ABSTRACT
Titanium-based composites have the potential to be utilized in a range of applications where mechanical strength and corrosion resistance coupled with low density are advantageous. Lightweight structural composites were processed using a melt approach. The microstructures of these novel materials were related to processing variables including chemistry and solidification parameters. Microstructural characterization was conducted using optical and scanning electron microscopy. Phase determination was carried out using X-ray diffraction. Microstructural analysis and corrosion tests relevant to maritime applications were conducted. The coupons produced with the highest molar ratio of titanium to boron carbide processed at the lowest temperature showed the highest nobility and lowest rate of electron transfer of all the coupons. All coupons produced using this method showed an increase in nobility relative to UNS R50400 titanium.
INTRODUCTION
The combination of ductile metals and hard ceramics offer advantages that neither class of materials affords individually. The combinations of materials with disparate properties can be based on a metal matrix or a ceramic matrix. Aluminum-based composite materials have been extensively studied; however, their titanium analogs have not been explored as much. Titanium-based composite materials1,2 can be engineered to have different combinations of high specific strength, stiffness, and wear resistance, thereby making them potential candidates for light-weighting applications, e.g., in the aerospace, high performance automobile and biomedical industries. These materials can also display excellent corrosion resistance, making them potential candidates for marine applications as well.
EXPERIMENTAL PROCEDURE
The titanium-based composites were formed in a high temperature graphite furnace. A 3:1 molar ratio of titanium to boron carbide was used to achieve minimal residual metal in the composite while operating within safe limits.2 A higher residual metal content was achieved by using a nominal metal to ceramic molar ratio of 4.5:1. The necessary amounts of boron carbide powder were weighed, placed in a graphite crucible and then slightly tapped to achieve a consistent uniform packing density. The predetermined amount of titanium was then weighed and layered over the surface of the boron carbide powder. The crucible was placed in the chamber of the furnace. The chamber’s atmosphere was evacuated and backfilled with argon. Once the chamber was filled, argon was allowed to continuously flow through the chamber. The furnace was then heated to temperatures of either 1750°C or 1850°C. The coupons were held at temperature for 15 minutes then furnace cooled through the use of a water jacket. The coupons were then ground and polished to a 0.05 µm surface finish. Polished test coupons were characterized and analyzed through scanning electron microscopy (SEM) and X-ray diffractography (XRD). The coupons were again polished to 0.05 µm 30 minutes prior to running electrochemical tests in order to minimize any oxidation on the surface.
