centrations of alloying additions and have lower
creep strength than PM alloys. They also are sig-
ni
fi
cantly less expensive to make and fabricate. As
was the case for Ti alloys, melting technology for
ingot metallurgy Ni base alloys also has evolved
over the past ~35 years. Today triple melting is
commonly practiced to ensure composition and
inclusion control and homogeneity of the
fi
nal pro-
duct. In the case of Ni base alloys, triple melt prac-
tice starts with a vacuum induction
fi
rst melt, fol-
lowed by an electroslag second remelt and a
fi
nal
VAR melt. This practice has led to a major
reduction of melt related defects such as freckles
and white spots and large inclusions that were the
cause of rotor failures during the early days of Ni
base alloy rotor development. As a consequence
of these improvements, with the exception of the
inherent temperature limitations of IM alloys, these
materials are excellent in terms of reproducibility
and reliability.
The use of powder metallurgy (PM) technology
to make turbine disks also has evolved since its
inception ~25 years ago. Alloys are made into
powder if they contain such high concentrations of
solute that large (
30 cm diameter) ingots cannot
be cast without having unacceptable levels of
freezing segregation. The use of PM methods for
producing rotor alloys solves this problem but
requires the utmost care. For example, a highly
disciplined powder handling process is essential to
avoid accidental incorporation of unwanted con-
taminants that can have deleterious effects on
fatigue behavior. A single, large inclusion in a high
stress region of a rotor can cause a rotor burst with
disastrous consequences.
A typical processing sequence is as follows:
1. Inert gas atomization to form powder;
2. Screening of the powder to a predetermined
maximum powder particle size, typically
270
mesh (44
μ
m);
3. Vacuum degassing of the powder
4. Placing the powder in an extrusion can
5. Extruding the powder into a billet using an
extrusion ratio that assures full density;
6. Forging the billet into a
fi
nal forged part, often
by an isothermal, hot die forging process.