JOURNAL OF MATERIALS SCIENCE 40 (2 0 0 5 ) 4173 – 4179 Dynamic properties characterization of metastable Al/Ti composites N. SRIKANTH, LOU KUM HOONG, M. GUPTA Department of Mechanical Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore, 117576 E-mail: mpegm@nus.edu.sg A new idea of using stiffer metallic reinforcement, such as titanium, in a ductile metallic matrix, such as aluminium, to enhance the dynamic properties (viz., stiffness and damping) is successfully attempted. The study focuses on the relationship between the stiffness and the damping capability of the aluminium matrix with the weight percentage of titanium added to it. Results of this study show that addition of about 3.2, 6 and 7.5 wt% of titanium increases the overall damping capacity of the Al matrix by 6, 17 and 24%, respectively. Particular emphasis is placed to rationalize the increase in damping in terms of the increase in dislocation density and presence of plastic zone at the matrix-particulate interface. C  2005 Springer Science + Business Media, Inc. 1. Introduction Accumulation of vibration energy increases the vibra- tion amplitude in a dynamic mechanical system. Hence damping, which refers to the dissipation of energy to the surrounding environment by a reversible microstruc- tural movement or an irreversible thermoelastic process inside the material during mechanical vibration, is gen- erally sorted as a solution in the design [1]. Parallel to this option, stiffness can be increased as it refers to the capacity of a mechanical system to sustain loads with- out excessive changes in its geometry (generally called deformations). Thus materials with increased stiffness and damping property are actively sought for the design of dynamic mechanical systems such as in spacecrafts, semiconductor equipments and robotics. Studies have demonstrated that it is possible to achieve significant improvements in the stiffness and damping behavior of metal matrix composites (MMCs), by incorporating certain types of dispersoids in the metal matrix [1–4]. Aluminium based formula- tions are one such category of light weight materials that has the capability to exhibit such properties espe- cially when it is unified with stiffer ceramic particulates [1, 2]. In related studies, it has been shown that the ad- dition of ceramic particulates to the aluminium matrix assists in improving damping properties of the overall composite [3]. Additionally, the authors presented the idea of interconnected metallic reinforcements that also resulted in the significant increase in damping and stiff- ness of the composite material [4]. Investigation related to machinability of MMCs has shown that the presence of hard ceramic phase decreases the tool life of cutting tools drastically [5]. Hence the idea of dis-continuous stiffer metallic phase in a ductile metallic phase adopted in the present study may enhance the machinability of this new MMC. The results of the literature search, however, reveal that no attempt has been made to in- vestigate such metallic reinforcements addition on the damping behavior of aluminium. Previous studies of Gupta et al. [6] has shown that titanium dissolves in aluminium at high temperatures and forms intermetallics with Al following solidifica- tion. The synthesis and processing of such material is extremely challenging due to the ability of Ti to raise the melting temperature of Al and extreme reactive na- ture of molten Al-Ti mixture. In order to avoid these limitations, Ti was subjected to surface modification so that it stays as elemental-Ti, mostly, following solid- ification. In addition, titanium being a stiffer material with a elastic modulus of 120 GPa, has a higher melt- ing point of 1667 ◦ C, and a hardness of 90 HV, and hence when left un-dissolved in molten aluminum ma- trix by proper surface modification on the particle sur- face, it can result in reinforcing the ductile Al matrix, which has a melting point of 660.3 ◦ C, elastic modulus of 72.2 GPa and a hardness of 15 HV [7]. But, in terms of density, Ti is heavier than Al by 1.6 times which can make the composite heavier, but when added in smaller volume fraction this effect may not be a significant fac- tor in the design. Hence the primary objective of this investigation was to investigate the energy dissipation of aluminium containing variable amounts of pure Ti phase. 2. Materials and processes In this study, pure Al (purity > 99.7%) was the base metal and titanium powders with an average size of 19 ± 9 μm were used as the starting materials. To pre- vent Ti dissolution in Al, the Ti particle’s surface was modified by preheating at 400 ◦ C for an hour in a ce- ramic container to produce a surface oxide layer and 0022–2461 C  2005 Springer Science + Business Media, Inc. 4173