Analyzing Boron in 9–12% Chromium Steels Using Atom Probe Tomography

Abstract

Small additions of boron can remarkably improve the long-term creep resistance of 9–12% Cr steels. The improvement has been attributed to boron segregation to grain boundaries during quenching, and subsequent boron incorporation into certain families of precipitates during tempering. However, the detailed mechanisms are not yet fully understood. Atom probe tomography (APT) is an excellent technique for gaining insights into boron distribution, however, in order to acquire accurate analysis of boron in 9–12% Cr steels using APT, there are several key challenges. In order to better understand and address these challenges, we developed a novel method for site-specific APT specimen preparation, which enables convenient preparation of specimens containing specifically selected grain boundaries positioned approximately perpendicular to the axis of the APT tip. Additionally, when analyzing boron at boundaries and in carbides (as diluted solute) and borides, a widening of the profile of boron distribution compared to other elements was repeatedly observed. This phenomenon is particularly analyzed and discussed in light of the evaporation field of different elements. Finally, the possible effects of detector dead-time on quantitative analysis of boron in metal borides are discussed. A simple method using 10B correction was used to obtain good quantification.

Atom Probe Tomography of Hard Nitride and Boride Thin Films

“…Deviations in the isotopic signature from that of the natural abundance (or from an abundance measured with another technique) in APT measurements suggests preferential losses of certain isotopes. However, the deviation can also be used to estimate the losses and correct the average composition, even though the spatial coordinates of the isotopes are lost [103,[121][122][123].…”

“…In the case of B, the losses of 10 B might not be insignificant since the 10 B/ 11 B ratio is 20/80. This can be corrected for by using an iterative correction of both 10 B and 11 B until the values stabilize at the selected isotope ratio [123].…”

“…The pile-up pairs correction is then used to correct the number of 10 B ions, which is used as input in the 10 B-method to correct 11 B. When compared in Paper IV, it yielded more accurate values than the 10 B-method, but no significant difference from the iterative 10 B-method [123] was found, and the benefits of the pile-up pairs correction, with higher precision and background correction independence, was not retained. It may, however, enable correction with 10 B-and 13 C-like methods for elements with more complex isotopic signature than C and B, i.e., with more than two isotopes, as each additional isotope makes the iterative method more troublesome to implement.…”

The Role of Deformation in Coarsening of M23C6 Carbide Particles in 9% Cr Steel

“…An increase in the boron content with a simultaneous decrease in the nitrogen content makes it possible to increase the operating temperatures of high chromium content steels of the martensitic class [1][2][3][4][5][6][7][8]. Using this approach to alloying, new thermal engineering steels MARBN, P93, and G115 for use as newgeneration thermal engineering materials have been developed in Japan and China [3,7].…”

“…It has previously been established [2,[4][5][6][7][8]13] that an increased boron content slows down the process of enlargement of M 23 C 6 particles. It is believed that doping with boron leads to a decrease in the interfacial energy of M 23 C 6 carbide, improves the stability against coagulation, and ensures structural stability due to the Zener pinning force [2,5,14,15].…”