Modes of failure can include fatigue cracks, brittle cracks produced by stress corrosion cracking or environmental stress cracking. The analysis and understanding of failure mechanisms helps engineers and materials scientists manage these risks and reduce the likelihood of failure.
Microstructure will have strong influence on a materials properties and its behaviour during service. Therefore, failure analysis typically includes microstructural examination, enabling an evaluation of the microstructural features of the failed component. This microstructure examination often identifies material processing errors, for example: heat treatment or welding, surface treatments and impurities etc.
EBSD is a powerful tool in the study of component life expectancy and understanding potential failure mechanisms. It readily provides valuable microstructural information such as phase identification, grain size/shape distribution, and grain orientation information. Thus it is commonly used to characterise phase/precipitates distribution, grain boundaries types and distribution, crack propagation, deformation, strain distribution and microtexture around cracks, fracture surfaces or facets. These are routine applications of EBSD which are used by researchers and product engineers to yield valuable information about the failure process.
Two case studies are discussed:
Example 1: EBSD characterisation of a crept Nickel alloy
Ni superalloys exhibit excellent mechanical strength and resistance to creep (the tendency for solids to deform under stress). These Superalloys generally have a gamma (γ) matrix coupled with a gamma prime (γ’) intermetallic phase Ni3(Al,Ti) which acts as a barrier to dislocation. Creep resistance is dependent on slowing the speed of dislocations within the crystal structure. Under creep conditions the γ’ particles tend to raft (rafting is directional coarsening of γ’, it takes place in the creep of Ni-base superalloys at high temperatures [F. Nabarro, 1995]) and deformation is seen largely at grain boundaries and the γ’/ γ interface.
EBSD is used to investigate the microstructure and damage developed in a Ni alloy after creep deformation.
A secondary electron image of the polished sample, Figure 1, illustrates that the microstructure is comprised of large grains containing γ’particles in a matrix. Two crack tips are observed, in the grain above the lower crack tip the gamma prime particles appear to have rafted. There are also some larger particles around the crack tip.
Combining EBSD and EDS the particles around the crack tip are identified as M23C6 carbides of chromium and molybdenum. This is achieved using Phase identification tool.
EBSD map data is shown in Figure 3. The pattern quality map, Figure 3a, illustrates the boundaries between the and γ’ phases and the rafting of the γ’ phase. The phase map, figure 3b illustrates the distribution of M23C6 carbides, the γ’ and γ phases have the same crystal structure and can be indexed with the fcc Ni phase.
Kernal Average misorientation (KAM) map: Figures 3c, illustrates the misorientation that exists at the γ phase grain boundaries and the interfaces between γ/γ’ phases. The higher degree of misorientation is shown by the bright green through yellow colour, shown on the key. The highest degree of misorientation measured (up to 2 degrees) is between the γ’ particles in region A, where the highest rafting is observed.
In this example combining EBSD with X-ray data enables the distribution of creep damage to be characterised. Here the local misorientation maps visually show extent and degree of creep damage between the phase interface, especially where the sample shows rafting.
Example 2: EBSD investigation of the Stress Corrosion Cracking (SCC) mechanisms of sigmatized Super Duplex Stainless Steel (SDSS) [C. Park, 2013].
Super duplex stainless steel (SDSS) products are widely used in the oil and gas industry for process equipment such as pipes, fittings, tubes, topside pipe-work, seawater handling systems and subsea umbilicals etc. These SDSS exhibit better mechanical and corrosion properties when compared to traditional austenitic grades of steel. The SDSS microstructure consists of austenite (γ) and ferrite (α) phases. However if it is heat treated, these materials can form intermetallic phase precipitation, such as sigma (σ), that will degrade both the mechanical and corrosion properties. Therefore it is important to understand the behaviour of the sigma phase relative to SDSS materials tested in a simulated oil field environment.
This study1 was conducted to investigate the potential susceptibility to SCC of sigmatized SDSS exposed to simulated North Sea platform production brine. A sample containing 20% of σ-phase was tested to understand the failure behaviour of SDSS. EBSD was used to investigate the cracking mechanism.
A phase map of a cross-section of a failed sample is shown in Figure 4a, it illustrates that crack nucleation occurs in the σ grains; σ-phase cracking is observed not just on the surface but also in the centre of sample. Regardless of grain size, cracks occurred in all brittle σ grains well away from the surface. Once cracks were initiated in the σ-phase, they primarily followed a path through the γ-phase, often being stopped by α and γ phases or were diverted into the σ-phase boundaries.
(a) EBSD phase map of the stigmatised SDSS sample tested in mineral oil: σ phase is in yellow, γ phase is in blue and a phase is in red. Some cracks are indicated by the arrow as examples to show cracks were initiated in the σ-phase, followed a path through the γ-phase and stopped by α and γ phases.
(b) EBSD strain map from the same area. Large amount of deformation was visualised in the microstructure around σ and a grains using a KAM map Figure 4b. This EBSD KAM maps reveals that high plastic strain around the cracks, with less plastic strain in γ phase and the crack propagation is associated with plastic strain at the crack tip.
Large amount of deformation was visualised in the microstructure around &gsigma; and α grains using a KAM map Figure 4b. This EBSD KAM maps reveals that high plastic strain around the cracks, with less plastic strain in γ phase and the crack propagation is associated with plastic strain at the crack tip.
In this study, EBSD gives an additional insight into complex behaviour of the phases in SDSS. It indicates the σ-phase is highly susceptible to cracking in an oil field environment.
C. J. Park Corrosion March 2013, Vol. 69, No. 3, pp. 276-285
F.R.N. Nabarro, “Rafting in Superalloys”, Met. Mat. Trans. A, 27A(1995), 513-529