Embrittlement

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Embrittled pinch roller

Embrittlement is a significant decrease of ductility of a material, which makes the material brittle. Embrittlement is used to describe any phenomena where the environment compromises a stressed material's mechanical performance, such as temperature or environmental composition. This is oftentimes undesirable as brittle fracture occurs quicker and can much more easily propagate than ductile fracture, leading to complete failure of the equipment. Various materials have different mechanisms of embrittlement, therefore it can manifest in a variety of ways, from slow crack growth to a reduction of tensile ductility and toughness.

Contents

Mechanisms

Embrittlement is a series complex mechanism that is not completely understood. The mechanisms can be driven by temperature, stresses, grain boundaries, or material composition. However, by studying the embrittlement process, preventative measures can be put in place to mitigate the effects. There are several ways to study the mechanisms. During metal embrittlement (ME), crack-growth rates can be measured. Computer simulations can also be used to enlighten the mechanisms behind embrittlement. This is helpful for understanding hydrogen embrittlement (HE), as the diffusion of hydrogen through materials can be modeled. The embrittler does not play a role in final fracture; it is mostly responsible for crack propagation. Cracks must first nucleate. Most embrittlement mechanisms can cause fracture transgranularly or intergranularly. For metal embrittlement, only certain combinations of metals, stresses, and temperatures are susceptible. This is contrasted to stress-corrosion cracking where virtually any metal can be susceptible given the correct environment. Yet this mechanism is much slower than that of liquid metal embrittlement (LME), suggesting that it directs a flow of atoms both towards and away from the crack. For neutron embrittlement, the main mechanism is collisions within the material from the fission byproducts.

Embrittlement of metals

Hydrogen embrittlement

One of the most well discussed, and detrimental, embrittlement is hydrogen embrittlement in metals. There are multiple ways that hydrogen atoms can diffuse into metals, including from environment or during processing (eg. electroplating). The exact mechanism that causes hydrogen embrittlement is still not determined, but many theories are proposed and are still undergoing verification. [1] Hydrogen atoms are likely to diffuse to grain boundaries of metals, which becomes a barrier for dislocation motion and builds up stress near the atoms. When the metal is stressed, the stress is concentrated near the grain boundaries due to hydrogen atoms, allowing a crack to nucleate and propagate along the grain boundaries to relieve the built-up stress.

There are many ways to prevent or reduce the impact of hydrogen embrittlement in metals. One of the more conventional ways is to place coatings around the metal, which will act as diffusion barriers that prevents hydrogen from being introduced from the environment into the material. [2] Another way is to add traps or absorbers in the alloy which takes into the hydrogen atom and forms another compound.

475 °C embrittlement

Electron backscatter diffraction map of 128hrs age hardened DSS with the ferrite phase formaing the matrix and austenite grains sporadically spread Aged DSS EBSD.png
Electron backscatter diffraction map of 128hrs age hardened DSS with the ferrite phase formaing the matrix and austenite grains sporadically spread

Duplex stainless steel is widely used in industry because it possesses excellent oxidation resistance, but it can have limited toughness due to its large ferritic grain size and embrittlement tendencies at temperatures ranging from 280–500 °C, especially at 475 °C, where spinodal decomposition of the supersaturated solid ferrite solution into Fe-rich nanophase () and Cr-rich nanophase (), accompanied by G-phase precipitation, occurs, [3] [4] [5] which makes the ferrite phase a preferential initiation site for micro-cracks. [6]

Radiation embrittlement

Radiation embrittlement, also known as neutron embrittlement, is a phenomenon more commonly observed in reactors and nuclear plants as these materials are constantly exposed to a steady amount of radiation. When a neutron irradiates the metal, voids are created in the material, which is known as void swelling. [7] If the material is under creep (under low strain rate and high temperature condition), the voids will coalesce into vacancies which compromises the mechanical strength of the workpiece.

Low temperature embrittlement

At low temperatures, some metals can undergo a ductile-brittle transition which makes the material brittle and could lead to catastrophic failure during operation. This temperature is commonly called a ductile-brittle transition temperature or embrittlement temperature. Research has shown that low temperature embrittlement and brittle fracture only occurs under these specific criteria: [8]

  1. There is enough stress to nucleate a crack.
  2. The stress at the crack exceeds a critical value that will open up the crack (also known as Griffith's criterion for crack opening).
  3. High resistance to dislocation movement.
  4. There should be a small amount of viscous drag of dislocation to ensure opening of crack.

All metals can fulfill criteria 1, 2, 4. However, only BCC and some HCP metals meets the third condition as they have high Peierl's barrier and strong energy of elastic interaction of dislocation and defects. All FCC and most HCP metals have low Peierl's barrier and weak elastic interaction energy. Plastics and rubbers also exhibit the same transition at low temperatures.

Historically, there are multiple instances where people are operating equipment at cold temperatures that led to unexpected, but also catastrophic, failure. In Cleveland in 1944, a cylindrical steel tank containing liquefied natural gas ruptured because of its low ductility at the operating temperature. [9] Another famous example was the unexpected fracture of 160 World War II liberty ships during winter months. [10] The crack was formed at the middle of the ships and propagated through, breaking the ships in half quite literally.

Embrittlement temperatures [11]
MaterialTemperature
[°F]		Temperature
[°C]
Plastics
ABS −270−168
Acetal −300−184.4
Delrin -275 to -300-171 to -184
Nylon -275 to -300-171 to -184
Polytron −300−184.4
Polypropylene -300 to -310-184 to -190
Polytetrafluoroethylene −275−171
Rubbers
Buna-N −225−143
EPDM -275 to -300-171 to -184
Ethylene propylene -275 to -300-171 to -184
Hycar -210 to -275-134 to -171
Natural rubber -225 to -275-143 to -171
Neoprene -225 to -300-143 to -184
Nitrile-275 to -310-171 to -190
Nitrile-butadiene (ABS) -250 to -270-157 to -168
Silicone −300−184.4
Urethane -275 to -300-171 to -184
Viton -275 to -300-171 to -184
Metals
Zinc −200−129
Steel −100−73

Other types of embrittlement

Embrittlement of inorganic glasses and ceramics

The mechanisms of embrittlement are similar to those of metals. Inorganic glass embrittlement can be manifested via static fatigue. Embrittlement in glasses, such as Pyrex, is a function of humidity. Growth rate of cracks vary linearly with humidity, suggesting a first-order kinetic relationship. The static fatigue of Pyrex by this mechanism requires dissolution to be concentrated at the tip of the crack. If the dissolution is uniform along the crack flat surfaces, the crack tip will be blunted. This blunting can actually increase the fracture strength of the material by 100 times. [13]

The embrittlement of SiC/alumina composites serves as an instructive example. The mechanism for this system is primarily the diffusion of oxygen into the material through cracks in the matrix. The oxygen reaches the SiC fibers and produces silicate. Stress concentrates around the newly formed silicate and the fibers' strength is degraded. This ultimately leads to fracture at stresses less than the material's typical fracture stress. [14]

Embrittlement of polymers

Polymers come in a wide variety of compositions, and this diversity of chemistry results in wide-ranging embrittlement mechanisms. The most common sources of polymer embrittlement include oxygen in the air, water in liquid or vapor form, ultraviolet radiation from the sun, acids, and organic solvents. [15]

One of the ways these sources alter the mechanical properties of polymers is through chain scission and chain cross-linking. Chain scission occurs when atomic bonds are broken in the main chain, so environments with elements such as solar radiation lead to this form of embrittlement. Chain scission reduces the length of the polymer chains in a material, resulting in a reduction of strength. Chain cross-linking has the opposite effect. An increase in the number of cross-links (due to an oxidative environment for example), results in stronger, less ductile material. [16]

The thermal oxidation of polyethylene provides a quality example of chain scission embrittlement. The random chain scission induced a change from ductile to brittle behavior once the average molar mass of the chains dropped below a critical value. For the polyethylene system, embrittlement occurred when the weight average molar mass fell below 90 kg/mol. The reason for this change was hypothesized to be a reduction of entanglement and an increase in crystallinity. The ductility of polymers is typically a result of their amorphous structure, so an increase in crystallinity makes the polymer more brittle. [17] In the case of polyethylene terephthalate, hydrolysis produces chain scission embrittlement. [18] It has been demonstrated that the degradation of the mechanical properties correlates with the reduction of the mobile amorphous fraction (MAF), and that the ductile-to-brittle transition occurs when the minimum MAF is reached. [19] This supports a micromechanical interpretation of the embrittlement mechanism rather than a molecular interpretation.

The embrittlement of silicone rubber is due to an increase in the amount of chain cross-linking. When silicone rubber is exposed to air at temperatures above 250 °C (482 °F) oxidative cross-linking reactions occur at methyl side groups along the main chain. These cross-links make the rubber significantly less ductile. [20]

Solvent stress cracking is a significant polymer embrittlement mechanism. It occurs when liquids or gasses are absorbed into the polymer, ultimately swelling the system. The polymer swelling results in less shear flow and an increase in crazing susceptibility. Solvent stress cracking from organic solvents typically results in static fatigue because of the low mobility of fluids. Solvent stress cracking from gasses is more likely to result in greater crazing susceptibility. [21]

Polycarbonate provides a good example of solvent stress cracking. Numerous solvents have been shown to embrittle polycarbonate (i.e. benzene, toluene, acetone) through a similar mechanism. The solvent diffuses into the bulk, swells the polymer, induces crystallization, and ultimately produces interfaces between ordered and disordered regions. These interfaces produce voids and stress fields that can be propagated throughout the material at stresses much lower than the typical tensile strength of the polymer. [22]

Related Research Articles

<span class="mw-page-title-main">Ductility</span> Degree to which a material under stress irreversibly deforms before failure

Ductility is a mechanical property commonly described as a material's amenability to drawing. In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stress before failure. Ductility is an important consideration in engineering and manufacturing. It defines a material's suitability for certain manufacturing operations and its capacity to absorb mechanical overload. Some metals that are generally described as ductile include gold and copper, while platinum is the most ductile of all metals in pure form. However, not all metals experience ductile failure as some can be characterized with brittle failure like cast iron. Polymers generally can be viewed as ductile materials as they typically allow for plastic deformation.

<span class="mw-page-title-main">Polymer degradation</span> Alteration in the polymer properties under the influence of environmental factors

Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours.

<span class="mw-page-title-main">Brittleness</span> Liability of breakage from stress without significant plastic deformation

A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a sharp snapping sound.

<span class="mw-page-title-main">Hydrogen embrittlement</span> Reduction in ductility of a metal exposed to hydrogen

Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed, hydrogen lowers the stress required for cracks in the metal to initiate and propagate, resulting in embrittlement. Hydrogen embrittlement occurs most notably in steels, as well as in iron, nickel, titanium, cobalt, and their alloys. Copper, aluminium, and stainless steels are less susceptible to hydrogen embrittlement.

<span class="mw-page-title-main">Reactor pressure vessel</span> Nuclear power plant component

A reactor pressure vessel (RPV) in a nuclear power plant is the pressure vessel containing the nuclear reactor coolant, core shroud, and the reactor core.

<span class="mw-page-title-main">Stress corrosion cracking</span> Growth of cracks in a corrosive environment

Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC is highly chemically specific in that certain alloys are likely to undergo SCC only when exposed to a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.

<span class="mw-page-title-main">Intergranular fracture</span>

Intergranular fracture, intergranular cracking or intergranular embrittlement occurs when a crack propagates along the grain boundaries of a material, usually when these grain boundaries are weakened. The more commonly seen transgranular fracture, occurs when the crack grows through the material grains. As an analogy, in a wall of bricks, intergranular fracture would correspond to a fracture that takes place in the mortar that keeps the bricks together.

In metallurgy and materials science, annealing is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for an appropriate amount of time and then cooling.

Liquid metal embrittlement is a phenomenon of practical importance, where certain ductile metals experience drastic loss in tensile ductility or undergo brittle fracture when exposed to specific liquid metals. Generally, tensile stress, either externally applied or internally present, is needed to induce embrittlement. Exceptions to this rule have been observed, as in the case of aluminium in the presence of liquid gallium. This phenomenon has been studied since the beginning of the 20th century. Many of its phenomenological characteristics are known and several mechanisms have been proposed to explain it. The practical significance of liquid metal embrittlement is revealed by the observation that several steels experience ductility losses and cracking during hot-dip galvanizing or during subsequent fabrication. Cracking can occur catastrophically and very high crack growth rates have been measured.

Radiation damage is the effect of ionizing radiation on physical objects including non-living structural materials. It can be either detrimental or beneficial for materials.

In geology, a deformation mechanism is a process occurring at a microscopic scale that is responsible for changes in a material's internal structure, shape and volume. The process involves planar discontinuity and/or displacement of atoms from their original position within a crystal lattice structure. These small changes are preserved in various microstructures of materials such as rocks, metals and plastics, and can be studied in depth using optical or digital microscopy.

Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the favorable properties of steel result from interstitial incorporation of carbon into the iron lattice. Brass, a binary alloy of copper and zinc, has superior mechanical properties compared to its constituent metals due to solution strengthening. Work hardening has also been used for centuries by blacksmiths to introduce dislocations into materials, increasing their yield strengths.

<span class="mw-page-title-main">Forensic polymer engineering</span> Study of failure in polymeric products

Forensic polymer engineering is the study of failure in polymeric products. The topic includes the fracture of plastic products, or any other reason why such a product fails in service, or fails to meet its specification. The subject focuses on the material evidence from crime or accident scenes, seeking defects in those materials that might explain why an accident occurred, or the source of a specific material to identify a criminal. Many analytical methods used for polymer identification may be used in investigations, the exact set being determined by the nature of the polymer in question, be it thermoset, thermoplastic, elastomeric or composite in nature.

<span class="mw-page-title-main">Environmental stress cracking</span> Brittle failure of thermoplastic polymers

Environmental Stress Cracking (ESC) is one of the most common causes of unexpected brittle failure of thermoplastic polymers known at present. According to ASTM D883, stress cracking is defined as "an external or internal crack in a plastic caused by tensile stresses less than its short-term mechanical strength". This type of cracking typically involves brittle cracking, with little or no ductile drawing of the material from its adjacent failure surfaces. Environmental stress cracking may account for around 15-30% of all plastic component failures in service. This behavior is especially prevalent in glassy, amorphous thermoplastics. Amorphous polymers exhibit ESC because of their loose structure which makes it easier for the fluid to permeate into the polymer. Amorphous polymers are more prone to ESC at temperature higher than their glass transition temperature (Tg) due to the increased free volume. When Tg is approached, more fluid can permeate into the polymer chains.

In polymers, such as plastics, thermal degradation refers to a type of polymer degradation where damaging chemical changes take place at elevated temperatures, without the simultaneous involvement of other compounds such as oxygen. Simply put, even in the absence of air, polymers will begin to degrade if heated high enough. It is distinct from thermal-oxidation, which can usually take place at less elevated temperatures.

<span class="mw-page-title-main">Photo-oxidation of polymers</span>

In polymer chemistry photo-oxidation is the degradation of a polymer surface due to the combined action of light and oxygen. It is the most significant factor in the weathering of plastics. Photo-oxidation causes the polymer chains to break, resulting in the material becoming increasingly brittle. This leads to mechanical failure and, at an advanced stage, the formation of microplastics. In textiles the process is called phototendering.

Low hydrogen annealing, commonly known as "baking" is a heat treatment in metallurgy for the reduction or elimination of hydrogen in a material to prevent hydrogen embrittlement. Hydrogen embrittlement is the hydrogen-induced cracking of metals, particularly steel which results in degraded mechanical properties such as plasticity, ductility and fracture toughness at low temperature. Low hydrogen annealing is called a de-embrittlement process. Low hydrogen annealing is an effective method compared to alternatives such as electroplating the material with zinc to provide a barrier for hydrogen ingress which results in coating defects.

Polymer fracture is the study of the fracture surface of an already failed material to determine the method of crack formation and extension in polymers both fiber reinforced and otherwise. Failure in polymer components can occur at relatively low stress levels, far below the tensile strength because of four major reasons: long term stress or creep rupture, cyclic stresses or fatigue, the presence of structural flaws and stress-cracking agents. Formations of submicroscopic cracks in polymers under load have been studied by x ray scattering techniques and the main regularities of crack formation under different loading conditions have been analyzed. The low strength of polymers compared to theoretically predicted values are mainly due to the many microscopic imperfections found in the material. These defects namely dislocations, crystalline boundaries, amorphous interlayers and block structure can all lead to the non-uniform distribution of mechanical stress.

Metal-induced embrittlement (MIE) is the embrittlement caused by diffusion of metal, either solid or liquid, into the base material. Metal induced embrittlement occurs when metals are in contact with low-melting point metals while under tensile stress. The embrittler can be either solid (SMIE) or liquid. Under sufficient tensile stress, MIE failure occurs instantaneously at temperatures just above melting point. For temperatures below the melting temperature of the embrittler, solid-state diffusion is the main transport mechanism. This occurs in the following ways:

References

  1. ↑ R.A. Oriani, "Hydrogen Embrittlement of Steels", Ann. Rev. Mater. Sci., vol 8, pp.327-357, 1978
  2. ↑ H. Bhadeshia, "Prevention of Hydrogen Embrittlement in Steels", ISIJ International, vol. 56, no. 1, pp. 24-36, 2016. Available: 10.2355/isijinternational.isijint-2015-430
  3. ↑ Örnek, Cem; Burke, M. G.; Hashimoto, T.; Engelberg, D. L. (April 2017). "748 K (475 °C) Embrittlement of Duplex Stainless Steel: Effect on Microstructure and Fracture Behavior". Metallurgical and Materials Transactions A. 48 (4): 1653–1665. Bibcode:2017MMTA...48.1653O. doi: 10.1007/s11661-016-3944-2 . ISSN   1073-5623. S2CID   136321604.
  4. ↑ Weng, K. L; Chen, H. R; Yang, J. R (2004-08-15). "The low-temperature aging embrittlement in a 2205 duplex stainless steel". Materials Science and Engineering: A. 379 (1): 119–132. doi:10.1016/j.msea.2003.12.051. ISSN   0921-5093.
  5. ↑ Beattie, H. J.; Versnyder, F. L. (July 1956). "A New Complex Phase in a High-Temperature Alloy". Nature. 178 (4526): 208–209. Bibcode:1956Natur.178..208B. doi:10.1038/178208b0. ISSN   1476-4687. S2CID   4217639.
  6. ↑ Liu, Gang; Li, Shi-Lei; Zhang, Hai-Long; Wang, Xi-Tao; Wang, Yan-Li (August 2018). "Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD". Acta Metallurgica Sinica (English Letters). 31 (8): 798–806. doi: 10.1007/s40195-018-0708-6 . ISSN   1006-7191. S2CID   139395583.
  7. ↑ Chopra, O.K. & Rao, A.S., A review of irradiation effects on LWR core internal materials – Neutron embrittlement. Journal of Nuclear Materials. 412. 195-208 (2011). 10.1016/j.jnucmat.2011.02.059
  8. ↑ Chernov, Vyacheslav & Kardashev, B.K. & Moroz, K.A.. (2016). Low-temperature embrittlement and fracture of metals with different crystal lattices – Dislocation mechanisms. Nuclear Materials and Energy. 9. 10.1016/j.nme.2016.02.002
  9. ↑ Edeskuty F.J., Stewart W.F. (1996) Embrittlement of Materials. In: Safety in the Handling of Cryogenic Fluids. The International Cryogenics Monograph Series. Springer, Boston, MA
  10. ↑ Benac, D.J., Cherolis, N. & Wood, D. Managing Cold Temperature and Brittle Fracture Hazards in Pressure Vessels. J Fail. Anal. and Preven. 16, 55–66 (2016). https://doi.org/10.1007/s11668-015-0052-3
  11. ↑ Gillespie, LaRoux K. (1999), Deburring and edge finishing handbook, SME, pp. 196–198, ISBN   978-0-87263-501-2.
  12. ↑ Ashby, M. F. (2019). Materials : engineering, science, processing and design. Hugh Shercliff, David Cebon (4th ed.). Kidlington, Oxford, United Kingdom. ISBN   978-0-08-102376-1. OCLC   1097951622.{{cite book}}: CS1 maint: location missing publisher (link)
  13. ↑ Courtney, Thomas H. Mechanical Behavior of Materials. McGraw Hill Education (India), 2013.
  14. ↑ Heredia, Fernando E., et al. "Oxidation Embrittlement Probe for Ceramic-Matrix Composites." Journal of the American Ceramic Society, vol. 78, no. 8, 1995, pp. 2097–2100., doi:10.1111/j.1151-2916.1995.tb08621.x
  15. ↑ Courtney, Thomas H. Mechanical Behavior of Materials. McGraw Hill Education (India), 2013
  16. ↑ Courtney, Thomas H. Mechanical Behavior of Materials. McGraw Hill Education (India), 2013.
  17. ↑ Fayolle, B., et al. "Mechanism of Degradation Induced Embrittlement in Polyethylene." Polymer Degradation and Stability, vol. 92, no. 2, 2007, pp. 231–238., doi:10.1016/j.polymdegradstab.2006.11.012
  18. ↑ W McMahon, HA Birdsall, GR Johnson, CT. Camilli, Degradation studies of polyethylene terephthalate, J. Chem. Eng. Data 4 (1) (1959) 57–79
  19. ↑ L Doyle, I Weidlich, Hydrolytic Degradation of Closed Cell Polyethylene Terephthalate Foams. The Role of the Mobile Amorphous Phase in the Ductile-Brittle Transition, Polymer Degradation and Stability, Volume 202, 2022, 110022, ISSN 0141-3910, https://doi.org/10.1016/j.polymdegradstab.2022.110022.
  20. ↑ Thomas, D. K. "Network Scission Processes in Peroxide Cured Methylvinyl Silicone Rubber." Rubber Chemistry and Technology, vol. 40, no. 2, 1967, pp. 629–634., doi:10.5254/1.3539077
  21. ↑ Courtney, Thomas H. Mechanical Behavior of Materials. McGraw Hill Education (India), 2013.
  22. ↑ Miller, G. W., et al. "On the Solvent Stress-Cracking of Polycarbonate." Polymer Engineering and Science, vol. 11, no. 2, 1971, pp. 73–82., doi:10.1002/pen.760110202
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