Lecture 7 - Nickel Alloys Flashcards
What is chromium’s effect on nickel?
oxidation and hot corrosion resistance, solid-solution strengthening
What is molybdenum and tungsten’s effect on nickel?
solid solution strengthening, M6C carbides
What is aluminum and titanium’s effect on nickel?
forms gamma’ hardening precipitate; Al enhances oxidation resistance and Ti forms MC carbides
What is cobalt’s effect on nickel?
raises gamma’ solvus temperature
What is boron, zirconium, and hafnium’s effect on nickel?
improves rupture life through increased ductility
B forms borides
Hf forms MC carbides and promotes eutectic gamma-gamma’ formation in cast alloys
What is carbon’s effect on nickel?
forms MC, M7C3, M23C6, M6C carbides
What is niobium’s effect on nickel?
forms gamma’, Ni3Nb, hardening precipitate; forms delta orthorhombic Ni3Nb
What is tantalum’s effect on nickel?
solid solution strengthening; forms MC carbides, enhances oxidation resistance
What are the microstructures of manufactured Inconel alloys like?
complex and messy
Why is post-processing treatment necessary?
since it is difficult to control the phase fraction of as-built materials, the microstructure has poor properties and HIPing is needed to tailor material properties
Which properties are critical for Inconel alloys?
fatigue and creep at elevated temperatures (porosity is a big concern)
What do the macro and micro residual stresses affect for printing Inconel 718?
Type I - part dimensional accuracy
Type II - mechanical properties
What is the difference between Inconel 625 and 718?
625 - solid-solution matrix-stiffened face-centered-cubic-alloy
718 - age-hardenable austenitic material, strength dependent on precipitation of gamma prime during heat treatment, higher value, adds niobium
What are topologically close-packed phases?
phases formed by high levels of refractory elements to increase high temperature creep and rupture properties
undesirable brittle phases formed during heat treatment or service
appear as discs and are easy crack origins
What phases do TCP’s include?
sigma (Fe-cr, 50-50) the worst phase for ductility
mu
Laves
What is the Ni3Al?
gamma’, geometrically close-packed
What are properties of Inconel 625?
melt point 1300C
thermal expansion coefficient of 1.28 10^-5/K
resists wide range of temperature extremes
What are applications for Inconel 625?
marine, aerospace, nuclear, industrial components
What are applications for Inconel 718?
jet engine, gas turbine
What are the current approaches to print 718?
L-PBF
L-DED
EB-DED
EB-PBF
Binder Jet
How high is the rupture strength for SLM alloys?
low
Why add Al to Ni?
Reduce weight
How can you control hardness with aging?
aim for peak aging (not over or under)
What are the two more important properties for elevated temperatures for Ni?
mechanical strength, creep
What is the typical application temperature for Ni?
600-700C
What is the relationship between gamma’ and creep rupture strength, and temperature?
creep rupture strength increases with gamma’
creep rupture strength decreases with temperature
Why is there a push to increase temperature limit for Ni alloys?
higher temperature limit -> higher operating temperature -> higher efficiency
What are the factors of the harsh operating environment?
high temperature, high stress, high oxidation
Why is Inconel 625 relatively easy to print?
no phase transformation, but some voids
What is the Laves phase
extremely brittle
What is a TTT diagram?
time temperature transformation that shows the phases transformed into based on operating temperature over time
Can you heat treat or add precipitates to Inconel 625?
no
Which is easier to print?
625
Why does porosity present an issue for 718?
Since applications are thin, need to control precision
Do laves phase and needle-like delta phase make the metal stronger?
yes - unless there are cracks
Do we want grain boundaries for IN718?
No
What limits ductility?
micro cracks, voids
What is HIP?
Hot isostatic pressing
What does an enhanced ductility after processing mean?
lots of pores initially
Why do SLM 718 have poor fatigue properties?
porosity
