Chip formation and microstructure evolution in the adiabatic shear band when machining titanium metal matrix composites

Titanium metal matrix composite (Ti-MMC) is a relatively new class of material, which has high potential applications in the aeronautical and biomedical sectors. Similar to titanium alloys, Ti-MMC produces segmented chips, which are characterized by adiabatic shear bands (ASB). Transmission Electron Microscopy (TEM) observations were performed and dislocations were observed on the atomic scale. Furthermore, the sheared surfaces, as well as the effects of the hard TiC particles on the ASB formation were investigated. It was shown that the grains located in the lightly strained areas within the chip segment are characterized by a high dislocation density. This is contrary to the highly strained areas inside the ASB, where the temperature was estimated to be close to the recrystallization temperature. Analysis of the results showed that no phase transformation took place inside the ASB. The strain and strain rate in the ASB were estimated to reach 7.5 and 4.5 x 10(5) s(-1), respectively. Using TEM and Focused Ion Beam (FIB) for sample preparation, the microstructure inside the ASB was found to be composed of elongated and equiaxed nano-sized grains. The segmentation mechanism of chips was observed to start from a crack on the material free surface ahead of the tool, and not at the tool tip. Furthermore, the hard particles inside the matrix were found not to be hindering, or retarding the ASB formation. A micro structural evolution model, based on these observations, has also been proposed. To the authors' best knowledge, TEM studies of ASB for Ti-MMC were never done previously for machining applications. (C) 2016 Elsevier Ltd. All rights reserved.