DEVELOPMENT OF YTTRIUM / TiB2 REINFORCED AA2024 MATRIX COMPOSITES WITH SUPERIOR MECHANICAL PROPERTIES

Abstract

Composites are a class of materials developed by reinforcing one or more materials to the matrix. The properties of a composite mainly depend on the factors like reinforcement material and processing method. Though the reinforcing material has the potential to improve the properties of a composite, the drawbacks and challenges encountered during the production stage deteriorate the properties of the end product. Hence, the present work is emphasized to investigate the influence of a few factors like processing techniques, minor reinforcements of a rare earth element (yttrium) and a ceramic material (TiB2) on the mechanical properties of AA2024 matrix composites. In the present work, initially, three batches of each comprising five different AA2024 matrix composites reinforced with varying amounts of micro yttrium ranging from 0.1 to 0.5 wt% are developed through stir casting, cold compaction and spark plasma sintering (SPS). It is revealed that the reinforcement of micro yttrium in small amounts has improved the mechanical properties of AA2024 matrix composites processed through stir casting, cold compaction and SPS. However, the composites developed through SPS showed better properties when compared to those of the composites developed through stir casting and cold compaction. Among the SPSed composite samples, the composite reinforced with 0.3 wt% micro yttrium showed highest values of hardness, UTS, YS and EL which are found to be 114 HV, 315 MPa, 262 MPa and 22.3% respectively. Hence, based on the present work, the processing methods can be rated as SPS > cold compaction > stir casting with respect to the mechanical properties attained in the composites. In a similar way, a batch of five different AA2024 matrix nano composites reinforced with varying amounts of nano yttrium ranging from 0.1 to 0.5 wt% are also developed through SPS. It is worthwhile to note that AA2024 matrix composite samples with minor reinforcement of nano yttrium showed better mechanical properties compared to those of the composites with reinforced with micro yttrium. From these investigations, it is also found that the composite reinforced with 0.3 wt% nano yttrium showed better properties among the other nano composite samples. Highest hardness, UTS, YS and EL are found to be 130 HV, 462 MPa, 382 MPa and 16.8% respectively for the composite reinforced with 0.3 wt% nano yttrium. These investigations confirmed that reinforcing rare earth element in small amounts to an aluminium iv matrix significantly improves its mechanical properties through grain refinement and acceleration of precipitation kinetics. Finally, a batch of five different AA2024 matrix hybrid composites reinforced with a fixed amount of TiB2 (1.0 wt%) and varying amounts of nano yttrium ranging from 0.1 to 0.5 wt% are developed through spark plasma sintering. These investigations revealed that the formation of the brittle Al3Ti phase is supressed and the ductility is retained. As an evidence of grain refinement, ultra fine grains, nano grains and nano precipitation are observed in the microstructures of the composite reinforced with 1.0 wt% TiB2 and 0.3 wt% nano yttrium. The composite reinforced with 1.0 wt% TiB2 and 0.3 wt% nano yttrium showed better mechanical properties. Highest hardness, UTS, YS and EL are found to be 137 HV, 496 MPa, 438 MPa and 15.8% respectively in the composite with 1.0 wt% TiB2 and 0.3 wt% nano yttrium. In addition, the composites developed through SPS reinforced with varying amounts of micro yttrium ranging from 0.1-0.5 wt% are further subjected to cryo rolling. The mechanical properties of the cryo rolled composites followed the same trend as those of the un rolled composites. Cryo rolling improved the strength of the composites remarkably, but at the cost of ductility. From the investigations carried out in the present work, it is found that 0.3 wt% yttrium (micro as well as nano) reinforcement seems to create a favourable condition in all the composite samples processed through stir casting, cold compaction and SPS to yield superior mechanical properties. In the composites reinforced with 1.0 wt% TiB2 along with varying amounts of nano yttrium, formation of Al3Ti phase is supressed and hence, the ductility of the composites is retained. It is also found that the nano yttrium reinforced composites processed through SPS attained improved mechanical properties when compared to their micro yttrium counter parts. Based on the present work, the processing methods can be rated as SPS > cold compaction > stir casting with respect to the mechanical properties attained in the composites.