Ab-initio study of alloying effects on structure stability and mechanical
properties of
a
-Nb
5
Si
3
Songxin Shi
a
, Linggang Zhu
a,b
, Lina Jia
a
, Hu Zhang
a,
⇑
, Zhimei Sun
a,b,
⇑
a
School of Materials Science and Engineering, Beihang University, Beijing 100191, PR China
b
Center for Integrated Computational Materials Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, PR China
article info
Article history:
Received 4 April 2015
Received in revised form 25 May 2015
Accepted 16 June 2015
Available online 3 July 2015
Keywords:
Ab-initio calculations
Nb–Si based superalloys
Site occupancy
Elastic properties
abstract
The tendency to dissolve in the matrix for alloying elements such as transition metals and some main group
elements in
a
-Nb
5
Si
3
phase as well as their effects on the structure stability and mechanical properties are
important for the performance of niobium-silicide based alloys, which, however, are not clear yet. In this
work, we unravel the above problems based on ab-initio calculations. Our results show that the alloyed
Nb
5
Si
3
systems become less stable as the alloying elements change from group IVB to VIB in the periodic
table. The occupation preferences of the alloying elements depend on their relative atomic radii respect to
either Nb or Si. Furthermore, the dissolution of the alloying elements is easier at high temperatures by the
Debye model analysis, from which the homogeni zation-treatment temperatures of alloyed Nb
5
Si
3
phases
are also deduced. All alloyed Nb
5
Si
3
phases are mechanically stable, even though their mechanical
properties like ductility are not improved. Finally, the electron localized function, Bader charge and
densities of states are used to understand the structure stability of alloyed Nb
5
Si
3
, and we find that ionic
bonding has quite significant effects on the stability of these intermetallic compounds.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
Among all the promising candidate materials that may replace
Ni-based superalloys for the application in turbine blades,
niobium-silicide based alloys have attracted growing attentions
due to their combinations of high melting point (>1750 °C), low
density (6.6–7.2 g cm
3
), attractive stiffness and excellent high
temperature strength [1–4]. The niobium-silicide based alloys are
a combination of niobium solid solution (Nb
SS
) and various
Nb-silicides, with the former providing fracture toughness and
good ductility while the latter offering excellent high temperature
strength and oxidation resistance [2].
a
-Nb
5
Si
3
with superior com-
pression resistance and high temperature strength is the mostly
obtained Nb-silicides after heat treatment [5–6]. However, the
unsatisfactory mechanical properties such as poor deformability
of
a
-Nb
5
Si
3
is one of the obstacles for the practical applications
of niobium-silicide based alloys [7].
Extensive experimental works have shown that adding alloying
elements is an effective way to improve the comprehensive prop-
erties of Nb–Si alloys [8,9]. Studies focused on alloying effects on
a
-Nb
5
Si
3
phase are very limited, and in most of the literatures mul-
ticomponent systems rather than ternary systems with a single
alloying element were used. For example, Li et al. studied the
Nb–18Si–5Ge–5Al and Nb–24Ti–18Si–5Ge–5Al alloying systems
and made the conclusion that the cooperative effect of Ti, Al and
Ge would increase the hardness of Nb
5
Si
3
by Vickers hardness tes-
ter [10]. Nevertheless, the above mentioned Nb–Si multicompo-
nent systems can not be used to distinguish whether the alloying
effects come from the contribution of certain single element or
the complex co-functionalization of all the alloying elements. In
fact, for the alloy designers, information about the effects of single
element on stability and mechanical properties should be more
useful. Therefore, in the present work, we perform a thorough
investigation on the influence of each frequently used alloying ele-
ment (Ti, Zr, Hf, V, Ta, Cr, Mo, W, B, Al or Ge) on the structure and
properties of
a
-Nb
5
Si
3
by using first-principles supercell calcula-
tions. Compared to experimental work, apparently, theoretical
study is much cheaper considering the time and material con-
sumed in experiments, for example, single crystal is needed to
measure the important mechanical parameters – elastic constants,
however, single crystal of
a
-Nb
5
Si
3
is difficult to fabricate due to its
high melting point. Moreover, simulations based on electronic
structure calculations can also provide some important physical
information, such as chemical bonding of the intermetallics, which
is helpful to understand their properties. In this work, we focus on
http://dx.doi.org/10.1016/j.commatsci.2015.06.019
0927-0256/Ó 2015 Elsevier B.V. All rights reserved.
⇑
Corresponding authors at: School of Materials Science and Engineering, Beihang
University, Beijing 100191, PR China (Z. Sun).
E-mail addresses: zhanghu@buaa.edu.cn (H. Zhang), zmsun@buaa.edu.cn
(Z. Sun).
Computational Materials Science 108 (2015) 121–127
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Computational Materials Science
journal homepage: www.elsevier.com/locate/commatsci