ULTRAFINE-GRAINED MATERIALS
Strength and fatigue properties enhancement in ultrafine-grained
Ti produced by severe plastic deformation
I. P. Semenova Æ R. Z. Valiev Æ E. B. Yakushina Æ
G. H. Salimgareeva Æ T. C. Lowe
Received: 19 June 2008 / Accepted: 2 September 2008 / Published online: 8 October 2008
Ó The Author(s) 2008. This article is published with open access at Springerlink.com
Abstract Severe plastic deformation (SPD) of titanium
creates an ultrafine-grained (UFG) microstructure which
results in significantly enhanced mechanical properties,
including increasing the high cycle fatigue strength. This
work addresses the challenge of maintaining the high level
of properties as SPD processing techniques are evolved
from methods suitable for producing laboratory scale
samples to methods suitable for commercial scale pro-
duction of titanium semi-products. Various ways to
optimize the strength and fatigue endurance limit in long-
length Grade 4 titanium rod processed by equal channel
angular pressing (ECAP) with subsequent thermal
mechanical treatments are considered in this paper. Low-
temperature annealing of rods is found to increase the
fatigue limit, simultaneously enhancing UFG titanium
strength and ductility. The UFG structure in titanium pro-
vides an optimum combination of properties when its
microstructure includes mostly equiaxed grains with high-
angle boundaries, the volume fraction of which is no less
than 50%.
Introduction
Investigations in recent years have shown that nanostruc-
turing titanium by severe plastic deformation (SPD)
techniques substantially increases its mechanical strength
[1–3]. This is attractive for its application in medicine and
engineering. Another important property for use of tita-
nium in structural applications is its fatigue limit. The
fatigue endurance limit in nanostructured titanium was
established [4] to increase, but not as much as might be
expected based on comparison of concurrent increases in
ultimate tensile strength or yield strength. Comparing the
titanium structure refinement obtained by various SPD
techniques, it should be noted that straining scheme, tem-
perature, applied load during treatment greatly impact the
size of formed grains and the associated grain boundary
types. The smallest grain sizes achieved via SPD process-
ing are produced by high pressure torsion (HPT), which
readily imparts grain sizes less than 100 nm [5]. However,
this SPD technique can be realized only on smaller sam-
ples. Bulk billets can be produced by omnifaceted forging
and ECAP, but the grain size will be 200 nm and larger
and, besides, there is often a substructure found inside the
grains [6, 7]. An important advantage of ECAP over om-
nifaceted forging is a more homogeneous structure
refinement in a billet and a greater process efficiency that is
important both for achievement of exceptional properties
and practical applications of nanomaterials.
Transition from laboratory scale SPD processing meth-
ods to commercial scale processing is a complex problem,
requiring development of ancillary processes to reduce
cost, improve operational efficiency, and ensure stable
reproducible properties. For example, to produce medical
implants by mechanical treatment, straight, round polished
rods 4–8 mm in diameter and 3000 mm in length are
I. P. Semenova (&) Á R. Z. Valiev Á E. B. Yakushina Á
G. H. Salimgareeva
Institute of Physics of Advanced Materials, Ufa State Aviation
Technical University, 12 K. Marx Str, Ufa 450000, Russia
e-mail: semenova-ip@mail.ru
R. Z. Valiev
e-mail: RZValiev@mail.rb.ru
T. C. Lowe
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
123
J Mater Sci (2008) 43:7354–7359
DOI 10.1007/s10853-008-2984-4