V. N. Gaidarova 1 , R. L. Lazarova 1 , S. V. Vaseva 2 ,
V. K. Manolov 1 , S. G. Konstantinova 1
1 Institute of Metal Science – Bulgarian Academy of Sciences
2 Space Research Institute – Bulgarian Academy of Sciences

The needs from cast details with special properties and increasing requirements to them demand an enlargement of the investigations and search for new approaches for improvement of the alloys and castings structure and properties. For solving this problem a new class of modifiers – ultradispersed nanomodifiers of high-meltingpoint compounds and nanodiamonds (ND) are experimented.
In the Space Research Institute – Bulgarian Academy of
Sciences detonation methods for producing of ultradispersed diamond powders have been developed and patented during last years. The carried out investigations ensure a wide range of technologies for application at different engineering fields. The nanodiamonds as modifiers act as crystallization nucleus, around which an alloy crystal phase is formed. In result the structure becomes finer. Moreover, the nanodiamonds particles prevent the movement of the dislocations and in this way promote an additional material strengthening.
In present work the modifying effect of nanodiamonds obtained by detonation technology on the structure and properties of AlSi7Mg alloy is investigated.

Alloy AlSi7Mg (Si – 7.00-7.20%, Mg – 0.3-0.4%, Ti – 0.02%, Fe – 0.6-0.8%) is modified by introducing in the melt of small quantity (0.1-0.15 %) of ND.
The wettability of the ND particles by the liquid metal is very low and their homogenous introduction in the melt is a difficult task. One of the methods for improving ND acquirement of the alloy is the chemical processing with appropriate cladding metal.
Three types of ND covered with different metals (~ 10 %) are used:

ND+Ag powder of ND and cladding metal Ag. ND concentrations of
0.1 % and 0.15 % from the melt weight;

ND+Cu -powder of ND and cladding metal Cu.ND concentration - 0.15 %;

ND+Ni powder of ND and cladding metal Ni. ND concentration - 0.15 %.
The alloy is melted in electric-resistance furnace with aluminum capacity of the crucible 0.500 kg. Degassing is carried out by the aid of argon at
730-740 o C during 5min. Nano modifiers packed in aluminum foil and encapsulated in aluminum cartridge are introduced in the melt at 720 o C with the aid of mechanical stirring by titanium mixer during 3-5 min.

Experimental castings (~0.250 kg) are produced in metal mold fig.1.
Samples for testing are taken from the cylindrical part. The structure and properties are investigated in cast condition.
Fig. 1.
The metallographic analysis is carried out by microscopes Reichert MeF
2 and PolyvarMet. The quantitative metallographic analysis is performed with automatic system Olympus MicroImage. The mean grain diameters and DAS are defined. The microhardness is measured by MicroDuromat
4000 with loud 10 g during 10 s. Mechanical properties: tensile strength
R m
, yield strength R
P0.2
and elongation A
5 are determined using standard method. The density is measured using Archimedes’ principle.

Investigation of ND+Ag powder by SEM
Powder of ND+Ag is dissolved in ethanol and dispersed by ultrasonic bath. A drop of this solution is fixed and dried on a supporting surface (carbon mesh or silicon plate). So obtained specimen can be studied in the vacuum conditions. Field
Emission SEM Hitachi S4800 is used for observation at 15 kV with 1nm resolution.
In fig. 2 it is seen that nanoparticles are of 5-20 nm size, most of them gathered in agglomerates.
X 200 000
X 3000
X 300 000
X 450 000
Fig. 2. SEM images of nanodiamonds covered with silver at different magnifications.

The microstructure of untreated alloy shows columnar morphology (fig. 3a).
Aluminum grains are with elongated forms, reaching 80100 µm in their maximum length. Addition of 0.1% ND+Ag) changes the form from columnar to more equiaxed (fig.3b-4b); the size rarely exceeds 5060µm (fig.3b - 4b) .
The volume fracture of the spherical α-cells in the modified castings is nearly two times bigger than in unmodified one. D mean decreases with 26.7 % and
DAS - with 6.6 %. The increasing of microhardness reaches 15.3 % (table 1).
a) unmodified b) modified with 0.1 % ND+Ag
Fig.3. Microstructure (LM) of AlSi7Mg alloy castings.

The silicon from massive long plates/rods changes to relatively uniformly distributed finer particles in the eutectic domain. The pores are decreased significantly (fig. 4).
a) unmodified (x 1000) b) modified with 0.1 % ND+Ag (x 1000) a) unmodified (x 10 000) b) modified with 0.1 % ND+Ag (x 10 000)
Fig. 4. SEM images of AlSi7Mg alloy at different magnifications.

The increase of ND additions up to 0.15 wt. %, regardless of the kind of cladding metal (table 1) leads to similar refining effect with decrease of
D mean and DAS, but to smaller increase of the microhardness. From visual observation of the microstructure a refinement of Si and intermetal crystals, including Fe-containing ones, is established as well (fig. 5).
a) unmodified b) modified with 0.15 % ND+Ag c) modified with 0.15 % ND+Cu d) modified with 0.15% ND+Ni
Fig. 5. Eutectic microstructure (LM) in AlSi7Mg alloy castings.

Experimental results of the mechanical tests are in agreement with the observations of the microstructure. The increase of the tensile strength reaches 8 % and of the elongation – more than 90 % (table 1).
The conclusion can be made, that addition of lower quantity of nanopowder (0.1 % ND) affects stronger the microhardness and yield strength than higher concentration (0.15 % ND). The latter causes significant increase of plasticity (above 40 %). The best result is registered in case of the alloy modified by ND+Ag (casting 3) – the elongation increases with
91.5 %. The increase of density can be related to the elimination of the porosity in the modified alloy.
Таble 1. Microstructural and physic-mechanical characteristics
Casting
№
Type of modifier
1 Unmodified
2 0.1 % ND+Ag
Change in %
3 0.15 % ND+Ag
Change in %
4 0.15% ND+Cu
Change in %
5 0.15% ND+Ni
Change in %
D mean
[μm]
17.93
13.15
-26.7
14.87
-17.1
12.92
-27.9
14.90
-16.9
DAS
[μm]
13.74
12.84
-6.6
13.17
-4.2
12.27
-10.7
11.97
-12.9
HV
[kg/mm 2 ]
58.2
67.1
15.3
62.5
7.4
60.6
4.1
62.0
6.5
R
P0.2
[MPa]
123
128
4.1
118
-4.0
118
-4.0
113
-8.1
R m
[MPa]
200
211
5.5
210
5.0
216
8.0
209
4.5
A
5
[%]
4.7
5.1
8.5
9.0
91.5
6.8
44.7
7.2
53.2
D
[g/cm 3 ]
2.6923
2.7103
0.7
-
-
-
-
-
-

The initial investigations convincingly confirm that modification with nanodiamonds significantly influences the microsructure and mechanical characteristics of AlSi7Mg alloy. The mean grain diameter decreases with
26-27 %, the form of aluminum grains alters together with the fragmentation of the silicon phase. These changes influence the mechanical properties of the modified alloy. The microhardness increases with 15 %. The increase of the tensile strength reaches 8 % and of the elongation – more than 90 % .
The present work is supported by the National Science
Fund (Bulgaria) – Project TK01/076 and Project DO 02-
93/2009. The usage of the facilities at Center for
Microanalysis of Materials, University of Illinois at Urbana-
Champaign, USA is highly acknowledged.