1 Confidential: not for distribution. Submitted manuscript to SURFCOAT for peer review (it is formatted by me, so please be patient). Please cite this article as: B.N. Mordyuk, et al., Structure, microhardness and damping characteristics of Al matrix composite reinforced with AlCuFe or Ti using ultrasonic impact peening, Surface & Coatings Technology 204 (2010) 1590–1598, http://dx.doi.org/10.1016/j.surfcoat.2009.10.009 Structure, microhardness and damping characteristics of Al matrix composite reinforced with AlCuFe or Ti using ultrasonic impact peening B.N. Mordyuk a∗ , M.O. Iefimov b , G.I. Prokopenko a , T.V. Golub a , M.I. Danylenko b a Kurdyumov Institute for Metal Physics, 36 Academician Vernadsky blvd., UA-03680, Kyiv, Ukraine b Frantzevich Institute for Problems of Materials Science, 3 Krzhyzhanivsky st., UA-03142, Kyiv, Ukraine Abstract Ultrasonic impact peening (UIP) is used to modify the near-surface layers of cp aluminum. The effects of icosahedral quasicrystalline (QC) AlCuFe or hcp Ti fine powders added to zone of severe plastic deformation at the UIP process on microstructure, phase composition, microhardness of near-surface layers and damping properties of aluminum are studied. The results show that composite layers, which are characterized by relatively uniform distribution of reinforcing particulates with similar volume fraction of about 0.17 are formed. While semi- coherent particulate/matrix interface is observed for QC reinforcements, the Ti particulates seem to be strongly adhered to the aluminum matrix due to formation of Ti 3 Al interlayer. While a dislocation-cell structure is formed after the UIP only, highly misoriented fine grain structure with mean grain size of 0.1 – 0.5 µm is observed in the AlCuFe reinforced composite layer, and the Ti reinforced layer is characterized by mean grain size of 0.5 – 2 µm. Observed microsructural features predetermine significant enhancement of microhardness and damping properties of as-treated aluminum specimens. Much higher magnitudes of microhardness (about 1.3 GPa) and logarithmic decrement (about 12x10 -4 ) are observed in Al specimens covered with the QC reinforced composite layer in comparison to those for specimens contained the Ti reinforced layer (about 1 GPa and 3.6x10 -4 ) and to the as-peened aluminum specimen (0.58 GPa and 1.4x10 -4 ). It is due to (i) the smallest grain size, (ii) semi-coherent particulate/matrix interface and (iii) high hardness and specific stiffness of the AlCuFe QC phase. Relatively high level of microhardness (about 1.1 GPa and 0.8 GPa) and logarithmic decrement (about 5 10 -4 and 2 10 -4 ) are conserved for Al specimens covered with the QC and Ti reinforced composite layers even after heating to 623 K. Keywords: Surface composite, Quasicrystalline particles, Microstructure, Aluminum, Ultrasonic impact peening, Microhardness, Damping 1. Introduction Comparing to the unreinforced aluminum and Al-based alloys, Al-based metal matrix composites (Al-MMCs) are known to possess a number of beneficial properties [1, 2]. Their high specific strength, increased hardness and good wear resistance promote wide use for structural applications in the aerospace and automobile industries. Bulk Al-MMCs can be produced by dispersing of hard particles into the aluminum matrix using solid or liquid techniques. One of the efficient methods to induce ultrafine grain structure in bulk materials including Al-MMCs is deforming it to large strains below recrystallization temperature without intermediate thermal treatment [3]. At the same time, such surface dependent properties as wear, fatigue etc. have also been found to be improved by a suitable modification of microstructure and/or composition of the surface layer’s matrix instead of bulk reinforcement [4]. Therefore, surface Al-MMCs reinforced by different particulates could also be quite attractive. Both a matrix material and reinforcement are usually selected on the base of the end application of composite. Hard particles such as carbides, oxides, nitrides or intermetallics are the conventional reinforcement materials for Al-MMCs [1-3]. Use of quasicrystalline (QC) particles as reinforcements for Al-MMCs seems to be also prospective because QC materials have unique combination of high hardness and modulus of elasticity, high wear resistance, low friction coefficient and relatively low density. Therefore, QC reinforcements are used to strengthening of Al-based alloys [5-7]. As QC particles are metastable in these alloys, they can be formed by means of rapid solidification techniques and sustain high strength of alloy just to temperature of 300-350 o C. Degradation of mechanical properties occurred at higher temperatures is due to transformation of QC particles into crystalline phases. Al-based alloys hardened with stable AlCuFe QC particulates can not be produced because of very thin existence domain of QC AlCuFe. Thus, composites reinforced with stable QC particulates can be produced only by means of mechanical treatment or severe plastic deformation. A nature and strength of particulate/matrix interface, which play an important role in the composite performance, seem to be dependent on a type of atomic structure of particulate (crystalline/quasicrystalline). Thus, a comparison of QC AlCuFe and hcp Ti, which therewith have equal density, would be quite informative. Presumably, the most known mechanical surface treatment for production of surface Al-MMCs is a friction stir processing (FSP), which is shown to be an effective method allowing significant improvement of different surface dependent properties by uniform distribution of the second phase [4,8,9]. Besides, such methods of severe plastic deformations as shot peening [10] or mechanical alloying in the vibration chamber [11] are also used to produce surface composites or composite coatings by means of repeated ball collisions by the treated surface. Recently, ultrasonic impact peening usually used for modification of structure, phase composition and macroscopic residual stress in surface layers of metallic materials [12-15] has been successfully employed to modify aluminum specimens by severe plastic deformation of near-surface layers and simultaneous embedment of fine powders [16,17]. Mechanically mixed surface layer is shown to be formed either by means of rotation of special tool penetrated into the surface layer in the case of FSP or by the severe deformation process in the case of UIP (or other peening techniques). ∗ Corresponding author: Telephone / fax: +38(044)424-0521. E-mail: mordyuk@imp.kiev.ua