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24Fe, Ti, AND Ni ALUMINIDS FOR MODIFICATION OF POWDER ANTIFRICTION MATERIALS
L. N. Dyachkova*", A. Ph. Ilyushchanka", L. Ya. Voronetskaya", and N. M. Parnitsky"
aO.V. Roman Powder Metallurgy Institute, Minsk, 220005, Republic of Belarus *e-mail: dyachkova@tut.by
DOI: 10.24411/9999-0014A-2019-10035
At the present time, powder antifriction materials based on iron and copper have found the widest application for parts of friction units, since the technology of powder metallurgy makes it possible to most fully realize the basic conditions for creating wear-resistant materials with a pronounced heterogeneous structure due to the introduction of various kinds of additives, both interacting and not interacting with the base metal. Due to the great possibilities in varying the chemical and phase composition, powder materials have a wide range of tribotechnical properties and can be used for light, medium and heavy loaded working conditions.
Iron graphite materials are most known among iron-based antifriction materials, in which graphite is partially dissolved in iron, while strengthening it, and the remaining part plays the role of solid lubricant [1].
Numerous additives in different concentrations and different dispersion are introduced into powder materials to increase the mechanical and tribotechnical properties. Improving tribotechnical properties of powder materials based on iron is achieved by the introduction of solid lubricants (sulfides, selenides, tin, lead), solid inclusions in the form of oxides, borides, carbides, intermetallides [2-5].
The properties of powder materials, including antifriction materials, are determined by the phase composition and morphology of the structure, which are formed during sintering and depend on the composition of the base and introduced additives. In this regard, the effect of the introduction of additives of ultrafine intermetallide powders of different phase composition produced by the method of mechanically activated self-propagating high-temperature synthesis (MASHS) [6] on the structure and properties of powder materials based on iron and copper was investigated.
Powders of aluminides of nickel, titanium, and iron of the following chemical and phase composition were used as additives in the antifriction material based on iron: Ni-14 wt % Al (NisAl); Ni-50 wt % Al (Ni5Ab-NiAl); Ti-21.5 wt % Al (TisAl); Ti-57.5 wt % Al (TiAh-TiAb); Fe-17 wt % Al (FesAl); Fe-57 wt % Al (Fe2Ab, FeAls).
Intermetallide powders were introduced in the amount of 0.2; 0.5; 1 wt % when mixed with iron powders and 0.6% graphite. Samples for the research were pressed at a pressure of 500 MPa to a relative density of 83-85% and were sintered in a protective-reducing atmosphere of endogas for 1 h at a temperature of 1100°C.
Three-point bending tests were performed on a Tinius Olsen testing machine (England) at a loading rate of 2 mm/min. The structure was studied on a MEF-3 metallographic microscope (Austria) and an electron scanning microscope "Mira" of Tescan Company (Czech Republic) with an X-ray microscope attachment "Oxford Instruments" (United Kingdom).
Studies have shown that the introduction of nickel-based intermetallides strengthens powder steel more effectively than titanium-based ones (Fig. 1). Strength improvement occurs only with the introduction of 0.2-0.5% and is 1.5-2 times compared with the strength of steel powder without an additive - 280 MPa. The increase in strength is explained not only by the modifying effect, but also by alloying the iron base with nickel formed during the decomposition of the intermetallide or in the elemental form in the intermetallide. This was confirmed by the X-ray
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microanalysis. The nickel content gradually decreases from maximum in the center of the intermetallide particle to zero in the base. The absence of aluminum in the structure is probably due to a small amount of it.
The greatest strengthening occurs with the introduction of intermetallides having a lower melting point or polymorphic transformation - Ni3Al and TiAh-TiAb. Hardness is higher for materials with the introduction of TiAb intermetallide, and unlike strength, the hardness increases with an increase in the amount of intermetallide additive up to 1 wt %.
The study of the structure showed that the degree of structure modification of powder steel depends on the composition of aluminide and its content (Fig. 2). With an increase in the amount of additive, its modifying effect occurs to a greater extent, regardless of the composition. This explains the increase in hardness of the material.
450 i 400 -350 -
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H 250 -c
" 200 -ra
^ 150 -<i>
m 100-
50 -0 -
Fig. 1. The effect of the content of the aluminide additive on the strength of powder steel FeGr0.6.
Fig. 2. The microstructure of powder steel PK50 with the introduction of 1% aluminide. (a) without additive; (b) NiAb; (c) NÏ5Ab-NbAl; (d) TbAl; (f) TiAh-TiAb; (g) Fe3Al.
The reduction in grain size, as a rule, has a positive effect on the strength of the material, but with the introduction of 1% aluminide, regardless of its composition, the strength of powder steel decreases. This is explained by the fact that with an increase in the content of the additive, particles of aluminides, which are located mainly along the grain boundaries of iron phase, lead
to their weakening. In addition, the decrease in strength is also due to the fact that aluminide powders have a high content of oxides. The presence of oxides leads to a decrease in the carbon content in steel, which is spent on their recovery. This occurs especially with the introduction of titanium aluminides, in which the oxygen content is higher than in nickel aluminides due to the higher sensitivity of titanium for oxygen.
X-ray microscopic analysis of steel powder with the additive of nickel aluminide of the Ni5Al3-Ni3Al phase composition revealed that, like in the single-phase Ni3Al intermetallide, in this case there is a coherent bond of intermetallide with the base. In the border region with the intermetallide, the presence of nickel in the amount of 0.6-1.4% and aluminum in the amount of 2-5.5% is identified.
The complex intermetallide is heterogeneous in composition. It contains regions with increased (up to 70-75%) nickel content and increased (28-34%) aluminum content, and nickel and aluminum content gradually decreases in the border region with iron base.
The greater alloying of the base with the introduction of single-phase intermetallide compared to two-phase one explains the greater strength of the material. The diffusion of nickel from the intermetallide into the iron base leads to the fact that the nickel content in the central region of the intermetallide is much higher than in the border region.
Titanium aluminide Ti3Al is also coherently bound to the iron base. The coherent bond of the intermetallide with the base is also confirmed by the fact that a transition layer is formed in the region of the intermetallide particle bordering the iron base. Its composition differs from that of the intermetallide. The titanium content in the center of the intermetallide particle is 11-17%, aluminum is 10-17%, the content of titanium and aluminum is 8 and 17% at the border with the base, respectively. Titanium content is 2-3%, aluminum is 5-6% in the base directly at the border with the inclusion of intermetallide. Further, titanium content is gradually reduced up to 0.3% and titanium is not identified after 20 p,m, aluminum is not identified after 10-15 ^m. This suggests that the diffusion of titanium into the iron base is much greater than aluminum.
Two phase titanium aluminide TiAl2-TiAl3 is heterogeneous in composition. Titanium content in the particle varies from 9 to 30%, aluminum changes from 8 to 21%. With the introduction of this aluminide, alloying of the iron base with titanium occurs to a lesser extent than with the introduction of the single-phase Ti3Al intermetallide. This explains the lower strength and hardness of the material.
The dependence of the strength of powder steel on the amount of iron aluminide additive is presented in Fig. 3. Increasing the amount of the additive up to 0.5 wt % leads to an increase in strength of 20-30 MPa; up to 1 wt %, to a slight decrease in strength with the addition of singlephase iron aluminide, and 60-65 MPa - with the addition of two-phase one. It should be noted that the addition of iron aluminide provides higher strength of powder steel than nickel and titanium aluminides.
650
1 2 3 4 5 6 7
Namber of composite
Fig. 3. The effect of the content of iron aluminide additive on the strength of powder steel PK50. 1 without additive; 2 0.2% FesAl; 3 0.5% FesAl; 4 1% FesAl; 5 0.2% Fe2Ab-FeAb; 6 0.5% Fe2Al5-FeAl3; 7 1% Fe2Ab-FeAb.
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