OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Mo-Si-B Alloy Development
J. H. SchneibelMetals and Ceramics Divison, ORNL
R. O. Ritchie, J. J. Kruzic Materials Science Division, LBNL
17th Annual Conference on Fossil Energy Materials Baltimore, MD, April 22-24, 2003.
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Why Ultra-High Temperature Materials?Vision 21 is about EFFICIENCY:Service temperature ↑ Efficiency ↑
Nickel-base superalloys: ≤ 1000ºCODS ferritic steels: up to 1400 º C(but low strength and high stress exponent)
Need ultra-high temperature, high strength materialsBest effort, high risk researchLong lead time from research to production (20 years)
Applications: sensor protection, heat exchangers, 1st stage vanes
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Ultra-High Temperature MaterialsBarriers: •Melting point•Oxidation resistance•Fracture toughness•Creep strength
Options:•Simple crystal structures – expensive (Pt, Ir)•Complex crystal structures – brittle
Scientific approach:macro- and microalloying Innovative processing to create optimum microstructure
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Why Mo-Si-B Alloys?
0.0 0.1 0.2 0.3 0.4 0.5
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1.0 0.0
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Molybd
enum BoronMo5SiB2
Mo3Si Mo5Si3Mo
•Melting point ≈2000ºC•High strength intermetallics•Equilibrium phases•Oxidation resistance: MoSi2•Toughening: metallic Mo phase
Berczik Akinc
Silicon
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Mo-Si-B alloys are finding interest
Molybdenum-Borosilicide WorkshopOrganized by Airforce, Navy, Pratt&WhitneyAnnapolis, Maryland, March 11 and 12, 2003
TMS Symposium “Beyond Nickel-Base Superalloys”Charlotte, NC, March 14-18, 2004
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Strategy for improving the mechanical properties
of Mo-Si-B Intermetallics
Unlikely that intrinsic brittleness of Mo3Si and Mo5SiB2can be alleviated in the near future
Engineer microstructure to minimize detrimental effect of brittle phases
Focus of this talk on:Microstructure topology and scaleMechanical properties of α-Mo
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
A sucessful microstructure for creep:nickel-base superalloys
CMSX-4
Single crystal superalloy
(Kazim Serin, Post-Doc at ORNL)
Continuous γ solid solution matrixCreep occurs in the γ channels between γ’ (Ni3Al) precipitates
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Optimizing Fracture Toughness
•High α-Mo volume fraction
•Continuous α-Mo
•Coarse α-Mo
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Mo-12Si-8.5B (at. %) cast&annealed (24h/1600ºC)≈40 vol. % discontinuous α-Mo
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Crush cast Mo-Si-B and “coat” powder with Mo
Evaporation of Si and/or: Mo3Si + 1/2O2 SiO↑ + 3 Mo
Mo5SiB2 + 1/2 O2 SiO ↑ + 2 Mo2B + Mo
Mo-20Si-10B powder particle after 16 h at 1600°C in vacuum
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Hot isostatic pressing of Mo-Si-B powderin niobium can (4h/1600°C/30ksi)
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
HIPed Mo-Si-B with continuous α-Mo matrix:nominal composition Mo-15Si-10B; 30 vol.% α-Mo.
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Mo-Si-B alloys with continuous α-MoHIPed from Si-evaporated Mo-20Si-10B
545-90 µmMedium_Low4990-180 µmCoarse3445-90 µmMedium
34≤ 45 µmFine
α-Mo volume fraction, %
Powder size prior to Si semoval
Specimen Designation
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Tensile Testing of Buttonhead Specimensat 3.3×10-3 s-1
Room temperature:premature fracture at 140 MPa:Need to eliminate flawsMo matrix needs improvement
1200ºC in vacuum:0.2% yield stress 336 MPamaximum stress 354 MPaductility 1.8%
“Coarse” microstructureContinuous α-Mo matrix
49 vol. %
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Creep strength
Determine compressive stress-strain curves at a strain rate of 10-5 s-1
Define creep strength as flow stress at 2% plastic strain
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600
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Stre
ss, M
Pa
Strain, %
Mo-9.5Nb-12Si-8.5B1300C
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Creep Strength at 1300°C(10-5 s-1, 2% plastic strain)
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Cre
ep S
treng
th, M
Pa
Cast&annealed40 vol. % α-Mo(discontinuous)
Fine34 v/o α-Mo
Medium34 v/o α-Mo
Medium_Low5 v/o α-Mo
Coarse49 v/o α-Mo
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Rigorous Fracture Toughness Testing
•Disc-shaped compact tension specimens
•Fatigue pre-cracking
•Cycling at low stress intensity:
remove bridging in crack wake
•Crack length from elastic compliance
•Determine resistance curve (R-curve)(Crack-growth resistance KR vs. crack extension ∆a
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Room Temperature Fracture Toughness
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Stre
ss In
tens
ity, M
Pa m
1/2
Crack Extension, mm Mo-12Si-8.5B40 vol% α-Mo
“medium”34 vol% α-Mo
“coarse”49 vol% α-Mo
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
What can we do to improve toughening efficiency of α-Mo?High toughening efficiency
less α-Mo, better oxidation resistance
• α-Mo ligaments < 1 µm
• Microalloying of α-Mo
• Ductilization of α-Mo by spinel particles
(Mike Brady: chromium)
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Fe-40 at. % Al/TiC:FeAl fractures usually by cleavage;Fracture mode of FeAl ligaments depends on thickness
Fracture stress of FeAl ligament: 500 MPa1 dislocation in pile-up: L=Gb/[πτ(1-ν)]=30 nm70 dislocations in pile-up: ideal cleavage stress for L=2 µmExpect ductile fracture for ligaments < 2 µm
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
FeAl ligaments show ductile fracture for thickness less than 2 µm:
Size scale important for fracture toughness
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Additions of Ti and Zr improvethe ductility and strength of Mo ( TZM):Try this approach with Mo-Si-B alloys
Screening tests:
Flexure tests with chevron-notched specimens
G=W/AKq=[(E×G)/(1-ν2)]1/20
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0.015
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Load
, kN
Displacement, mm
4 mm
3 mm
notch
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Zr additons improve room temperature fracture toughness,
probably by improving properties of α-Mo
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Mo-12Si-8.5B Mo-12Si-8.5B-1.5Ti Mo-12Si-8.5B-1.5Zr
K q, MPa
m1/
2
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Ductilization of Mo by adding spinel particles (MgAl2O4)
M. P. Brady recently revisited the Scruggs mechanism(1965) for the ductilization of Cr by spinel particles
Scruggs showed that mechanism works for Mo as well
Consolidate Mo powder (2-8 µm) and 3.4 wt% MgAl2O4spinel powder (1- 5 µm) in graphite hot-press:4h/1800°C/20MPa/vacuum
Carry out room temperature flexure tests
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Ductilization of molybdenumby spinel particles (MgAl2O4)
Optimum particle size and volume fraction?Collaboration with Bruce Kang, WVU
OAK RIDGE NATIONAL LABORATORYU. S. DEPARTMENT OF ENERGY
Summary and Conclusions
•Processing of Mo-Mo3Si-Mo5SiB2 with continuous α-Mo matrix•Control of microstructural scale and α-Mo volume fraction•Limited tensile ductility at 1200C•Qualitative correlation between microstructure and creep strength•High room temperature fracture toughness & R-curve behavior•Zr microalloying additions improve fracture toughness•Spinel particles improve ductility of Mo
Future work: Continue to focus on the mechanical properties of α-Mo phase