Abstract
During machining, the cutting surface of a metal is subjected to high strain rates and temperatures. Due to the small mass of the formed chip, the metal is rapidly quenched, preserving the as-machined microstructure. These extreme conditions are reported to be favorable to form nanograin or ultrafine-grain microstructures. However, detailed investigation of this region is problematic due to the size of the chips and the difficulty in preserving the cutting surface microstructure during traditional transmission electron microscopy (TEM) preparation. This study investigates the use of focused ion beam (FIB) specimen preparation to preserve and TEM to image the microstructure of the secondary deformation zone (SDZ) at the cutting surface in chips of Ti-6Al-4V formed during machining. Use of the FIB allowed precise extraction of a side or transverse view specimen, which preserved the cutting surface to reveal an inhomogeneous microstructure resulting from the nonuniform distribution of strain, strain rate, and temperature. Initial imaging of a conventional TEM foil prepared from the plan view of the cutting surface revealed microstructures ranging from heavily textured to regions of fine grains. Using FIB preparation of a transverse foil, a layered microstructure was observed revealing a variation of fine grains near the cutting surface, which transitioned to coarse grains toward the free surface. At the cutting surface, a 10-nm-thick recrystallized layer was observed capping a 20-nm-thick amorphous layer.
| Original language | English |
|---|---|
| Pages (from-to) | 3527-3533 |
| Number of pages | 7 |
| Journal | Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science |
| Volume | 42 |
| Issue number | 11 |
| DOIs | |
| State | Published - Nov 2011 |
Funding
The authors acknowledge the financial support provided, in part, by AFOSR Grant No. FA9550-07-1-0282, under the direction of Dr. Joan Fuller, and the Lockheed Martin/MAF, under the direction of Messrs. Randy Brown and Zhixian (Tim) Li. A portion of this research was conducted at the SHaRE User Facility, which is sponsored by the Division of Scientific User Facilities, Office of Basic Energy Sciences, United States Department of Energy. The FE-SEM at MSU was purchased under NSF-IMR Grant Nos. DMR.0216703 and 02070615.