2. Experiments
2.1. Material
Near alpha titanium alloy with a nominal composition (Ti–5Al–
3.5Zr–2 Sn–0.5 Nb–0.5Mo–0.5Si) was used as the starting material.
The microstructures of the as-received material are shown in Fig. 1
(a and b) at two different magnifications. The alloy mainly
consisted of primary alpha, with a small fraction of secondary α
structure (10%). The primary alpha (αp), secondary alpha (αs) and
beta (β) phases are marked in Fig. 1(b). The shape of the primary
alpha phase varies between globular and lenticular structures. The
average grain size was 35 μm with a standard deviation of 12.
2.2. Mechanical testing
To analyze the orientation dependent hardness variation, 400
indentations were made on a metallographically polished sample
using an Instron micro Vickers hardness testing machine by
applying a load of 10 mg for 10 s. The indentations which were
close to grain boundaries and in the secondary alpha region were
neglected and approximately 200 indentations were considered to
analyze orientation dependent hardness of the primary alpha
grain. The orientation analyses were carried out using the electron
back scattered diffraction (EBSD) technique. Further, to understand
the orientation dependent strain accumulation, the as-received
material was subjected to quasi- static tensile tests, compression
at a cross head speed of 0.1 mm/s and Charpy impact test at room
temperature and at 650 1C. The quasi-static tensile tests were
conducted in an AG-IS 250 kN testing machine following standards
JIS Z2241 for the room temperature measurements and JIS G
0567 for the high temperature measurements. Compression tests
were carried out in a Schimadzu 250 kN universal testing machine
using cylindrical samples having diameters of 4 mm and heights of
6 mm. Impact toughness was estimated from the Shimadzu Impact
Testing instrument following the standards JIS Z2242 for the
testing procedure and JIS Z2202 for V-notch specimen dimensions.
2.3. Microstructure and microtexture analyses
The microstructural characterizations of the samples were
carried out using HITACHI S4700 and JEOL JSM7001F field
emission gun scanning electron microscope (FEG-SEM). The crystallographic
orientation of individual indentation was determined
by EBSD using a TSL-EBSD detector attached to JEOL JSM 7001.
EBSD generated data were utilized to determine the orientation of
individual indentation, strain accumulation through grain average
misorientation (GAM) and grain reference orientation deviation
(GROD) and further to plot image quality (IQ) and orientation
maps. GAM represents the average nearest neighbor misorientation
between pixels in a given grain. GROD refers to misorientations
between the orientation of the pixel and the average
orientation of the grain to which the pixel belongs. GAM values
are plotted on an inverse pole figure (IPF) to analyze orientation
dependent strain accumulation. Microstructural parameters were
calculated using orientation information from at least 500 grains.
Slip activity in a given grain was identified by slip plane trace
analysis. The orientation gradient analysis was performed on
selected samples.