We continue our discussion on liquid-metal embrittlement (LME) by defining the most common types of embrittlement observed with heat-treated parts, namely:
- Environmentally induced cracking – Tensile stresses developed in the material from operations such as cold work (residual stress), welding, grinding, thermal treatments and service conditions.
- Stress corrosion cracking (SCC)– Induced from the combined influence of tensile stress and a corrosive environment. Forms include sulfide stress cracking, chlorine-induced SCC, caustic-induced SCC and hydrogen-induced cracking.
- Hydrogen embrittlement– The ingress of hydrogen into the metal that causes reduced ductility and load-bearing capacity, subsequent cracking and catastrophic brittle failures at stresses below the yield stress of susceptible materials.
- Temper embrittlement– A microstructural condition in which toughness and fracture resistance are lowered in hardened steels as a result of tempering. Two distinct types are tempered-martensite embrittlement (TME) and temper embrittlement (TE), which can occur in the ranges of 200-400°C (390-750°F) and 375-575°C (710-1050°F) respectively.
- Quench embrittlement– An intergranular mechanism of brittle fracture, developing in high-carbon steels during austenitizing or quenching.
- Corrosion fatigue– Loss of mechanical (fatigue) properties when a material is exposed to a corrosive environment under the joint action of corrosion and cyclic loading.
- Liquid-metal embrittlement– Exposure of a particular metal to another metal in the liquid or near-liquid state in combination with a state of tension (applied or internal), causing loss of mechanical properties due to intergranular attack.
Liquid-Metal Embrittlement Mechanics
Many theories have been proposed for LME. The major ones are:
- The dissolution-diffusion model of Robertson and Glickman says that adsorption of the liquid metal on the solid metal induces dissolution and inward diffusion. Under stress, these processes lead to crack nucleation and propagation.
- The brittle-fracture theory of Stoloff and Johnson, Westwood and Kamdar proposed that the adsorption of the liquid-metal atoms at the crack tip weakens inter-atomic bonds and propagates the crack.
- Gordon postulated a model based on diffusion penetration of liquid-metal atoms to nucleate cracks, which grow under stress to cause failure.
- The ductile-failure model of Lynch and Popovich predicted that adsorption of the liquid metal leads to weakening of atomic bonds and nucleation of dislocations, which move under stress, pile up and work-harden the solid. Dissolution also helps in the nucleation of voids, which grow under stress and cause ductile failure.
All of these models utilize the concept of an adsorption-induced surface energy lowering of the solid metal as the central cause of LME.
Fastener Case Study
As most fastener designers know, fastener failures can, in most instances, be avoided through careful consideration of:
- Mechanical property requirements such as proof or tensile strength, shear strength, tensile ductility, hardness, impact toughness, creep and stress relaxation and fatigue properties
- Physical property requirements such as thermal expansion/contraction, magnetic properties and elastic modulus
- Service requirements such as correct preload/torque level, method of tightening (manual vs. automatic), lubricants, thread fit, tightening speed, surface finish, plating and galling behavior
- Environmental requirements such as corrosive state of the environment, hydrogen embrittlement, stress corrosion cracking, crevice corrosion and pitting, corrosion fatigue, high-temperature effects and liquid-metal embrittlement
We’ll continue in Part 3.
- de Rosset, William S., “Use of Liquid Metal Embrittlement (LME) for Controlled Fracture,” Army Research Laboratory, ARL-TR-4976, September 2009.
- C. J. McMahon, Jr., “Brittle Fracture of Grain Boundaries,” Interface Science 12, 141- 146, 2004.
- Bogner, B., G. Rorvik, and L. Marken, “Bolt Failures – Case Histories from the Norwegian Petroleum Industry,” Microscopy and Microanalysis, Volume 11 Supplement S02, August 2005.
- Wikipedia (www.wikipedia.com)
- Krauss, George, Steels: Processing, Structure, and Performance, ASM International, 2005.
- Kolman, D. G., “Environmentally Induced Cracking, Liquid Metal Embrittlement,” ASM Harndbook, Volume 13A, Corrosion: Fundamentals, Testing and Protection, ASM International, 2003., pp 381 392.