Hot Forging of IN718 with Solution-Treated and Delta-Containing Initial Microstructures
Metallogr. Microstruct. Anal.
Hot Forging of IN718 with Solution-Treated and Delta-Containing Initial Microstructures
H. M. Lalvani 0 1 2
J. W. Brooks 0 1 2
0 The University of Birmingham , Edgbaston, Birmingham B15 2TT , UK
1 Advanced Forming Research Centre, University of Strathclyde , Glasgow PA4 9LJ , UK
2 & H. M. Lalvani
A systematic study of the effect of d phase precipitate morphology on the hot deformation behavior and microstructural evolution in nickel superalloy Inconel 718 is presented. Isothermal compression tests at fixed nominal strain rates and temperatures relevant to industrial forging (0.001-0.3 s-1 and 990-1040 C) were used. Three distinct initial microstructures have been examined: (I) solution treated, (II) a microstructure with finely dispersed particulate d precipitates, and (III) a microstructure containing dense network of intragranular and grain boundary d platelets. The peak flow stress associated with these various microstructures has been rationalized using a single, temperature-compensated power law. This clearly demonstrates opposition of the external applied stress by an internal back stress related to the initial d phase morphology and apparent delta solvus temperature. Post-peak flow softening is attributed to dynamic recrystallization, aided by the dissolution of finer precipitates in material containing particulate d phase, and to a certain degree of mechanical grain refinement caused by distortion and offsetting of grain segments where a dense d-platelet structure exists.
Bulk deformation; Delta phase; Nickel-based superalloys; Thermomechanical processing; Recrystallization
Introduction
IN718, a nickel-based superalloy, is widely used in
aeroengine applications due to its strength and stable
microstructure at elevated temperatures. These
high-temperature properties of IN718 are attributed to slow growth
kinetics of c00 precipitates and make IN718 a prime
candidate for forged turbine disks.
Significant research has been directed toward
characterizing the precipitation and dissolution kinetics of the d
phase in IN718 during heat treatment and aging, e.g., [
1–5
],
and several experimental investigations have explored
recrystallization associated with high-temperature
compressive flow in solution-treated material, e.g., [
6–10
], but
there has been rather less focus on the specific role of d
precipitates during hot deformation. However, it is known
that prior aging in the d stability field can have a significant
effect on stress–strain response [
11, 12
] and recent studies
have highlighted the complexity of the mechanisms
involved, including dynamic dissolution and
reprecipitation [13], sub-grain formation [
14
], and platelet
spheroidization [
15
].
A key concern in the manufacture of critical components
is how the variability in initial and evolving microstructure
might be reasonably characterized and quantified for
incorporation into process models of high-temperature
forging operations. A systematic study of the flow behavior
and microstructural evolution in IN718 obtained during
small-scale compression experiments using three distinct
microstructures has been presented in this paper. A brief
synopsis of flow stress and microstructure comparison
between a solution-treated and acicular d-containing
microstructure has been published by authors [
16
].
However, the current paper provides an in-depth analysis of the
role of the d phase in influencing dynamic recrystallization
and flow softening in IN718. Furthermore, a constitutive
law has been formulated in terms of an internal back stress
that takes into account the initial d phase morphology and
distribution.
Material and Experimental Methodology
The material used in the present work was supplied as
standard billet of 178 mm diameter that had undergone a
typical annealing and aging cycle. It was heat-treated at
980 C for 1 h and water-quenched followed by an aging
treatment at 720 C for 8 h. The billet was then
furnacecooled to 620 C, aged for a further 8 h, and finally
aircooled. The chemical composition (in weight %) of the
billet contains 50.50 Ni, 19.10 Cr, 18.78 Fe, 5.28 Nb, 4.15
Mo, 1.06 Ti, 0.613 Al, 0.130 Co, 0.115 W, 0.039 V, and
0.033 C. Microstructure analysis of the billet material
revealed a c-matrix grain size of 40–60 lm and densely
distributed d precipitates with varying morphology across
the billet diameter: predominantly fine, blocky d particles
near the outer billet diameter, becoming increasingly
acicular toward the billet center. Since the precipitation and
dissolution kinetics of the d phase have a complex
dependence on the time, temperature, phase morphology,
and local Nb concentration [
2
], a single value for the solvus
temperature is difficult to determine [
4
]. The equilibrium
delta solvus calculated using thermodynamic software
JMatPro was 1033 C. However, in order to plan a testing
program, an approximate ‘apparent’ d solvus temperature,
derived using material con (...truncated)