Microstructure and hardness of Co-Cr-Ti alloys for dental castings

Abstract

Effect of adding titanium on the microstructure and hardness of as-cast Co-Cr base dental alloys have been investigated with the purpose to determine the region of their optimal characteristics. Twelve samples of Co-Cr-Ti alloys with increasing titanium content (4 to 12 at.%), which were prepared by arc-melting technique in argon atmosphere, revealed similar two-phases dendritic microstructure. It consist of dendrites and interdendritic region, which differ in crystal structures (fcc, hcp) and slightly in compositions. The hardness of alloys increases significantly with titanium and chromium content. The as-cast alloys with smaller titanium content (samples 3 to 5) have promising features for dental applications, because their hardness values are in good agreement with that of similar commercial dental alloys.

Full text

METALURGIJA 43 (2004) 4, 273-277 273
LJ. SLOKAR et al.: MICROSTRUCTURE AND HARDNESS OF Co-Cr-Ti ALLOYS FOR DENTAL CASTINGS
Received - Primljeno: 2003-11-05
Accepted - Prihvaæeno: 2003-03-25
Original Scientific Paper - Izvorni znanstveni rad
ISSN 0543-5846
METABK 43 (4) 273-277 (2004)
UDC - UDK 669.255´26´295:620.18=111
LJ. SLOKAR, T. MATKOVIÆ, P. MATKOVIÆ
Lj. Slokar, T. Matkoviæ, P. Matkoviæ, Faculty of Metallurgy University
of Zagreb, Sisak, Croatia
INTRODUCTION
Since stainless steel was first used as an implant mate-
rial in surgery in the early 1930s, a number of metallic
materials, such as cobalt base alloys, titanium or titanium
alloys, have been developed for biomedical applications
[1]. They belong to the wide group of biomaterials, which
are the tools leading the way in the battle to make the life
longer, healthier and more complete for many individuals.
Each biomaterial has advantages as well as disadvantages,
and no one is the best material, but suited for different
circumstances. Therefore, research and development of
new metallic materials with higher quality in terms of both
biomedical and mechanical properties, are of interest.
Cobalt-chromium alloys are used over 60 years as im-
plant materials in dentistry and medicine owing to their high
corrosion resistance and biocompatibility. Commercial den-
tal as-cast alloys (with optimal composition approx. 70 at.%
Co - 30 at. % Cr) contain small quantities of alloying ele-
ments as carbon, molybdenum, wolfram, nickel, etc. The
outstanding properties of Co-Cr alloys result from the crys-
tallographic nature of cobalt, the solid- solution- strength-
ening effect of chromium and alloying elements, the forma-
tion of extremely hard carbides and corrosion resistance
imparted by chromium. The problem, however, is rather low
ductility of these base-metal alloys. Therefore considerable
research activities take place with the aim of improving
ductility by the alloying elements addition [2, 3].
In recent years, titanium and its alloys have received
much attention. They are currently the most commonly used
dental and orthopedic implant materials because of their
excellent biocompatibility, high corrosion resistance, low
density, good balance of mechanical properties and relati-
vely low price. However, this alloys have low shear strength,
wear resistance, castability and formability [4 - 6].
Considering that Co-Cr alloys and titanium belong to a
few metallic materials which fulfill the strong requirements
MICROSTRUCTURE AND HARDNESS OF Co-Cr-Ti ALLOYS FOR DENTAL CASTINGS
Effect of adding titanium on the microstructure and hardness of as-cast Co-Cr base dental alloys have been
investigated with the purpose to determine the region of their optimal characteristics. Twelve samples of Co-Cr-
Ti alloys with increasing titanium content (4 to 12 at.%), which were prepared by arc- melting technique in argon
atmosphere, revealed similar two-phases dendritic microstructure. It consist of dendrites and interdendritic re-
gion, which differ in crystal structures (fcc, hcp) and slightly in compositions. The hardness of alloys increases
significantly with titanium and chromium content. The as-cast alloys with smaller titanium content (samples 3 to
5) have promising features for dental applications, because their hardness values are in good agreement with
that of similar commercial dental alloys.
Key words: Co-Cr-Ti alloys, microstructure, hardness, dental alloys
Mikrostruktura i tvrdoæa Co-Cr-Ti legura za dentalne odljevke. Ispitivanje utjecaja dodatka titana na
mikrostrukturu i tvrdoæu lijevanih dentalnih legura na osnovi kobalta i kroma provedeno je s ciljem, da se ustanovi
podruèje njihovih optimalnih karakteristika. Dvanaest uzoraka Co-Cr-Ti legura s rastuæim udjelom titana (4 -12
at.%), koji su preparirani u elektro-luènoj peæi pod zatitnom atmosferom argona, pokazuje sliènu dvo-faznu
dendritnu mikrostrukturu. Ona se sastoji od dendrita i meðudendritnog podruèja, koji se razlikuju u kristalnim
strukturama (fcc, hcp) i neznatno u sastavima. Tvrdoæa legura znaèajno raste s udjelima titana i kroma. Lijevane
legure s manjim sadrajem titana (uzorci 3-5) mogle bi se primijeniti u stomatologiji, jer je njihova tvrdoæa slièna
vrijednostima za komercijalne dentalne legure.
Kljuène rijeèi: Co-Cr-Ti legure, mikrostruktura, tvrdoæa, dentalne legure

METALURGIJA 43 (2004) 4, 273-277274
LJ. SLOKAR et al.: MICROSTRUCTURE AND HARDNESS OF Co-Cr-Ti ALLOYS FOR DENTAL CASTINGS
for corrosion resistance and biocompatibility, the good per-
formances for Co-Cr-Ti alloys can be also expected. The
available data confirm that the addition of titanium to Co-
Cr alloy can improve some mechanical and physical prop-
erties [7 - 9] but on the other side, they show insufficiency
in the investigation of this topic. Therefore, the purpose of
this study was to prepare and investigate some selected Co-
Cr-Ti alloys as potential for biomedical application.
MATERIALS AND METHODS
Alloys of cobalt-chromium and titanium were prepared
by melting the elements (with purity better than 99,9 %) in
a laboratory arc-furnace in argon atmosphere.
The samples were remelted three times in order to
achieve homogeneity. Their casting was performed using
specially constructed cooper anode which served also as a
casting mould (Figure1.).
The samples of cylindrical shape (dimensions about
7×12 mm) were obtained after melting. They were em-
bedded in an epoxy resin and metallographically prepared
with wet grinding on 120 - 1200 grit SiC paper and final
polishing with Al2O3 water suspension. Etching was ac-
complished with solution of 50 ml HCl, 5 ml HNO3 and
50 ml H2O warmed on 50 °C, or with Krolls reagent (1-3
ml 40% HF, 2 - 6 ml HNO3 conc., 100 ml H2O).
The microstructure of the alloys was investigated us-
ing quantitative optical metallography and scanning elec-
tron microscopy. The etched samples were observed by
optical microscope (Leitz, Ortholux) at the magnification
of 280 ×. Optical micrographs taken with digital camera
(Olympus DP11), were analysed with the corresponding
programmes (Olympus DP-Soft, UTHSCA Image Tool).
Selected sample of Co-Cr-Ti alloy was analysed by scan-
ning electron microanalyser (Jeol, JXA 50 A).
X - ray diffraction pattern are taken on polished disc
surface from 10 to 100° of 2G value using CuK= - radia-
tion (Philips, PW 3710).
Hardness was measured by Vickers method (30 N, 10
s) on the equipment of Otto-Wolpert-Werke. Microhard-
ness measurements were performed on the microhardness
tester Leica, VHMT (0,49 N, 10 s). Heat treatment was
performed on the sample discs approximately 5 mm thick,
which were cut from the as-cast cylindrical samples.
RESULTS AND DISCUSSION
Twelve samples of Co-Cr-Ti alloys have been prepared
to examine the effect of titanium on the microstructure and
hardness of cobalt-chromium alloys. Their chemical com-
positions with increasing titanium content from 4 at.% to
12 at.%, were selected to be close to that of similar com-
mercial as-cast dental alloys (Table 1.). Positions of experi-
mental alloys are illustrated on the isothermal section of
ternary system Co-Cr-Ti at room temperature (Figure 2.).
Figure 1.
Slika 1.
Ex
p
erimental arc-meltin
g
furnace
Eks
p
erimentalna
p
eæ za luèno tal
j
en
j
e
Metal vacuum
j
ar
Vacuum seal
Vacuum
or inert
g
as
suppl
y
Vacuum
seal
Handle
Water
Water
Water Water
Cu-anode
Cu-mold +

W-cathode
Illumination
Sample No.
Composition
/ %
Interdendritic
region light
/ %
Dendrites
region dark
/ %
Average area
of grains
/ m
µ
1 Co%$  "
Cr Ti
Co% # "
Cr Ti
Co$$ ! "
Cr Ti
Co$ !# "
Cr Ti
Co%  &
Cr Ti
Co$% # &
Cr Ti
Co$ ! &
Cr Ti
Co#% !# &
Cr Ti
Co$&  
Cr Ti
Co$! # 
Cr Ti
Co#& ! 
Cr Ti
Co#! !# 
Cr Ti
33,28 66,72 53,65
2 36,79 63,21 130,14
3 30,83 69,17 172,54
434,18 65,82 310,06
5 38,89 61,11 192,06
6 29,18 70,82 175,10
7 59,04 40,96 607,26
8 54,01 45,99 422,73
9 58,13 41,87 151,42
10 42,68 57,32 67,01
11 64,31 35,69 55,72
12 74,25 25,75 79,99
Average peri-
meter of grains
/ m
µ
40,48
83,35
67,49
100,17
79,15
78,85
189,21
143,19
60,29
54,31
54,76
72,35
Table 1.
Tablica 1.
Com
p
ositions of as- cast Co-Cr-Ti allo
y
s and the results
of
q
uantitative o
p
tical metallo
g
ra
p
h
y
Sastav li
j
evanih Co-Cr-Ti le
g
ura i rezultati kvantitativ-
ne o
p
tièke metalo
g
rafi
j
e

METALURGIJA 43 (2004) 4, 273-277 275
LJ. SLOKAR et al.: MICROSTRUCTURE AND HARDNESS OF Co-Cr-Ti ALLOYS FOR DENTAL CASTINGS
The microstructural examination of the as-cast Co-Cr-
Ti alloys by optical microscopy shows that all samples
have nearly the same typical dendritic two phases micro-
structure. This type of microstructure can be seen on the
microphotograph of representative sample 4 (Figure 3.)
with the composition Co61Cr35Ti4, which is close to that of
commercial dental alloys. It consists of dendrites (dark
phase) and interdendritic region (light phase), with two
types of Co-Cr-Ti solid solutions [9]. Figure 4. shows the
microstructure of sample 12 with the highest content of
titanium. The results of quantitative metallography (Table
1.) indicate that the content of phases depend on alloy com-
positions, so that the percentage of dendrites increases
with titanium content.
Compositions of dendrites and interdendritic region in
sample 4 were determined by the method of point analy-
sis in EDX. The non-etched samples were used for obtain-
ing the correct average concentration of elements. The
selective chemical
etching results in
enrichment or im-
poverishment of
elements on the
sample surface and
therefore, to the
ambiguous results
of the phase com-
positions (Table 2.)
and subsequently
to the wrong con-
clusions in [10].
Correct data from
Table 3. show that
chemical composi-
tions of dendrites
and interdendritic
region are very
close. That suggests, that not the composition, but the crys-
tal structure is the reason for different properties of these
phases [11].
X-ray diffraction data for sample 4 (Table 4.) confirm
that dendrites and interdendritic region are Co-Cr-Ti solu-
tions with hcp and fcc type of crystal structures.
Cr / %, at.
Cr
Co
1
5
9
2
6
10
3
7
11
4
8
12
CoTi
Co Ti
Cr Ti
Co Cr
!Co Cr
%&
Ti
Co / %, at.
Ti / %, at.
Figure 2.
Slika 2.
Com
p
ositions of ex
p
erimental allo
y
s
p
resented on the iso-
thermal section of ternar
y
Co- Cr-Ti s
y
stem at room tem-
p
erature
Prikaz sastava eks
p
erimentalnih le
g
ura na izotermièkom
p
res
j
eku ternarno
g
Co-Cr-Ti sustava kod sobne tem
p
e-
rature

100 m
µ
Figure 3.
Slika 3.
O
p
tical micro
g
ra
p
h of as-cast Co Cr Ti allo
y
Metalo
g
rafsk a snimka li
j
evane Co Cr Ti le
g
ure
$ !# "
$ !# "
Figure 4.
Slika 4.
O
p
tical micro
g
ra
p
h of as-cast Co Cr Ti allo
y
Metalo
g
rafsk a snimka li
j
evane Co Cr Ti le
g
ure
#! !# 
#! !# 
100 m
µ
Alloy
Co Cr Ti
$ !# "
Composition / %
Interdendritic
region
(light phase)
Des
dark
endrit
( phase)
62,35 35,52 2,42
51,05 45,38 3,55
Table 2.
Tablica 2.
Chemical com
p
ositions of
dendrites and interdendritic
re
g
ion for as-cast Co Cr Ti
etched allo
y
Kemi
j
ski sastav dendrita i
meðudendritno
g
p
odruè
j
a li-
j
evane Co Cr Ti na
g
riene
le
g
ure
$ !# "
$ !# "
Co Cr Ti

METALURGIJA 43 (2004) 4, 273-277276
LJ. SLOKAR et al.: MICROSTRUCTURE AND HARDNESS OF Co-Cr-Ti ALLOYS FOR DENTAL CASTINGS
The results of hardness measurement (Table 5.) indi-
cate significant hardness change with the alloy composi-
tion. Hardness of as-cast alloys increases primary with ti-
tanium content and than with chromium content. The high-
est hardness of 1012 HV has very brittle sample 12 with
the greatest contents of titanium and chromium. This value
is triple compared to the hardness of sample 1, with the
minimum contents of mentioned elements. Microhardness
measurements on sample 4 (Table 3.) indicate that
interdendritic region has greater microhardness, as noble
phase with fcc structure, which etched more slowly (light
phase). Dendrites which etched easier (dark phase) have
lower microhardness, as a less noble phase.
Effect of two different heat treatments on the micro-
structure and hardness of all as-cast alloys have been in-
vestigated. After heat treatment at 800 °C for 10 hours and
quenching into water all samples show higher hardness
values (average 10 %). Another heat treatment was per-
formed at 800 °C for 10 hours followed by slow cooling
in the furnace. Comparing with the first treatment, no
change of microstructure and hardness was observed. That
suggests an important role of cooling conditions during
alloy solidification, which determine the morphology of
the grains. Therefore, the subsequently heat treatments of
as-cast alloys, without preliminary deformation, can not
induce nucleation and crystal growth i.e. recrystallization
and thereby decrease of hardness. Among twelve samples
of as-cast Co-Cr-Ti alloys only the samples 3 to 5 have
hardness values in good agreement with that of commer-
cial Co-Cr base dental alloys (for example Wironit) i.e.
about 350 HV (MPa) [9].
CONCLUSIONS
The investigations of the microstructure and hardness
of as-cast Co-Cr-Ti alloys resulted with the following con-
clusions:
1. Optical metallography showed that all twelve samples
of the as-cast Co-Cr-Ti alloys have similar two-phases
dendritic solidification microstructure, typical for com-
mercial dental alloys.
2. The chemical compositions of dendrites and interden-
dritic region in sample Co61Cr35 Ti4 were found to be
very close to each other using scanning electron mi-
croanalyser and method of point analysis.
3. X - ray diffraction indicates that dendrites and inter-
dendritic region are Co-Cr-Ti solid solutions with dif-
ferent, hcp and fcc type of crystal structures.
4. The results of hardness measurement revealed strong
influence of different alloy chemistry, so that hardness
increases with titanium and chromium content. Among
all experimental alloys, only the samples 3 to 5 with
smaller titanium content have hardness values in agree-
ment with that of similar commercial dental alloys.
Table 4.
Tablica 4.
The results of diffraction analyses for alloy Co Cr Ti in
the as-cast condition
Rezultati difrakcijske analize za lijevanu Co Cr Ti le-
guru
$ !# "
$ !# "
No. 2 / °
θ
d(obs.)
d -solid
solution
Co(hcp) d -solid
solution
Co(fcc)
1 42,1 2,15 2,17 -
2 43,0 2,10 - 2,05
3 45,7 1,99 2,02 -
4 46,7 1,94 1,91 -
5 50,8 1,79 - 1,77
6 90,7 1,08 - 1,07
7 91,0 1,08 1,07 -
Table 5.
Tab li ca 5 .
The results of Vickers hardness (30 N, 10 s) for Co-Cr-Ti
allo
y
s
Rezultati Vickersove tvrdoæe (30N, 10 s) za Co-Cr-Ti le-
g
ure
Sample
No.
Alloy
/ % HVHV
1 298 307 302 302
Co%$  "
Cr Ti
Co% # "
Cr Ti
Co$$ ! "
Cr Ti
Co$ !# "
Cr Ti
Co%  &
Cr Ti
Co$% # &
Cr Ti
Co$ ! &
Cr Ti
Co#% !# &
Cr Ti
Co$&  
Cr Ti
Co$! # 
Cr Ti
Co#& ! 
Cr Ti
Co#! !# 
Cr Ti
2 299 308 307 305
3 318 314 314 315
4 364 370 356 363
5 373 366 379 373
6 396 397 382 392
7 518 486 504 503
8 660 665 661 662
9 513 506 514 511
10 555 561 555 557
11 689 681 699 690
12
Co Cr Ti
Alloy
Co Cr Ti
$ !# "
Composition / % HV
Microhardness
HV
459
Interdendritic
region
(light phase)
Des
dark
endrit
( phase)
63,07 34,65 2,27 446 452
450
61,67 35,51 2,80
248
236
261
248
Table 3.
Tablica 3.
Chemical com
p
ositions of dendrites and interdendritic
re
g
ion for as-cast Co Cr Ti non-etched allo
y
and mi-
crohardness HV values (0,49 N, 10 s)
Kemi
j
ski sastav dendrita i meðudendritno
g
p
odruè
j
a za
li
j
evanu Co Cr Ti nena
g
rienu le
g
urui HV vri
j
ednosti
mikrotvrdoæe (0,49 N, 10 s)
$ !# "
$ !# " 

METALURGIJA 43 (2004) 4, 273-277 277
LJ. SLOKAR et al.: MICROSTRUCTURE AND HARDNESS OF Co-Cr-Ti ALLOYS FOR DENTAL CASTINGS
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