TWI Industrial Member Report Summary 945/2010
A Bourgeon
Background
The drive to operate in harsher environments and with greater loads has led to the development and adoption of a range of high strength, corrosion resistant alloys (CRAs), eg in the oil and gas industry. As with most applications of new materials in demanding industrial applications there have been successes and failures, some of the latter involving unanticipated failure modes (eg Taylor et al, 1999).
One feature of many of the high strength CRAs, which are exposed to high stresses, is time-dependent plastic strain accumulation under constant load, ie low temperature creep. This strain development under sustained load may not in itself be a problem, but when it acts in combination with hydrogen migration/ingress and/or corrosion mechanisms, it can lead to time-dependent cracking (Woollin and Murphy, 2001). Some offshore failures of high strength CRA components, including hydrogen embrittlement of duplex stainless steel and Alloy 718, have challenged the existing understanding of the failure modes. Data are required to allow design rules to be formulated to reliably avoid such cracking mechanisms. This report explores the application of electron backscatter diffraction (EBSD) to characterise the development of plastic strain with respect to the various microstructural features of these complex alloys.
Electron backscatter diffraction (EBSD) is performed with the scanning electron microscope (SEM) to provide a wide range of analytical data ranging from the crystallographic orientation studies (texture analysis, strain determination), to phase identification to grain size measurements. What makes this technique so interesting is that it allows rapid and automatic diffraction analysis to give crystallographic data and imaging with a spatial resolution of less than 0.5µm, together with the regular capabilities of an SEM, such as capacity for large specimens and option of simultaneous chemical analysis.
This report describes work undertaken to try to understand and evaluate to what extent it is possible to determine the strain distribution in a metallographic sample on a microstructural scale, in an SEM, using EBSD. EBSD is an advanced tool that is becoming increasingly accessible for routine investigations due to advances in automation and software to analyse the data. Crystallographic information is derived from EBSD by analysis of diffraction patterns using automated software that produces maps from a large volume of complex data. The objective of the work was to gain a comprehensive understanding of the qualitative analysis of strain that can be performed.
The specimen is positioned in the specimen chamber with an angle of 70° to the horizontal, to enhance the proportion of backscattered electrons able to undergo diffraction. The resulting diffraction pattern is captured and indexed in real time by the data collection software, generating the raw data. This raw data is then processed by analysis software, which gives access to drawing various maps to display the relevant information, such as inverse pole figure maps, phase maps or kernel average misorientation (KAM) maps.
Objective
To establish the feasibility of determining strain distribution in high strength CRAs on a microstructural scale using EBSD.