Cobalt-Based Alloys

Cobalt--Based Alloys
Cobalt
History of CoCo-based Alloys
• Originated from investigations in the early 1900s,
when p
patents covering
g alloys
y from the systems
y
CoCr and Co-Cr-W were issued.
• The resulting alloys have been under development
every since.
• C
Common trade
t d names such
h as Stellite
St llit and
d Hastelloy
H t ll
(both developed by the Haynes Corp.).
• Very important for cutlery, machine tools, and wearresistant hardfacing applications.
History
• 1930’s
– Co-Cr-Mo alloy Vitallium developed for dental prosthetics.
• 1940
1940’s
s
– HS-21 (derived from Vitallium) becomes important for
turbochargers and gas turbine applications.
– Wrought Co-Ni-Cr
Co Ni Cr alloy S-816
S 816 used for gas turbine blades and
vanes.
– Co-Ni-Cr-W alloy X-40 developed in 1943. Still used in gas
turbine vanes.
• 1950-1970
1950 1970
– Ni-based superalloys strengthened by γ′ developed. Rapidly
surpassed capabilities of Co alloys. Co-based alloys lack
comparable precipitation hardening mechanism (“doesn’t
( doesn t this
sound similar to Mg alloys?”)
• 2000’s
– Discovery of something new and exciting!
Some typical CoCo-based Alloys and uses
Co
FSX-414
Bal.
Stellite 21
Bal.
Stellite 31
Bal.
MarM302 Bal.
MarM509 Bal.
Haynes-188 Bal.
Ni
10.5
2
10
–
10
22
Cr
29.5
28
20
21.5
23.4
22
Al
–
–
–
–
–
–
Ti
–
–
–
–
0.25
–
Mo
–
5.5
–
–
–
–
W
7
–
15
10
7
14.5
Ta
–
–
–
9
3.5
–
B
0.012
–
–
0.005
–
–
*maximum amount
Characteristics & Uses
FSX-414
Stellite 21
Stellite 31
MarM302
MarM509
Haynes-188
Gas turbine vanes
Wear resistance
Wear resistance
Jet engine blades
blades, vanes
Jet engine blades, vanes
Better oxidation resistance than Hastelloy X
Predominantly composed of Co, Ni, Cr, W
Zr
–
–
–
0.015
0.35
–
C
0.25
0.3
0.1
0.85
0.6
0.1
Other
2 Fe
–
–
–
–
3 Fe*
0 90La
0.90La
Advantages vs. NiNi-based Alloys
• Higher melting points and flatter stress-rupture
curves.
curves
– Results in higher stress capability to higher absolute
temperatures than Ni-base (or Fe-base) alloys.
• Better hot corrosion resistance in contaminated
gas turbine atmospheres due to their higher Cr
contents.
• Better weldability and better thermal fatigue
resistance than Ni-base alloys.
Disadvantages vs. NiNi-based Alloys
• Lower strength
• Lower ductility and fracture toughness at
ambient
bi t ttemperatures.
t
• Li
Limited
it d opportunity
t it ffor iimprovementt off currentt
alloys.*
Chemistry of CoCo-Based Alloys
• Chemical compositions are analogous to
stainless steels
steels.
• Th
The roles
l off the
th major
j and
d minor
i
alloying
ll i
elements are virtually identical as both are
“austenitic”
austenitic (i.e.,
(i e FCC-based)
FCC based) alloy systems
systems.
General Characteristics
• Austenitic matrix (FCC crystal structure)
• 20 – 30% Cr is added to provide oxidation and hot
corrosion resistance. This is the most important
addition.
dditi
• C
Cr also
l provides
id some solid
lid solution
l ti strengthening
t
th i and
d
is of vital importance in precipitation strengthening.
• Additional solid solution strengthening is provided via
additions of Ta,, W,, Nb,, Mo.
General Characteristics – cont’d
• Precipitation hardening via carbide formation.*
– Co alloys generally contain 0.25-1.0%C.
– Co-base
C b
alloys
ll
are h
heatt ttreated
t d tto control
t l precipitation,
i it ti
which controls properties.
• Ni or Fe (up to 20%) are added to stabilize the FCC
phase, thus suppressing the transformation to HCP
Co at low temperatures.
• Can be prone to TCP phase formation, in particular for
high Cr contents (>58%)
*NOTE: Nitrogen is often substituted for carbon in the “carbide” phases.
Physical Properties of Co
HCP when T<421°C
FCC when T>421°C
Co is also ferromagnetic
Tmp is 50°C greater than Ni
Alloying Element
Change in Melting Temperature (°F)
Raise
Tungsten
+1
Lower
Nitrogen
‐1
Iron
‐1
Chromium
‐5
Molybdenum
‐8
Vanadium
‐15
Manganese
‐15
Aluminum
‐20
Tantalum
‐30
Zirconium
‐30
Sulfur
‐40
Titanium
‐65
Niobium
‐70
Silicon
‐75
75
Boron
‐115
Carbon
‐120
Phase Equilibria
BCC
FCC
HCP
A.M. Beltran; “Cobalt‐Base Alloys;” in Superalloys II, C.T. Sims, N.S. Stoloff, A
M B l
“C b l B
All
”i S
ll
II C T Si
N S S l ff
and W.C. Hagel, editors; (John Wiley & sons, New York, 1987) p.139.
Predel, B.: Co‐Cr (Cobalt‐Chromium). Madelung, O. (ed.). SpringerMaterials ‐
The Landolt‐Börnstein Database (http://www.springermaterials.com). DOI: 10 1007/10086082 907
10.1007/10086082_907
Most alloys are predominantly composed of Co + Cr, Ni, and/or W
Functions of Alloying Elements
Nickel
Chromium
Tungsten
g
Ti, Zr, Nb,
, , , Ta C
Principal function
Austenite stabilizer
Surface stability + carbide former
Solid‐solution strength
MC formers
Carbide formation
Problems Lowers corrosion resistance
Forms TCP phases
Forms TCP phases
Harms surface stability
Decreases ductility
X‐40
10
25
7.5
‐‐‐
0.45
MM‐509
MM
509
10
24
70
7.0
3.5 Ta, 0.5 Zr, 3
5 Ta 0 5 Zr
0.2 Ti
0 60
0.60
L‐605
10
20
15.0
‐‐‐
0.10
HS‐188
22
22
14.0
‐‐‐
0.08
when added in excess
Examples
• Small amounts of Aluminum (~ 5 wt.%) has been added to improve oxidation and hot corrosion
resistance. Used extensively in coatings.
• Additions of Titanium,
Titanium have been shown to form a γ′ phase,
phase (Co
(Co,Ni)
Ni)3Ti.
Ti but to also stabilize undesirable
HCP-Co3Ti or Co2Ti-Laves phases).
• Rare earth additions (0.08-0.15 wt.%) increases oxide scale adhesion and reduces oxidation
kinetics(“RE effect”).
Phase Equilibria
BCC
FCC
HCP
Predel, B.: Co‐Cr (Cobalt‐Chromium). Madelung, O. (ed.). SpringerMaterials ‐
The Landolt‐Börnstein Database (http://www.springermaterials.com). DOI: 10.1007/10086082_907
A.M. Beltran; “Cobalt‐Base Alloys;” in Superalloys II, C.T. Sims, N.S. Stoloff, A
M B l
“C b l B
All
”i S
ll
II C T Si
N S S l ff
and W.C. Hagel, editors; (John Wiley & sons, New York, 1987) p.141.
Alloying additions selected to stabilize FCC or HCP phases
oy g add o s se ec ed o s ab e
o
p ases
Remember our discussion of alloy design.
FCC/HCP transition  Possibility of Stacking Faults
Alloying
A.M. Beltran; “Cobalt‐Base Alloys;” in Superalloys II, C.T. Sims, N.S. Stoloff, and W.C. Hagel, editors; (John Wiley & sons, New York, 1987) p.142.
• Dislocation interaction w/ faults produces strengthening.
• Faults are also preferential nucleation sites for carbides; can degrade properties.
Faults are also preferential nucleation sites for carbides; can degrade properties.
• FCC‐stabilizing additions (e.g., Ni) are added to inhibit SF formation & carbide formation during HT exposure.
Carbides in CoCo-based alloys
• Primary
Pi
strengthening
t
th i precipitate:
i it t incoherent
i
h
t cubic
bi carbides.
bid
• Comparative C contents:
– Austenitic stainless steel
– Ni-base superalloy (cast)
– Co-base superalloy (cast)
0.02-0.20 wt.%
0.05-0.20 wt.%
0.25-1.0 wt.%
Solubility limits at 1260°C
A.M. Beltran; “Cobalt‐Base Alloys;” in Superalloys II, C.T. Sims, N.S. Stoloff, and W.C. Hagel, editors; (John Wiley & sons, New York, 1987) 144
1987) p.144.
Carbides in CoCo-based Alloys
A.M. Beltran; “Cobalt‐Base Alloys;” in Superalloys II, C.T. Sims, N.S. Stoloff, and W.C. Hagel, editors; (John Wiley & sons, New York, 1987) p.145.
• Carbide
Carbide formers come from groups to the left of Co in the periodic table formers come from groups to the left of Co in the periodic table
(Goldschmidt’s criteria). These elements are more electronegative and thus more reactive than Co.
Classes of Carbides
• M3C2, M7C3, and M23C6 Carbides.
– These are basically chromium carbides containing Co
Co, W
W, or Mo
in place of Cr.
– M3C2 has a rhombic crystal structure and forms via a peritectic
reaction with Cr. It has been observed in some of the early
superalloys with low Cr contents.
– M7C3 has a trigonal crystal structure and forms at low Cr/C
ratios. It can be dissolved during solution treating and in some
cases transforms to M23C6 during aging
aging.
Classes of Carbides
• M3C2, M7C3, and M23C6 Carbides.
– M23C6 generates very potent precipitation strengthening
strengthening. It is the
result from decomposition of the M7C3 carbides via reactions of
the form:
23C 7 C3  7C
23Cr
7Cr23C6  27C
6C+23Cr  Cr23C6
decomposition
Re‐precipitation
– In M23C6 carbides, some heavier atoms often substitute for Cr
yielding the following formulae:
(Co,Ni) x (W,Mo) y (C,B,Si) z
Cr18Co3Mo 2 C6
Cr17 Co 4 W2 C6
Classes of Carbides
• M3C2, M7C3, and M23C6 Carbides.
– Tend to form via a eutectic type reaction:
– In
I castt alloys
ll
M23C6 comes outt interdendritically.
i t d d iti ll
– Specific carbide morphology depends upon cooling rate.
•M3C2, M7C3, and M23C6 Carbides.
– Morphology will depend on alloy
chemistry and cooling rates.
M3C2, M7C3, and M23C6 Carbide Morphology
•
Cast alloys:
Interdendritic M23C6
In interdendritic areas, slow cooling results in a eutectic
microstructure consisting of alternating plates of  (FCC) and 
((M23C6) p
phases.
Fast cooling leads to change in morphology of M23C6.
Classes of Carbides
• M6C and MC Carbides
– These carbides are rich in refractory elements. They are used to
strengthen wrought and investment cast Co
Co-base
base alloys.
– MC carbides are typically of the form: TaC, HfC, NbC, etc.,
– Similar to Ni-base alloy systems, M6C carbides are generally
found in low-Cr alloys with Mo and/or W levels > 4–6 at.%.
– M6C carbides are typically of the form:
M 3 M3C or M 4 M3C
(Co 0.45Cr0.3Ta 0.15 W0.1 )C
Classes of Carbides
• M6C and MC Carbides
– M6C carbides often form during
g service via a decomposition
p
reaction:
MC + austenite  M 6 C
– Example:
TaC + (Co,Ni,Cr,C)  (Co,Ni) 4 (Cr,Ta) 2 C
Microstructural Variants MC Carbides
Chinese script eutectic carbide
Blocky or acicular carbides
Blocky carbides tend to develop when N2 is present.
Other phases
• We stated that it was possible to form ′, just not the same ′ as
a Ni-base superalloy.
• In
I these
h
alloys
ll
γ′′ = (Co,Ni)
(C Ni)3Ti.
Ti
However, the phase is unstable
above 760°C.
Also, once you add Cr, it no
longer forms.
Mechanical Working
• We want to break down the coarse carbides that
form during
g solidification. This can be done by
y
mechanical working.
Figure taken from Co: Cobalt in Superalloys, The Cobalt Development Institute: 1985; page 10
Solution Treatment and ReRe-PPTn.
Figure taken from Sims, Figure
taken from Sims
Stoloff, and Hagel; Superalloys II, John Wiley & Sons: 1987; page 155
Can work with certain drawbacks.
It is also possible to form TCP phases (, , Laves). Laves)
Steps must be taken to avoid them. They are undesirable as they are sites for the initiation d
bl
h
f h
of fracture. Carbides strengthen these alloys, but are also potential sites for fracture initiation.
Something New!
• Co-based -′ Alloys
• Based on ternary Co-Al-W
Co Al W
• Off
Offer potential
t ti l for
f superalloy
ll type
t
performance
f
at higher temperature.