Poster over-viewing initial results of TEM study on grade 2 titanium which received a novel heat treatment resulting in a alpha+beta phase microstructure.

1. HR-TEM study of β phase lamellae in Ti grade 2 [E163]
Walter Guy, Karel Tesař, Aleš Jäger, Viera Gärtnerová
Institute of Physics AS CR
Department of Advanced Structural Materials
Na Slovance 1999/2, 18221 Praha 8, Czech Republic
In this work a HR-TEM
study of a duplex phase microstructure, formed
by heat treatment of CP grade 2 titanium, was
performed. In particular, β phase lamellae were
studied and the results showed that the lamellae
are comprised of both β phase titanium and δ
phase titanium hydride.
These hydride formations were unexpected for
this purity of titanium, and were likely formed
during annealing by the diffusion of hydrogen into
the bulk from EDM cut surfaces. Future work will
determine whether the hydrides play a significant
role in the formation and/or stabilization of the β
phase lamellae observed in this material.
Titanium and its alloys find use in a number of fields including hydrogen storage, biomedical implants, and aerospace
applications among others. Commercially pure (CP) titanium is particularly interesting for its high strength-to-weight ratio, corrosion resistance, and
bio-compatability. It is also well known that Ti interacts strongly with hydrogen, therefore the goal of this work was to analyze the content and
distribution of hydrogen, introduced by EDM with subsequent annealing, and its relation to β phase present in heat treated CPtitanium.
BCC: Body Centered Cubic
BF: Bright Field
CP: Commercial Purity
EDM: Electro-Discharge Machining
FCC: Face Centered Cubic
FFT: Fast Fourier Transform
HCP: Hexagonal Close Packed
HR: High Resolution
TEM: Transmission Electron Microscopy
SAED: Selected Area Electron Diffraction
Glossary of Acronyms
The financial support provided by GACR
GBP108/12/G043 is greatly appreciated.
Motivation
Titanium grade 2 was
machined by EDM and then annealed slighty
above the transus temperature to yield an α+β
phase microstructure which was subsequently
studied via TEM. EDM followed by annealing
likely diffused surface hydrogen contamination
into the sample bulk. Lamellar structure of β
phase was observed and analyzed by HR-TEM
techniques. It was found that the lamellae were
composed of an ordered layering of β phase and
δ phase titanium hydride within the α phase.
Extruded titanium grade
2 was heat treated at 900°C for 1 hour to produce
an α+β duplex microstructure, with the β phase
being confined to grain boundaries and triple
junctions. Asummary of the preparation follows:
 CP grade 2 titanium (99.6 wt.% Ti)
 Received in extruded state
 Cut by wire EDM into billets
 Heat treated at 900°C for 1 hour and cooled
at a rate of 5°C/s to room temperature
 Cut by wire EDM into 3mm TEM discs
 Thinned by mechanical grinding to a
thickness of 100 microns
 Final thinning by twin-jet electropolishing
 Light microscope observation
 Observation by FEI X-Twin S/TEM at 200keV
 Standard TEM bright field/dark field
 FFT analysis of HR-TEM images
Abstract
Material
Methods
The focus of this study was on the lamellar
structures seen in approximately 5% of grains
observed by TEM. Due to the fine structure of the
lamellae (seen in (b)), characterization by SAED
was impractical. For this reason the FFTs of HR-
TEM images were used for phase identification,
as exemplified by images (c) and (d) of α phase
titanium. Three phases of titanium were
identified in the lamellar regions, and are listed in
the following table.
The lamellar structure present in our material,
with a “sandwich” of α/δ/β/δ/α, has been observed
previously in other alloys such as Ti-6Al-4V, but it
was not expected to be present in CPTi.
Image (b): BF-TEM
image of a region
with lamellar α/β
s t r u c t u r e . T h e
sample is oriented
with the lamellae in a
zone axis ([110] ) forβ
optimal contrast.
Image (a): Light microscopy
image of the sample showing microstructure and β
phase distribution. Regions containing β phase had a
lamellar structure
as seen in (b).
Image (e): Detail of the lamellar
structure in (b), with labeled phases.
(c) (d)HR-TEM of α-Ti FFT of (c)
2 nm
β
δ
(f)
4 nm
α
α
δ
β
δ
(e)
40 nm
(b)
1 μm
α
α
α
β
(a)
50 μm
α
β
α
The microstructure was composed of α phase
(α-Ti) grains, while the lamellae consisted
primarily of the expected β phase (β-Ti) but
flanked by δ phase titanium hydride (δ-TiH) which
acted as an interface phase between the α and β
titanium, as seen in image (e). All of the phase
boundaries were seen to be coherent
(exemplified by image (f)) with distinct orientation
relationships, identical to those observed in other
titanium alloys. The orientation relationships are
listed below.
Image (f): HR-TEM image of a phase boundary
between β-Ti and δ-TiH in (e), with inset FFTs of the
respective areas.
Summary of the Phases Identified
Phase Lattice Parameters
Name Structure a/b [Å] c [Å] α [°] β [°] γ [°]
α-Ti HCP 2.95 4.68 90 90 120
β-Ti BCC 3.28 3.28 90 90 90
δ-TiH FCC 4.44 4.44 90 90 90
Results
Conclusion
Acknowledgment
Image (c): HR-TEM of α-Ti from the
lamellar region in image (b).
Phase Orientation Relationships
[0001] // [001] // [110] , (010) // (220) // (112)α δ β α δ β
[2110] // [110] // [111] , (0001) // (002) // (110)α δ β α δ β
[2113] // [112] // [311] , (0110) // (220) // (112)α δ β α δ β
Image (d): FFT calculation of
image (c), indexed as [0001] .α