residual particles of copper alloy powder.
2. Copper-gallium IMC CuGa2 (gray regions, Fig. 1b).
3. Copper particles which don’t contain tin but include 1-2 at.% of
gallium (more dark surface areas inside the gray regions (Fig. 1b).
4. Multi-phase regions including Cu, Ga, In, Sn and Bi (e.g., at. %:
Bi – 31; Cu - 7; Ga – 9; Sn – 21, In – the rest). The elements ratios in
such a regions can be varying (light regions, Fig. 1b).
5. Pure InBi intermetallide that doesn’t contain another metals and
shows almost equiatomic indium-bismuth ratio (light regions, Fig. 1b).
So, the composition of the surface and core layers of the samples
is somewhat different. The most important and specific phases that
present in all the alloys studied in a big quantity are the intermetallides
CuGa2 and InBi.
The phases like gallium solutions in tin (indium) and another
alloys like mentioned have low strength. These phases play the role of
softening components and their amount in the hardened composite alloy
must be as little as possible. The Bi addition to the DHA has the goal to
associate indium or tin with bismuth and form the IMC like InBi or SnBi alloy which demonstrate better mechanical properties as the part of
DHA structure. The literature data concerning Sn-Ag-Cu lead free
solders show the strength increasing when some amount of Bi have been
added to the alloy [2].
The EDX analysis demonstrated that InBi intermetallic compound
forms in significant amount in the alloys investigated. Further the results
of compression strength studying will be presented.
To carry out the compression strength test we prepared the
samples with the diameter of 4 mm and of about 7 mm height. After the
exposition of the hardened alloy during 7 days at room temperature the
surface of the sample ends were polished. Then the compression strength
() was measured for 5 samples using Zwick/Roell equipment. The
composition of the samples tested was the following (wt. %): Cu – 45.6;
Sn – 2.4; Bi – 12; In – 8.6; Ga – the rest. The average compression
strength measured is equal to 116 MPa.
The typical plots for DHA in the coordinates “compression stress”
vs “compression deformation” is shown in Fig. 2 and Fig. 3.
Fig. 2. Compression stress – sample deformation curve for InBi – containing
Fig. 3. Compression stress – sample deformation curve for “usual” diffusivehardened alloy.
One can see the area with slight slope between maximum stress
point (about 3.5 % deformation) and the point at approximately 6 %
deformation in Fig.2. Usually such curves demonstrate another behavior
for DHA samples. It can be demonstrated for Cu-Ga-Sn hardened alloy
(Fig. 3). The plasticity of Bi-containing samples (Fig. 2) is better than
those for the samples which don’t contain Bi (Fig. 3).
At the same time, the numeric value of the compression strength
of about 116 MPa is comparatively low. Evidently the further task is to
increase the compression strength and keep the “plasticity” of the DHA
material to improve the exploitation characteristics of the composite.
Authors are grateful for support to the Presidium of RAS (research
program 12-P-3-1032).
Shubin A.B., Shunyaev K.Yu., Yamshchikov L.F. The Diffusion of
Gallium into Copper-Tin Alloy Particles// Defect and Diffusion
Forum. 2009. V.283-286. P. 238-242.
Matahir M., Chin L.T., Tan K.S., Olofinjana A.O. Mechanical
strength and its variability in Bi-modified Sn-Ag-Cu alloy// J. of
Achievements in Materials and Manufacturing Engineering. 2011.
V.46. Issue 1. P. 50-56.