Most heat treaters are aware of many failure phenomena, perhaps the most well-known of which is hydrogen embrittlement (aka hydrogen-assisted cracking). But there are several other types, such as liquid-metal embrittlement, that we must understand as well. Let’s learn more.
It is known that certain metals when exposed to other metals in the liquid (or solid) state are susceptible to a phenomenon known as liquid-metal embrittlement (LME), also known as liquid-metal-induced embrittlement (LMIE) or liquid-metal cracking (LMC). The embrittlement of aluminum in contact with liquid mercury is a classic example, which is why any mercury-filled device is prohibited on aircraft due to concerns over loss of structural integrity of the aircraft in flight.
Other examples include carbon and stainless steels, which are susceptible to LME by zinc and lithium. Copper and copper alloys are susceptible to LMC by mercury, and lithium, aluminum and aluminum alloys are susceptible to LME from mercury and zinc (Table 1). While the exact mechanisms of embrittling are complicated, the penetration by the embrittling agent is normally intergranular, and the requirements for embrittlement tend to vary depending on the materials involved.
The minimal conditions required for LME of steels are:
- The alloy must be in a state of tension (applied or internal).
- The surface must be clean and free of oxides (which act as barriers).
- The embrittling species (liquid) must intimately wet the metal surface.
The embrittling effect is most severe when high hardness exists, typically above 40 HRC (Fig. 1). How fast a material will fail due to LME depends on many factors. Under certain conditions, fracture can take place in seconds (Fig. 2, online). Crack-growth and propagation rates in the range of 0.82-3.3 feet/sec (0.25-1.0 m/s) have been measured. An incubation period and a slow pre-critical crack-propagation stage generally precede final fracture.
It is not uncommon for certain steels to experience ductility losses and cracking during manufacturing processes such as hot-dip galvanizing or during subsequent fabrication. LME effects can also be observed even in the solid state when one of the metals is brought close to its melting point. This type of phenomenon is also known as solid-metal-induced embrittlement.
Many theories have been proposed for LME. The major ones are listed here.
- 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 interatomic bonds and propagates the crack.
- Gordon postulated a model based on diffusion-penetration of liquid-metal atoms to nucleate cracks, which under stress grow 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. Also, dissolution 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 the following:
- 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 versus 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 LME
Despite our best efforts, failures do occur – as seen in the example involving steel bolts (150 mm long x 1 mm diameter) used to hold a cover on an automotive air-conditioning compressor. Leaks were observed in service, and closer inspection revealed the leakage to be in the contact area of a washer to the bolt flange where the attachment to the cover is made (Fig. 3).
The material in question was SCM435 (0.33-0.38% C, 0.15-0.35% Si, 0.60-0.90% Mn, ≤ 0.030% P, ≤ 0.030% S, ≤ 0.30% Cu, ≤ 0.25% Ni, 0.90-1.20% Cr, 0.15-0.30% Mo), a chromium/molybdenum steel heat treated to achieve a hardness in the range of 35-39 HRC. The bolts were then run in an exothermic-gas atmosphere, which produced a 0.01-mm (0.0004-inch) oxide layer. The application required that the bolt flange be mated to a tin-coated brass washer at assembly (Fig. 4, online).
A metallurgical investigation followed and revealed the presence of a tin-rich phase (Fig. 6, online), which was confirmed under a scanning electron microscope to be LME with a brittle intergranular fracture mode (Fig. 7). The tin was found at the grain boundaries, both those already exposed by fracture and internally that were not opened. Energy Dispersive X-Ray Spectroscopy (EDS) analysis was used to confirm the presence of tin-rich areas.
Liquid-metal embrittlement is one of the least expected failure modes experienced by products in service applications. It is often catastrophic when it occurs, but it is correctable by proper matching of materials with their environment as well as controlling the stress state and hardness of the material.