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Experimental Temperature Dependence of Stress-Elongation, Young’s

Modulus and Fracture Stress for two Max Phase Bulk Materials

C. Sylwan*1, S. Reichardt and T. Eriksson

Royal Institute of Technology - KTH, Teknikringen 42, SE-100 44 Stockholm,

Sweden

Abstract.

A recently discovered material, the Max phases, has properties between metals and ceramics.

In order to judge their usefulness for achieving higher temperatures in the hot zones in gas

turbines, some mechanical properties have been determined at various temperatures. The

stress for 0.2% elongation, Young’s modulus and fracture stresses were calculated from

stretch tests over the temperature range 22-1400 ºC.

The Max phases were Maxthal 211 with and without niobium addition. In some regions

comparisons were made with Inconel. The Young modulus was calculated by equation fitting

from the elongation curves at several temperatures.

Keywords: Young’s modulus, fracture stresses, Maxthal, Maxphases, rods, stress-elongation,

creep specimen, surface study

Introduction. This work is the core part of the Mistra-financed project “Fundamental study on the use of

Max phases as material in micro turbines”, carried out at KTH by the Departments of

Chemical Engineering Technology (KET), Chemical Technology (KT) and Heat and Power

Technology (EGI). The project main goal was to investigate the possibility to employ

Maxthal, Ti2AlC (211) titanium aluminium carbide and Ti2AlC + Nb (211Nb) consisting of

niobium-doped titanium aluminium carbide, two newly discovered ceramic materials in gas

turbines.

Today, super alloys are usually used in high-temperature locations in gas turbines, interlaced

with some sophisticated internal cooling designs. Static parts in the hottest places, e.g. guiding

vanes without cooling air, would allow the turbine to run hotter with increased average engine

temperature which, according to the Carnot rule, would lead to improved turbine efficiency.

Based on 40 recorded, out of about 50 in all, stress-elongation tests at Swerea Kimab, at

temperatures of 22 to 1400 ºC, stress for 0.2% elongation, Young’s modulus and also fracture

stresses were studied. Our findings from these experiments were used for estimation of how

well Maxthal could fit into turbines.

Specimens. The test pieces were rods made of bulk material manufactured and supplied by Kanthal AB in

Hallstahammar, Sweden. The bulk material was then machined into two different sizes of test

rods which subsequently were stretch tested at several temperatures at Swerea Kimab in

Stockholm, Sweden.

The two different sizes were 7mm TMF rods, length 130mm, and SIMR creep specimen SK

25, which is 5mm across in the centre part and has a length of 50 mm. Since the material was

scarce by the time the tests were performed, the rod sizes were chosen so that the two 10 mm

diametre ends of a ruptured large rod could be utilized for the manufacture of two small rods

and hence the material in a large rod could be tested three times.

1 *Corresponding author: csylwan@kth.se

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The Max phases were the commercially available Maxthal 211 with a density of 4.04-4.06

and a variety with addition of 5-10 % niobium, Maxthal 211 Nb, density 4.15-4.16.

The tests were performed in special devices at Swerea Kimab where a surrounding coil heats

the rod by induction. The Maxthal materials have very high electrical conductance which

makes it possible to use this method which normally is utilized for metal alloy specimens.

While heated, the elongation due to stress is measured.

Results.

Below are stress-elongation diagrams for Maxthal 211 and Maxthal 211 Nb.

Diagram 1, stress-elongation diagram from various Maxthal 211 tests.

Diagram 2, stress-elongation diagram from various Maxthal 211 Nb tests.

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Diagram 3, stress-elongation diagram from Maxthal 211 and 211 Nb tests at 1300 and

1400º C

Stress for 0.2% elongation. The stress values have in this case been read from Figures 1, 2 and 3 in the Excel display

which permits distinguishing values of curves by pointing at them. The results are displayed

in the diagrams below together with values for Inconel 792 as deciphered from a curve

received from Siemens Turbomachinery in Finspång [1]. The point “Inconel Kimab”

represents a test ordered by us using an existing Inconel test rod, of the large type.

The absence of values below 1200º C for 211 and below 1000 ºC for 211 Nb is due to the fact

that those specimens were already broken before attaining 0.2% elongation.

Diagram 4, stress for 0.2% elongation Inconel versus Maxphases

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Diagram 5, stress for 0.2% elongation, detail at higher temperatures.

Young’s modulus.

In order to evaluate the E moduli at different temperatures from the diagrams, a special

procedure was utilized: Using Excel, second degree polynomials were fit to the individual

curves. Special attention was given to the part of the curves approaching zero strain, because

the curves often do not give zero stress for zero strain. The reason for this is that a stress is

sometimes deliberately applied in order to enhance stability when strain is set to zero. The

curve fitting of Excel cannot then always deal well with this part of the curve. For that reason

Swerea Kimab recommends that all these curves be drawn by hand. In order to avoid a

transport of data out of and into the computer, we have settled for an adjustment of some

curve stress values so that there is zero stress when there is zero strain, so that the curves look

good to the eye. This adjustment is possible in Excel to an extent where the curves become

similar to hand drawn.

After this treatment the polynomials were differentiated so that the inclinations at zero strain

(the values of the E moduli) could be listed. See diagrams below.

Diagram 6, E-modules for Maxthal 211 at different temperatures.

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Diagram 7, E-modules for Maxthal 211 Nb at different temperatures.

Since it has been stated by Kanthal [2] that the manufacture of Maxthals still is at a prototype

stage which may result in imperfections in the materials, we should use only the best values if

more than one is available. We have chosen to omit values lower than the highest for each

temperature studied. The result is a new diagram where only the highest value for each

temperature is shown; regardless of whether it was achieved with a small or a large rod.

These values have also been used to fit curves and functions (evaluated with Excel).

For Maxthal 211 we found E = -9E-11T4 + 9E-08T3 - 2E-05T2 - 0,1021T + 291,44 and for

Maxthal 211 Nb E = -1E-07T3 + 5E-05T2 - 0,1007T + 337,91 where E (bold) is the

E modulus and E is the exponent. The functions could be used to transfer Young’s modulus

data into the simulation program Femlab.

Diagram 8, Highest values E-modules for Maxthal and Inconel at different

temperatures.

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Table 1, E-values for some different temperatures, as calculated from the polynoms.

Temperature [C] E-211 E-211 Nb [GPa]

0 291 338

20 289 336

500 241 288

1000 169 188

1350 40 48

Polynomials for two of the curves, as well as the corresponding curves, in figure 8 above were

derived using Excel. The Inconel X-750 points are derived from a polynomial in the program

library of Comsol’s Femlab [3]. This program library contained, in May 2009, twenty

polynomials for the variation of Young’s modulus with temperature in Kelvin. The X-750-

curve had the closest fit to our experimental point for Inconel Kimab at 1200 °C.

Fracture stress.

The fracture stress was also evaluated from the curves of diagrams 1,2 and 3, simply as the

maximum stress obtained at different temperatures for the individual test rods. See figures

below.

Diagram 9, Fracture stress for Maxthal 211 at different temperatures.

Diagram 10, Fracture stress for Maxthal 211 Nb at different temperatures.

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Those values have also been used to fit curves and functions (evaluated with Excel).

For Maxthal 211 the fracture stress is - 3E-07T3 - 0,186T + 134.5 and for

Maxthal 211 Nb the fracture stress is -4E-07T3 - 0,319T + 218.1 These functions could be

used in Femlab.

Fracture–surface studies of Maxthal up to 1200 °C Fracture surfaces were studied using microscope, involving both Maxthal 211 and Maxthal

211 Nb. The test series stretched from room temperature to 1200 °C with different

magnifications up to 500 X. Some of the 500X-SEM pictures are presented below.

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Picture 1gives a clear view of a brittle* ceramic at room temperature. The ceramic’s large

“flakes” are clearly visible at 500 X enlargement.

Picture 2, however, is slightly different since the surface seems smoother. The reason for that

might be the inversion of some niobium in the ceramic matrix in order to make it stiffer and

more durable, hence giving it longer elongation curves. This trend is even more prominent in

the following pictures.

Picture 1. Maxthal 211, room temperature. Picture 2. Maxthal 211 Nb, room

temperature.

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From picture 3, without niobium in the ceramic matrix, the “flakes” are still clearly visible*

when fractured at 800 °C, but that is not the case in picture 4. With niobium in the structure

the absence of brittleness is striking *.

The following pictures show 1000 °C fractures.

Footnote: *) The nature of brittleness of the materials was established during microscopic

evaluation by WenLi Long at Department of Materials Science and Engineering; KTH.

The following pictures show 1000 °C fractures.

Picture 6, with niobium in the matrix, show signs of transformation towards ductile state. The

next sets of pictures are from 1200 °C fractures.

Picture 3. Maxthal 211, 800°C.

Picture 4. Maxthal 211 Nb, 800°C.

Picture 5. Maxthal 211, 1000°C.

Picture 6. Maxthal 211 Nb, 1000°C.

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The results for Maxthal 211 were analysed at KIMAB. Brittle facture appeared on virtually

every surface we tested up to 1200°C, where suddenly a ductile transition took place.

The above points to a DBTT (ductile-brittle transition temperature) at around 1100 °C for this

ceramic, compared to –70, –80 °C for steel. The same could be said for Maxthal 211 Nb.

However while niobium noticeably strengthens the ceramic it does little for lowering its

DBTT.

Another conclusion is that while niobium favourably influences tensile strength it is less

successful on strain.

Longitudinal cut-surface studies of Maxthals at 1300 °C and 1400 °C

At the temperatures of 1300 and 1400°C, no fracture surfaces were studied by microscope.

Instead, some rods were cut longitudinally through the narrow part and the cut surfaces were

analysed at KTH using SEM and back scatter detector, see below

Picture 7. Maxthal 211, 1200°C. Ductile State. *)

Picture 8. Maxthal 211 Nb, 1200°C. Ductile

state. *)

Picture 18. Same as for 17 but specimen 211 Nb-

A, (Maxthal 211 Nb).

Picture 17. Specimen 211-A2, (Maxthal 211)

1300°C. The thin line is thought to be oxidation.

The dots are empty cavities or they could be

locally Al2O3 corns.

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After the specimens were examined, two possible explanations to interpret the pictures

emerged. According to one view, the oxidation of the ceramic is negligible. The “dots”

observed within them are simply empty cavities or local Al2O3 grains, already present after

the fabrication process. The second version is less favourable. According to this view the

oxidation is mild at low temperatures, but as the heat rises, the corrosion increases; thus the

Al2O3 grains were not present at the beginning, but when high temperatures are reached, small

portions, observed as “dots”, begin to migrate deeper and deeper into the ceramic causing the

material to be weakened.

Discussion.

Elongation:

Note that there are no 0.2 % elongation values for 211 below 1200 ºC since the specimens

were broken already before attaining 0.2 % elongation, whereas for 211 Nb there are no

corresponding values below 1000 ºC.

Maxthal demonstrates a higher dimensional stability than Inconel above 1150 ºC and this

seems to be the most obvious advantage of Maxthal when taking to account the results from

stress-strain measurements.

Young’s modulus:

Young’s modulus is generally higher for Maxthal 211 Nb than for Maxthal 211 at all

temperatures. Compared to Inconel, Maxthal has considerable higher E values up to 1100 ºC,

according to our experiments and literature [3]. Our own experiment at 1200 ºC shows equal

E values for Inconel and Maxthal. Between 1200 ºC and 1400 ºC we have no E values for

Picture 20. Specimen 211-C, (Maxthal 211)

1400°C. The marked area with red line

implies some broader oxidation in the matrix.

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Inconel, but E probably falls rapidly in this region since 0.2 % elongation values for Inconel

point towards zero stress around 1250 ºC in diagram 4.

Fracture stress:

There is a clear tendency for Maxthal 211 Nb to be tougher at most temperatures except for

the highest. However, Maxthal 211 Nb is more brittle at room temperature: When machining

the last batch of test rods when material was being prepared for 8 specimens of Maxthal 211

Nb and 20 specimens of Maxthal 211, all of the latter were finished whereas two of the

former broke in the lath. There is also a clear tendency for the small rods to be less tough than

the large ones, especially in the case of 211 Nb.

Fig. 11, Fracture stress for Maxthal 211 Nb, best values at each temperature according

to fig. 10 and ultimate strength of 718 according to Duck-Hoi Kim et al. [4]

Fig.9 shows the typically much higher ultimate strength of an Inconel at lower temperatures

compared to Maxthal but also the steep decline beginning under 800ºC, while the fracture

stress of 211 Nb starts to decline slower but from a much lower level between 1000ºC and

1200 ºC. This is a similar behaviour as displayed in fig. 4, stress for 0.2% elongation, where

the Maxthals demonstrate a higher ability to withstand stress above 1150ºC. In some sense

this comparison fills in the gap for Maxthal in fig. 4 at temperatures below 1000ºC.

References.

[1] Xin-Hai Li of Siemens Turbomachinery, Finspång, personal communication.

[2] Jens-Petter Palmqvist of Kanthal, Hallstahammar, personal communication.

[3] Femlab Chemical Engineering Module, version 2.3 by COMSOL AB, Stockholm

[4] Duck-Hoi Kim, Jae-Hoon Kim, Jeong-Woo Sa, Young-Shin Lee, Chui-Kyu Park and

Soon-Il Moon, Materials Science and Engineering:A

Volumes 483-484, 15 june 2008, Pages 262-265

14th International Conference on the Strenght of Materials.