Microstructure, mechanical performance and oxidation mechanism
of boride in situ composites
Chang-Ming Chen
a,b,
*, L.T. Zhang
a
, W.C. Zhou
a
,
Z.Z. Hao
c
, Y.J. Jiang
c
, S.L. Yang
c
a
National Laboratory of Solidi®cation Processing, Northwestern Polytechnic University, Xi'an 710072, PRChina
b
Joint Open Laboratory of Nuclear Analysis, Shanghai Institute of Nuclear Research, Chinese Academy of Sciences,
Shanghai 201800, PRChina
c
Baotou Institute of Rare Earth Researche, Baotou 014010, PR China
Received 16 July 1999; received in revised form 2 June 2000; accepted 15 August 2000
Abstract
In situ composite of LaB
6
±ZrB
2
eutectic prepared by ¯oating zone melting is an interesting candidate for the manufacture of
turbine blades because of its excellent mechanical properties. Studies of a composite prepared by ¯oating zone melting showed that
the average diameter of the ZrB
2
®bers was $0.2±1.2 mm, with ®ber lengths up to 100 mm. LaB
6
was the ®rst phase to nucleate when
eutectic growth occurred, and ZrB
2
shows non-faceted growth. In an ingot solidi®ed with planar growth, the orientation relation-
ship of the phases was: growth direction [001]
LaB6
//[00.1]
ZrB2
, interfacial plane (110)
LaB6
//(12.0)
ZrB2
. Crack extensions along and
perpendicular to the direction of ®bers exhibit toughness of 8.7 and 17.8 MPa m
1/2
, respectively. The oxidation behavior of LaB
6
±
ZrB
2
eutectic in situ composites was studied in the temperature range of 912±1223
C and in the pressure range 1.5Â10
4
±7.2Â10
4
Pa.
The rate of weight change is described by k
p
=9.71Â10
À3
exp(À31000/RT)mg
2
cm
À4
min
À1
in the range 912±1094
C. Above
1094
C, rapid oxidation kinetics were observed, La
2
O
3
.
B
2
O
3
and La
2
O
3
.
3B
2
O
3
being the primary scale constituent with ZrO
2
de®-
ciency in the outer scale. # 2001 Elsevier Science Ltd. All rights reserved.
Keywords: LaB
6
±ZrB
2
eutectic; Microstructure; Mechanical performance; High temperature oxidation; In situ composite
1. Introduction
LaB
6
has received much attention as a good cathode
material. Recently, Ukrainian scientists [1] have fabri-
cated a LaB
6
±ZrB
2
eutectic in situ composite using a
radio-frequency heating zone melting method, and have
found that the composite not only reveals unusual
thermionic properties, but also has good fracture
toughness (up to 20.3±27.8 MPa m
1/2
), very high bend
strength (up to 1200 MPa), excellent thermal shock
resistance (500 K/min), and good microstructural stabi-
lity over a large temperature range. It appears that the
composite is potentially a new type of structural mate-
rials for use at elevated temperature up to 1650
C, and
is expected to be usable for the manufacture of turbine
blades [1]. Although good mechanical properties of
these composites have been reported, the morphology,
toughening mechanisms and oxidation behavior of the
LaB
6
±ZrB
2
eutectic are not clear. The aim of this work
is to investigate the microstructure and performance of
the directionally solidi®ed LaB
6
±ZrB
2
eutectic using a
¯oating-zone melting furnace.
2. Experiment
The LaB
6
and ZrB
2
powders used in this experiment
were produced at the Institute of Materials Problems in
the Ukraine. The purity of the LaB
6
was >99.4%, and
the purity of the ZrB
2
was >98.2%. The powders were
weighted in a ratio of 20 wt.% ZrB
2
, and mixed in an
ethanol slurry for 1 h, then uniaxially hot pressed at
1700
C and a pressure of 30 MPa for 30 min. The hot
pressed LaB
6
±ZrB
2
bars were directionally solidi®ed by
¯oating zone melting in a 0.3±0.5 MPa argon atmo-
sphere to form rods in a diameter of 8 mm [2]. Because
the directional solidi®ed composite yields an anisotropic
0266-3538/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0266-3538(00)00187-1
Composites Science and Technology 61 (2001) 971±975
www.elsevier.com/locate/compscitech
* Corresponding author. Fax: +86-21-5955-3021.
E-mail address: cmchen@online.sh.cn (Chang-Ming Chen).