High Temperature Fracture

By: Chris Cantoni, Nate Fenstemaker, and Austin Ryan

Intro:

High-temperature fracture is the crystallographic and diffusional flow of atoms in a material’s lattice based on the temperature, applied stress level, and stress state conditions of the overall material. This is different from fracture, which is only defined by crystallographic flow. Three primary modes of high-temperature fracture exist: rupture, intergranular creep fracture, and transgranular creep fracture. Each mode involves void growth and nucleation as being the causes for fracture occurring. These modes of high temperature fracture are especially evident in the failure of turbine blades, which are exposed to elevated temperatures for long intervals of time.

For different crystalline structures, different conditions for rupture, intergranular creep fracture, and transgranular creep fracture exist.

Contents:

- Introduction (Chris Cantoni)

- Modes of High Temperature Fracture (Chris Cantoni)

- Rupture, Transgranular Creep Fracture, and Intergranular Creep Fracture

- Void growth (Nate Fenstemaker)

- Calculations

- High Temperature Fracture in Crystal Structures (Chris Cantoni)

- FCC, BCC, HCP, Ionic solids, Covalent solids

- Materials that Undergo High temperature Fracture (Austin Ryan)

- Steel, Ceramics

- Materials Application for High Temperature Fracture: Turbine Blade (Nate Fenstemaker)

Modes of High-Temperature Fracture:

A polycrystalline material will fracture differently at high temperatures than it will at low temperatures. Low temperature fracture involves the formation of voids in a material, the coalescence of these voids to form a crack, and the propagation of this crack, eventually leading to material failure14. High temperature fracture, on the other hand, occurs through three different modes, each based on the growth and coalescence of voids.

Three modes of high-temperature fracture exist for different stress levels and temperatures. Each mode is characterized by a reduction in area in the sample during tensile elongation due to voids nucleating around second phase particles and areas of strain. For all modes, an external stress of greater strength than the surface energy stress must be applied for voids to grow and cause fracture to occur.

Pictured are rupture (a), transgranular creep fracture (b), and intergranular creep fracture (c), which are the three modes of high-temperature fracture1. [Figure 1]

Rupture, which takes place at high stress levels and high temperatures, is characterized by a near 100% reduction in area. This is due to the dynamic recrystallization that occurs during rupture as voids that are being created in the material undergoing tensile elongation are being suppressed by the new grains growing in the material. Essentially, the growth of internal cavities and voids during tensile elongation is overcome by recrystallization so that the voids do not grow or connect with other voids to form cracks2. This explains why metals that are experiencing high-temperature fracture are noticeably malleable, as no cracks emerge during rupture and the metal therefore cannot fracture as easily.

Transgranular creep fracture, which occurs at temperatures lower than those associated with rupture, is characterized by a finite reduction in area. This finite reduction in area results from nucleation of internal voids at lower temperatures. Recovery processes take place at higher temperatures that allow for accommodation of nucleation sites. However, at lower temperatures the growth of nucleation sites overcomes the recrystallization rate and voids form1. Combination of surrounding nucleation sites eventually leads to fracture, but not without a reduction in area.

Intergranular creep fracture, which occurs at stress levels and strain rates lower than those for transgranular creep fracture and rupture, is characterized by little to no reduction in area. This is a result of voids forming along grain boundaries from the presence of second-phase particles. The voids that form on grain boundaries that are normal to the tensile axis grow to cause fracture. Because of this, only local deformation occurs and voids cannot combine with other voids as easily1. This results in minimal macroscopic deformation, which explains why there is almost no reduction in area in intergranular creep fracture. This mode of fracture occurs in all crystalline solids experiencing fracture.

Void Growth:

Void growth is the primary means by which intergranular creep fracture and transgranular creep fracture occur. Understanding the mechanisms behind void growth can help engineers avoid catastrophic failure by removing a mechanical part from service before the voids reach a critical size.

Wedge-void type crack formed from unaccommodated grain-boundary sliding1. [Figure 2]

Void growth occurs by deformation involving diffusional flow and/or dislocation creep. The driving force for void growth is the applied stress. As the voids grow, they comprise a larger portion of the cross sectional area of a material. This increases the stress upon the area that does not contain voids for intergranular creep fracture or on the area of the grain boundary for transgranular creep fracture. The creep rate is accelerated by void growth. When the voids grow large enough to comprise a critical area that is normal to the applied stress, shearing or micro rupture occurs, causing voids to combine and leading to eventual catastrophic failure.

Cavity-void type crack formed from an inclusion interacting with the grain boundary1. [Figure 3]

Voids can be present in a material initially but are also created through void nucleation. There are a few mechanisms through which void nucleation is thought to occur. Wedge shape cracks are formed by grain boundary sliding that is not accommodated for by other rearranging in the

material’s lattice. At grain boundaries, triple points exist where the stress is parallel and opposing cracks form into a wedge shape, or “w” shape. Spherical cavities also form and their growth and coalescence can lead to fracture. Often times, a precipitate along the grain boundary will be a nucleation point for a void during intergranular creep fracture. Incompatible strain can also lead to void nucleation. Sharp discontinuities along the grain boundary may cause grain boundary sliding that is not accommodated for, leading to void nucleation.

Formation of wedge-shaped cracks produced by unaccommodated grain boundary sliding. The type of sliding at the triple point of the grain boundaries determines the orientation of the

cracks1. [Figure 4]

The tendency toward wedge or cavity voids based on stress vs. grain diameter1. [Figure 5]

Cracks can also form due to external stress. An inclusion along the grain boundary will create voids which grow by unaccompanied grain boundary sliding. If the inclusion is located within the grain or if the void is the result of flow that is not accommodated, the void will have a different nucleated shape.

Void growth calculations:

The rate at which voids grow can help determine the useful operation life of a part. By calculating the change in volume with time of the void, the rate of change of void growth volume can be estimated1:

DB: boundary diffusion coefficient; δB: grain boundary thickness; rh: crack radius; σ: stress applied; Ω: atomic volume; kT: dimensions of energy; l: intervoid spacing

Damage and tertiary strain rates depend on material parameters, applied stress, and the instantaneous value of the damage. The dominant void-growth mechanism also depends on these factors.

High-Temperature Fracture in Crystal Structures:

Materials with different periodic arrangements of atoms will experience rupture, intergranular creep fracture, and transgranular creep fracture at different temperatures and tensile stress levels. This fracture information can be interpreted using fracture-mechanism maps, which plot the homologous temperature and normalized stress that a material experiences.

High temperature mechanism map for nickel, which is characteristic of a typical FCC metal1. [Figure 6]

FCC1: For nickel, which is a typical FCC pure metal, intergranular creep fracture dominates the fracture and occurs at all values of homologous temperature and withstands normalized stress from 10-2 N/m2 to about 5*10-4 N/m2 at a T/Tm value of 1.0. For an FCC crystal, transgranular creep fracture occurs above intergranular creep fracture at mid-range homologous temperatures, and rupture occurs at higher homologous temperatures.

High temperature mechanism map for molybdenum, which is characteristic of a typical BCC transitional metal1. [Figure 7]

BCC1: For molybdenum, which is a BCC transitional metal, low-temperature fracture dominates from the 10-2 N/m2 region to negligible normalized tensile stress values around 0.6 for homologous temperature. intergranular creep fracture occurs after the homologous temperature becomes 0.7, and transgranular creep fracture and rupture occur around 10-3 N/m2 at 0.4 and 0.9 homologous temperatures, respectively.

High temperature mechanism map for magnesium, which is characteristic of a typical HCP metal1. [Figure 8]

HCP1: For magnesium, which is a typical HCP metal, the modes of high-temperature fracture resemble the modes for high-temperature fracture in BCC crystal structures, with low temperature fracture homologous temperature only extending to about 0.5.

High temperature mechanism map for sodium chloride, which is characteristic of an ionically bonded material1. [Figure 9]

Ionically bonded1: For sodium chloride, low temperature fracture dominates up to 0.8 for the homologous temperature. The transgranular creep fracture region for this ionic material is much smaller compared with its others, which is due to the domination of low-temperature fracture in the material.

High temperature mechanism map for alumina, which is characteristic of a polycrystalline covalent solid1. [Figure 10]

Covalent solids1: For alumina, low-temperature fracture dominates up to 0.7 for the homologous temperature, but the intergranular creep fracture and transgranular creep fracture regions are drastically reduced.

High temperature mechanism map for silicon nitride, which is a counterexample to the covalent solid example above1. [Figure 11]

Covalent solids (2)1: For silicon nitride, low-temperature fracture completely dominates the plot with only intergranular creep fracture taking up the homologous temperature region from 0.7 to 1. This makes silicon nitride the counter-example to covalently bonded materials, as it has no rupture or transgranular creep fracture regions.

Materials that Undergo High Temperature Fracture:

Steels:

Steel is the most common metal used in industry, and is often put under stresses at high temperatures. It is important to know the difference in steel’s characteristics at low temperatures and at elevated temperatures. At room temperature, permanent deformation can occur when an effective applied stress exceeds the yield strength of the steel. At high temperatures, permanent deformation can occur at stress levels below the typical yield strength of the steel over a period time, called creep. The steel’s performance under stress changes with increased temperature, and can influence how types of steel are chosen for certain applications. 1

The stress needed for plastic flow to occur at an elevated temperature is much less than that of steel which is cold worked. The amount of creep and flow that occurs depends on the

temperature and duration of the test. The higher the temperature and the longer the duration, the greater amount of flow occurs. Though a prominent mechanism in high temperature fracture is creep, it also occurs at low to moderate stress levels being applied over long testing durations. To help eliminate creep fracture from manufactured parts with preexisting voids, recovery at high temperatures can be implemented to alleviate strain gradients.1

During short term tensile tests, intergranular dislocations increase. There is a temperature where, below it, transcrystalline fracture occurs and the metal is ductile. Above this temperature, the metal becomes brittle and breaks intergranularly. This temperature is related to the testing speed, and it is just above the recrystallization temperature of the given metal. A larger grain size will have increased strength but will also have a decreased ductility. 1,4

The alloying of the metal can greatly change the ductility and the fracture type of the steel. An example of the different effects that alloys experience can be found in an alloy containing Cr, Mo, and Si. As stated in “Fracture of Steels at Elevated Temperatures After Prolonged Loading”, this alloy experiences ductile transcrystalline fracture as opposed to an alloy containing only Cr, and Mo, which fractures intergranularly. Without the silicon, there is a much greater oxidation on the surface of the steel; this means that the combination of 5% Cr, 0.5% Mo, and 1.5% Si at 1200o F allows for a greater creep rate, eventually leading to transcrystalline fracture. This increases the temperature where the metal becomes brittle compared to that of the alloy with Cr and Mo, and becomes ductile at 1200o F. 4

Steel, at high temperatures, is much less resistant to oxidation. During oxidation, an oxide layer forms on the surface of the steel, leading to corrosion. With more oxidation, there is a thicker layer of oxide present on the metal. This thick layer of oxide on the metal protects it from further corrosion. These oxide layers can used to coat turbine blades, as well as car engines, to protect against corrosion at high temerperatures.14

In general, higher temperatures and lower strain rates favor intergranular cracking of steels, where dislocations travel along grain boundaries. Intergranular cracking first occurs very close to the recrystallization temperature of the given alloy and does not occur when there is a high strain rate. While intergranular fracture can function without surface oxidation, transcrystalline fracture occurs with oxidation. It is assumed that transcrystalline fracture results from the shearing of deformed grains along the crystal lattice and slip planes, as well as being accompanied by oxidation on the surface of the steel. 1, 14

Ceramics:

Ceramic materials are widely used at elevated temperatures because of their low thermal conductivity and their high resistance to temperature. At low temperature, ceramics are typically brittle and fail after about 0.1% strain. At elevated temperatures, ceramics often do not behave any different than at low temperatures until the temperature is high enough for plastic flow to occur. 14, 11

The dislocation widths in ceramic solids at high temperature are considerably smaller than those in a FCC metal. In a material with a large dislocation width, dislocations can move freely and the yield stress depends upon the interactions between dislocations and other defects. This allows the material to plastically deform easily, which makes it more ductile. There is low dislocation mobility in ceramics and they experience virtually no plastic deformation.10

At room temperatures, it is typical for a ceramic to fracture with little to no plastic deformation. The covalent bonds, in nature, have very high strength bonds, which cause a ceramic to have high hardness, and high compressive strength. At elevated temperatures however, there is a dramatic decrease in the modulus, and the ceramic passes through a brittle-to-plastic transition.

This exists at room temperature as well, but the modulus values of typical ceramics are very high, and the plastic deformation that occurs is negligible. At elevated temperatures, there are secondary slip systems that form, where the plastic deformation occurs. 10

In a ceramic, high temperature components typically experience very little yielding and can cause stress concentrations to develop. Although, after reinforcing the ceramic with composite materials, its fracture toughness increases, even under high temperature stresses. It can be assumed that the fracture toughness of a ceramic based composite material will be greater than a ceramic with no composite components, regardless of temperature.10

It is noted, however, that the base ceramic and composite component must have similar thermal expansion characteristics. This is the case because, if there is too much of a difference in thermal expansion, there will be stresses created by thermal expansion within the material. Compared to a non-composite ceramic, the composite material has greater toughness, but typically less tensile strength. This does not change with an increase of temperature, but the overall tensile strength decreases for both materials. 5

Materials applications for high-temperature fracture:

The following case study focuses on the failure of a turbine blade due to high temperature fracture and describes how the material’s microstructure responded to elevated temperatures. The phenomenon of high temperature fracture has resulted in many investigations of material failure at elevated temperatures. Several investigations involving the failure of turbine blades are located below in the “See also / further reading” section.

Turbine blades require good thermal stability, high tensile and fatigue strength, and resistance to creep and hot corrosion requiring the use of nickel-based superalloys.

4000 ton General Electric E H System gas turbine engine13. [Figure 12]

The turbine blades are typically protected from hot corrosion with a thermal barrier coating. Most turbine blades utilize γ′ precipitates which are carefully controlled during manufacturing to attain a fine ordered quasi-cubic precipitate to optimize high temperature strength by increasing the resistance to both diffusional and power law creep7. The blades operate at temperatures in excess of 600ºC and under high stress, which leads to micro-cracking. If the turbine blades are used at temperature for too long of a period the y' precipitates will begin to grow with shape tending towards spherical, through an Ostwald ripening mechanism7. The creep resistance of the blade will diminish as a result. If the blade is not removed from service following thermally induced damage, damage and even destruction of the gas-turbine may result. In the case of a power plant, this would reduce the output of the generating facility. If fracture were to occur in an aircraft turbine engine, it would likely result in a loss of propulsion at the very least and destruction of the aircraft at the very worst. Using the following relation, one may determine the remaining life of the turbine:

texp= time exposed to service stress; trem= remnant life; n = stress index for rate of strain;

λ=tertiary creep factor; and A = number fraction of cavitating boundaries. n will typically have a value of 3, but may be higher depending upon the stress involved3.

To inspect the failed blade, the microstructure in three different sections was examined: the trailing edge, the leading edge, and the center of the blade. The results of the analysis showed that the leading edge and center had negligible variations in the microstructure while the trailing edge showed evidence of coarsening of y' precipitate particles resulting in lower resistance to high temperature creep. It was determined that the trailing edge failed due to high-temperature creep damage. The sizes of the precipitate particles in the trailing edge were significantly larger at 1.31μm in comparison to the particles in the leading edge and center which had a value of 0.80μm and .78μm respectively3.

An optical micrograph showing cavities formed on the trailing edge of a turbine blade3. [Figure 13]

The SEM micrograph at bottom left shows cavities formed on the trailing edge of the turbine blade. The optical micrograph a bottom right shows voids and cavities along the grain boundaries.

By testing the hardness in the regions examined above, the lowest values were found at the trailing edge of the turbine blade further indicating that the blade failed due to creep. The hardness values were found via the Vickers hardness test. Testing showed the VHN values for the trailing edge to be nearly 150 less than that of the next lowest section3.

Optical micrograph showing voids and cavities along grain boundaries of a turbine blade3. [Figure 14]

The results of the investigation determined that improved cooling of the turbine blade was required or a more efficient thermal barrier coating should be used. Another recommendation was to modify the composition of the superalloy be modified to better resist the particle

coarsening and that the grain boundaries be parallel to one another to avoid creep damage due to cavities at the grain boundaries.

See also / further reading:

Viswanathan, R. "An Investigation of Blade Failures in Combustion Turbines." Engineering Failure Analysis. 8.5 (2001): 493-511.

Barella, S.; Boniardi, M.; Cincera, S.; Pellin, P.; Degive, X.; Gijbels, S. "Failure Analysis of a Third Stage Gas Turbine Blade." Engineering Failure Analysis. 18.1 (2011): 386-393.

Das,Goutam; Ghosh Chowdhury,Sandip; Kumar Ray,Ashok; Kumar Das,Swapan; Kumar Bhattacharya,Deepak "Turbine blade failure in a thermal power plant." Engineering Failure Analysis. 10.1 (2003): 85-91.

References:

1Thomas H. Courtney. Mechanical Behavior of Materials. Long Grove, IL: Waveland Press, Inc., 2000. Textbook.

2Hermann Riedel. Fracture at High Temperatures. Dusseldorf, Germany: Max-Planck-Institut fur Eisenforschung, 1986. Textbook.

3Huda, Zainul. “Metallurgical failure analysis for a blade failed in a gas-turbine engine of a power plant.” Materials & Design 30.8 (September 2009): Pages 3121-3125

4Jeffries: Effect of Temperature, Deformation, and Grain Size on the Mechanical Properties of Metals. Trans. A.I.M.E. (1919) page 60, 474.

5R. H. Thielemann and E.R. Parker. “Fracture of Steels at Elevated Temperatures after Prolonged Loading” Detroit Meeting, October, 1938.

6Armanios A. Erian. “Fracture of Composites”. CRC Press, 1996. Page 227.

7Carter, Tim. “Common Failures in Gas Turbine Blades.” Engineering Failure Analysis 12.2 (April 2005): Pages 237-247

8Dutta Sunil. “Fracture Toughness and Reliability in High-Temperature Structural Ceramics and Composites: Prospects and Challenges for the 21st Century”. John Glenn Research Center.

9 Ken Goto, Hiroshi Ohkita, Hiroshi Hatta, Hiroaki Iseki, and Yasuo Kogo. “Tensile Strength and Creep Behavior of Carbon-Carbon Composites at Elevated Temperatures”. Tokyo University of Science.

10C. Barry Carter, M. Grant Norton. “Ceramic materials: science and engineering”. Springer 2007.

11Ralf Riedel, I-Wei Chen. “Ceramics Science and Technology”. Wiley-VCH, 2010.

12Bažant Z. P. “Fracture mechanics of concrete structures”. Taylor & Francis, 1992. Page 106

13"DOE-Fossil Energy: World's Most Advanced Gas Turbine Now Ready to Cross Commercial Threshold." National Energy Technology Laboratory. National Energy Technology Laboratory, 18 FEB 2000. Web. 27 Apr 2011. <http://www.netl.doe.gov/publications/press/2000/tl-gasturb.html>.

14William D. Callister. Fundamentals of Materials Science and Engineering. Hoboken, NJ: John Wiley & Sons., 2008. Textbook.