MECHANICAL BEHAVIOR OF UDIMET 720LI SUPERALLOY
S. Chiozzi, V. Dattoma and R. Nobile
Università degli Studi di Lecce, Dipartimento di Ingegneria dell’Innovazione
Via per Arnesano, 73100 Lecce, Italy
[email protected] , [email protected] , [email protected]
ABSTRACT
The aim of the present work is to study the static tensile strength of Udimet 720Li superalloy at room and working temperature.
Slow strain rate tests have been performed on smoothed and notched cylindrical specimens, in order to evaluate material
ductility and notch sensitivity. Besides, a microstructural analysis has been done through Scanning Electron Microscope (SEM)
to analyze fracture surface and crack nucleation points, evidencing so the alloy brittle behavior and possible heat treatments
modifications in order to obtain better results at working temperature.
Introduction
Superalloys are the most diffused material for aeronautical and aerospace applications, mainly for turbines and compressors
production, because of their excellent resistance at high temperature. Among them, polycrystalline superalloy Udimet 720Li is
an interesting candidate for turbine disc application, due to its high temperature strength, good corrosion resistance and
excellent workability [1, 2].
This alloy has been developed starting from Udimet 720, which has excellent stress corrosion and high temperature
resistance, but which was found to be prone to sigma phase. Therefore, Li (“Low interstitial”) version has been developed to
minimize this problem. The main difference between Udimet 720 and Udimet 720Li consists in lower levels of carbon,
chromium and boron [3 - 6].
The aim of the present work is to study the static tensile strength of Udimet 720Li superalloy at room and working temperature.
Slow strain rate tests have been performed on smoothed and notched (kt ≈ 3.5) cylindrical specimens, in order to evaluate
material ductility and notch sensitivity. Tests have been performed at two different working temperature (650 and 700°C),
which covered all possible high temperature alloy employments.
This test plan is inserted in an extensive research project carried out in collaboration with Avio S.p.A., in which creep and high
temperature low cycle fatigue tests are planned also. Material is provided by Avio S.p.A., which is also responsible for heat
treatments, according to ASM standards for aeronautical components.
Moreover, a microstructural analysis has been performed through Scanning Electron Microscope (SEM) to analyze fracture
surfaces and crack nucleation points, evidencing alloy brittle behavior and possible heat treatments modifications in order to
obtain better results at working temperature.
Udimet 720Li superalloy
The alloy chemical composition in wt% is reported in Table 1.
Table 1
Al
B
C
Co
Cr
Mo
2.25 0.01 0.01 14.0 15.5 2.75
Ti
W
Zr
Ni
4.75
1.00
0.025
bal
This alloy has been developed from Udimet 720, which has excellent high temperature (< 700°C) mechanical properties due to
its structure and chemical composition [3].
The microstructure of Low interstitial (Li) version principally consists of two phases: a fcc γ matrix with a uniform grain size of
about 11.5µm and a large volume fraction of γ’ Ni3Al (about 3-5µm size). The γ’ exists in two forms. Coarse precipitates of
primary γ’, distributed mainly on the grain boundaries, and fine cuboidal γ’ embedded coherently through the γ matrix. In
addition to these precipitates, irregularly shaped TiN particles are present randomly scattered through the cross section [2, 3].
Experimental procedure
Tested specimens are reported in Figure 1.
Figure 1. Tested specimens
Tests have been performed both at room and working temperatures (650 and 700°C), so completely studying the alloy tensile
behavior.
Slow strain rate tests have been performed, according to ASTM E 8 for room temperature and ASTM E 21 for working
temperatures. A servo-hydraulic MTS 810 test machine has been used; for high temperatures tensile tests an electrical
resistance furnace has been employed (Figure 2), using a further thermocouple to measure the temperature on the specimen
surface. Tensile tests started when the temperature reached the test value, with a maximum difference of ±2°C. Strain has
been measured through an high temperature extensometer, with a gage length of 25mm.
Figure 2. Furnace employed
The same test procedures have been conducted both for smoothed specimens that for notched ones. A total number of 32
specimens have been tested: 16 smoothed and 16 notched.
Results
The typical qualitative stress-strain curves obtained for the three different tests temperature are following reported: room
temperature (Figure 3); 650°C (Figure 4); 700°C (Figure 5). Graphics have been normalized with respect to maximum values
of obtained stress and strain.
Figure 3. Room temperature tests
Figure 4. Tests at 650°C
Figure 5. Tests at 700°C
For notched specimens, stress has been evaluated on the base of the narrow section area, without considering the stress
concentration coefficient; the strain reported in x-axis was the nominal strain measured by the extensometer. This way, being
the notched specimens narrow section equal to the smoothed specimens one, it is possible to compare the two specimen
typologies.
From the curves analysis it is evident the different material behavior, due to the presence of the notch. In the case of notched
specimens, in fact, there is an evident reduction of the maximum elongation and a correspondent increase of the maximum
stress. Besides, there are differences between the material behavior at 650°C and the behavior at 700°C. In particular, it is
evident a greater elongation at 650°C. At room temperature material behavior is rather bilinear, with stress increase up to
fracture.
Microstructural analysis
Specimens fracture surfaces have been observed through Scanning Electron Microscope (SEM), in order to analyze cracks
initiation points. The fracture surface of a specimen tested at room temperature is reported in Figures 6 and 7.
Figure 6. Fracture surface, room temp.
Figure 7. Fracture surface, room temp.
Fracture results completely intergranular; it suggests the influence of “air environment” during crack propagation. This fracture
process can be identified as “cleavage”, because of impurities presence (metallic inclusions) inside the material. Besides,
fracture happens on 45° slanting surfaces.
Images relative to a tensile test at 650°C are reported in Figures 8 and 9.
Figure 8. Fracture surface, 650°C
Figure 9. Fracture surface, 650°C
Fracture directionality is evident from the previous micrographies. It is intergranular, as in the case of tests performed at room
temperature. Crack initiation started in the point indicated by the arrow in Figure 10, continuing then according to the direction
evidenced by the "lines".
Finally, fracture surfaces relative to a tensile test at 700°C are reported in Figures 10 and 11.
Figure 10. Fracture surface, 700°C
Figure 11. Fracture surface, 700°C
As for tests at room temperature and 650°C, fracture at 700°C is intergranular and it is evident its directionality. In no case
microcavities presence was evidenced, so we can conclude that there wasn’t the intervention of creep phenomenon.
Notched specimens: prevision model
There are several prevision models for mechanical behaviour of notched specimens. Generally, stress, ultimate and yield
values obtained for notched specimens depend from constitutive and strength material parameters (Young modulus E;
Poisson ratio ν; yield strength σy; strain hardening exponent n; tensile strength σr) for a given geometry; so they could be
expressed as a function of these five constants. These conclusions, taken in conjunction with considerations of dimensional
analysis, allow the functions for the yielding and maximum loads (Py and Pr) to be expressed in dimensionless form [7]:
σ y σ y

= f y 
,
,ν , n 
πa σ y
 E σr

Py
2
0
(1)
σ y σ y

Pr
,
,ν , n 
= f r 
2
πa 0 σ r
 E σr

(2)
where a0 is the radius of the minimum cross-section before the specimen is deformed; fy and fr are specific to each notch
geometry.
The experimental values are directly obtained by combining the results of the tension test performed with notched and
smoothed specimens for the same temperature and state of the material. Experimental values obtained from each notched
specimen are resumed in Table 2:
Table 2
Room temperature
650°C
700°C
1
1
2
3
4
5
Mean
1
2
3
4
5
Mean
fy
1.61
1.59
1.73
1.74
1.62
1.62
1.66±0.07
1.70
1.70
1.73
1.69
1.68
1.70±0.02
fr
1.18
1.42
1.39
1.45
1.33
1.37
1.39±0.05
1.56
1.57
1.55
1.57
1.53
1.56±0.01
Besides, it is possible to theoretically evaluate functions fy and fr and, consequently, notched specimens behaviour. For
example, Mackenzie [8] and Ritche [9] have identified the stress and strain distributions at the minimum cross-section with the
Bridgman’s analytical solution for the neck of a round bar under tension [10]:
 2R
P
≅ 1 + 0
2
a0
sπa 0 
 
a 
 ⋅ ln1 + 0 
  2 R0 
(3)
where s represents the engineering stress and R0 is the curvature radius of the notch profile at the minimum cross-section. In
this case we have obtained fy ≈ fr = 2.07.
As it is evident, theoretical value is rather different from experimental ones previous calculated. This is probably due to the fact
that the Bridgman’s solution is rigorously applicable only to blunt notches [7], while in this case we have sharp notches.
Experimental data show a dispersion, especially at 650°C. At 700°C there is an evident increase of fy and fr values, that
suggests the elasto-plastic material modification.
As analytical and experimental prediction differ significantly, a deeper analysis of the local stress state in the neighbourhood of
the notch has been carried out through FEM calculations. An elasto-plastic model of notched specimen has been built by
means of Ansys 10.0, by using axial symmetric element having a quadratic shape function. An isostropic hardening model has
been used and a Von Mises plastic flow rule has been assumed. Material data have been deduced by tensile tests executed
on smoothed specimen respectively at room temperature, 650 °C and 700 °C. Figure 12 reports the behaviour of local stress
in the notched specimen against radial distance. Trend is obtained in correspondence of the load that provokes failure for the
notched specimen tested at 650 °C. By the examination of this figure it is evident the effect of plasticization and subsequent
stress relaxation in the proximity of notch toe.
1
POST1
STEP=2
SUB =27
TIME=1.614
PATH PLOT
SY
1.000
0.949
0.897
0.846
σ/σmax
0.794
0.742
0.690
0.638
0.587
0.535
0.483
0
.636
.318
1.272
.954
1.908
1.59
r [mm]
2.544
2.226
3.175
2.862
Figure 12. Stress behaviour in the notched section for 650°C test
Numerical calculation allows to determine the local stress state in correspondence of failure. These local conditions are briefly
resumed in the following Table 3. The effect of temperature is to reduce the stress conditions that provokes failure in terms of
plastic zone extension, maximum stress and maximum plastic strain. Moreover, the stress state ratio, that is the ratio between
hydrostatic pressure and Von Mises equivalent plastic stress is reduced by increase in temperature.
room temperature
650 °C
700 °C
plastic zone extension
[mm]
0.41
0.38
0.35
Table 3
normalized
maximum stress
1.0725
1.0005
0.9495
maximum plastic strain
[mm/mm]
0.0736
0.05115
0.03903
stress state
ratio
2.396
2.020
1.585
Conclusions
In the present work a static tensile strength evaluation of Udimet 720Li superalloy was carried out. The study has been
conducted through slow strain rate tests on smoothed and notched specimens. Tests have been performed both at room
temperature and at 650 and 700°C, evidencing the different material behavior.
Besides, a prevision study of the notched specimens behavior on the basis of results obtained with smoothed specimens has
been performed. A numerical elasto-plastic model has been developed to calculate local stress field at the notch before failure
occurs.
Currently, creep and low cycle fatigue tests are being performed; results will be explained in next papers.
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