Diamonds from the Mir Pipe (Yakutia): Spectroscopic Features and Annealing Studies

For this study, 21 samples of colorless octahedral diamonds (weighing 5.4–55.0 mg) from the Mir pipe (Yakutia) were investigated with photoluminescence (PL), infrared (IR), and electron paramagnetic resonance (EPR) spectroscopies. Based on the IR data, three groups of diamonds belonging to types IIa, IaAB, and IaB were selected and their spectroscopic features were analyzed in detail. The three categories of stones exhibited different characteristic PL systems. The type IaB diamonds demonstrated dominating nitrogen–nickel complexes S2, S3, and 523 nm, while they were less intensive or even absent in the type IaAB crystals. The type IIa diamonds showed a double peak at 417.4+418.7 nm (the 418 center in this study), which is assumed to be a nickel–boron defect. In the crystals analyzed, no matter which type, 490.7, 563.5, 613, and 676.3 nm systems of various intensity could be detected; moreover, N3, H3, and H4 centers were very common. The step-by-step annealing experiments were performed in the temperature range of 600–1700 °C. The treatment at 600 °C resulted in the 563.5 nm system’s disappearance; the interstitial carbon vacancy annihilation could be considered as a reason. The 676.5 nm and 613 nm defects annealed out at 1500 °C and 1700 °C, respectively. Furthermore, as a result of annealing at 1500 °C, the 558.5 and 576 nm centers characteristic of superdeep diamonds from São Luis (Brazil) appeared. These transformations could be explained by nitrogen diffusion or interaction with the dislocations and/or vacancies produced.

Understanding the Efficacy of Concentrated Interstitial Carbon in Enhancing the Pitting Corrosion Resistance of Stainless Steel

By introducing a high fraction of interstitial carbon through low temperature carburization, the pitting corrosion resistance of austenitic stainless steel can be significantly improved. Previous work attributed this enhancement to the improvement of passive film properties. However, we show here that interstitial carbon actually weakens the passive film on stainless steel. In fact, the enhancement in pitting resistance is a result of carbon reducing the metal dissolution rate in a local pit environment by many orders of magnitude, which extremely decreases the growth stability of a pit and prevents it from transitioning into stable growth. Electronic structure calculations show that carbon bonds to the metal atoms and that the metal–carbon bonds are 1.6 to 2.0 times stronger than the metal–metal bonds. Different from prior theories, we show that the significant increase of pitting resistance originates from the formation of covalent bonds between interstitial carbon and its neighboring metal atoms, resulting in a significantly reduced dissolution rate. This study indicates a new strategy for the design of corrosion resistant alloys, namely alloying with concentrated interstitials that form strong bonds with the matrix atoms.