Effect of processing history and aging temperature on age-hardening kinetics of 2014-Al alloy-SiC whisker composite

Abstract

Aluminum alloys reinforced with SiC whiskers (SiCw) or particulates are attractive for applications requiring higher stiffness and specific strength than traditional aluminum alloys. These composites exhibit isotropic properties like conventional aluminum alloys and can be fabricated or processed by conventional metal working techniques [1]. The properties of the matrix alloy play important role in dictating the properties of the particulate or whisker reinforced composite. For use in structural applications the matrix alloy of the composites are generally chosen from age hardenable alloy such as 2xxx, 6xxx and 7xxx. This led us to examine the issue of enhancement of strength of these composites due to age hardening. It has been reported that discontinuously reinforced aluminum metal matrix composites (AMMCs) age considerably faster than the unreinforced base alloy [2–12] and the aging kinetics vary significantly from one system to the other. For example, Neih and Karlak [10] reported that 6061 Al-alloy reinforced with B4C particles reached peak hardness in 3 h at 177◦C whereas the unreinforced alloy took 10 h. Suresh et al. [11] also found that the peak aging time of Al-3.5 Cu/SiCp composite to be 16–24 h at 190◦C, whereas that of unreinforced alloy was 60 h. This accelerated aging behavior of composites has been primarily attributed to increased dislocation density and the existence of stress gradient within the matrix surrounding the reinforcing agent [4– 6]. Additionally, DSC studies [4, 7] have indicated that the nature of precipitates and the sequence of precipitation during aging remain similar irrespective of the fabrication techniques and processing history. However, the aging kinetics of powder metallurgy (P/M) processed composite are reported to be considerably faster than those observed in composites processed by solidification technique (vertex method) [7]. This difference is attributed to the possibility of generation of higher dislocation density in P/M processed composite materials [7]. Most of these reports on accelerated aging in AMMCs are confined to AMMCs synthesized either by P/M process or by gravity casting technique. To date systematic investigation of the aging response of squeeze infiltrated SiCw reinforced aluminum matrix composites is limited, even though this process is emerging as a promising technique for synthesizing AMMCs. In order to extend the use of AMMCs as structural components, secondary processing like extrusions, rolling and forging of composites becomes a necessary step. Such thermomechanical treatment is expected to alter the matrix microstructure [8] as well as cause fracture and fragmentation of the reinforcing phase of the composite which may further lead to significant change in aging kinetics. However, systematic investigation, to examine the effect of secondary processing on the aging kinetics of AMMCs, is yet to be carried out. The purpose of this present article is to document (i) the effect of SiCw reinforcement and (ii) the effect of aging temperature on the aging response (aging kinetics) of 2014 aluminum alloy matrix composite as compared to that of base alloy before and after secondary processing such as extrusion. To examine their age-hardening kinetics the changes in hardness of AMMCs and the base alloys have been monitored as functions of aging time, temperature and processing history. The composite selected in this investigation is 2014 aluminum alloy reinforced with silicon carbide whiskers to 15% volume fraction and made through squeeze infiltration technique. The liquid metal was infiltrated into preheated silicon carbide whisker preforms located in a preheated cylindrical die and solidified under applied pressure to synthesize AMMCs. Both the alloy and the composite were hot extruded using a horizontal extrusion press at Nuclear Fuel Complex, Hyderabad, India. Age-hardening treatment has been conducted on metallographically polished samples of 15 mm diameter and 5 mm thickness. The samples were solutionized at 495◦C for 2 h followed by water quenching and aging isothermally at 120◦C, 150◦C, 170◦C and 200◦C for various time intervals until over aging is confirmed. The Vickers hardness measurement on the polished surfaces was performed as a function of aging time using a load of 2.0 N with the help of Zwick Type 321200 hardness tester. The hardness readings were plotted as functions of aging time at a selected temperature in order to obtain the age-hardening curve for each set of samples. The aging characteristics (hardness vs. aging time) of the 2014 Al alloy and 2014 Al/SiCw alloy composite both in extruded condition at different temperatures (120◦C, 170◦C and 200◦C) is shown in Fig. 1. It may be noted from this figure that the time required to reach