Synergic Role of Self-Interstitials and Vacancies in Indium Melting

Wire Tool Electrode Behavior and Wear under Discharge Pulses

This work is devoted to researching the tool electrode behavior and wear under discharge pulses at electrical discharge machining. The experiments were conducted on the workpieces of 12Kh18N10T (AISI 321) chrome-nickel anti-corrosion steel and D16 (AA 2024) duralumin by a 0.25-mm-diameter CuZn35 brass tool in a deionized water medium. The developed diagnostic and monitoring mean based on acoustic emission registered the oscillations accompanying machining at 4–8 kHz. The obtained workpiece and non-profiled tool surfaces were investigated by optical and scanning electron microscopy. Calculated volumetric and mass removal rates showed the difference in the character of wear at roughing and finishing. It was shown that interaction between material components in anti-corrosion steel machining had an explosive character between Zn of brass and Ni of steel at a micron level and formed multiple craters of 30–100 µm. The secondary structure and topology of worn tool surfaces were caused by material sublimation, chemical interaction between material components at high heat (10,000 °C), explosive deposition of the secondary structure. Acoustic diagnostics adequately registered the character of interaction. The observed phenomena at the submicron level and microstructure of the obtained surfaces provide grounding on the nature of material interactions and electrical erosion wear fundamentals.

Mechanical Spectroscopy Investigation of Point Defect-Driven Phenomena in a Cr Martensitic Steel

The paper presents and discusses results of mechanical spectroscopy (MS) tests carried out on a Cr martensitic steel. The study regards the following topics: (i) embrittlement induced by Cr segregation; (ii) interaction of hydrogen with C–Cr associates; (iii) nucleation of Cr carbides. The MS technique permitted characterising of the specific role played by point defects in the investigated phenomena: (i) Cr segregation depends on C–Cr associates distribution in as-quenched material, in particular, a slow cooling rate (~150 K/min) from austenitic field involves an unstable distribution, which leads to Cr concentration fluctuations after tempering at 973 K; (ii) hydrogen interacts with C–Cr associates, and the phenomenon hinders hydrogen attack (HA) because hydrogen atoms bound by C–Cr associates are not able to diffuse towards grain boundaries and dislocation where CH4 bubbles may nucleate, grow, and merge to form the typical HA cracks; (iii) C–Cr associates take part in the nucleation mechanism of Cr7C3 carbides, and specifically these carbides form by the aggregation of C–Cr associates with 1 Cr atom.

Experimental evidence for thermal generation of interstitials in a metallic crystal near the melting temperature

The only intrinsic point defects of simple crystalline metals known from solid state physics are vacancies and interstitials. It is widely believed that while vacancies play a major role in crystal properties and their concentration reaches relatively big values near the melting temperature T m, interstitials essentially do not occur in thermodynamic equilibrium and their influence on properties is minor. Here, taking aluminum single crystals as an example, we present compelling experimental evidence for rapid thermoactivated growth of interstitial concentration upon approaching T m. Using high precision measurements of the shear modulus we found a diaelastic effect of up to [Formula: see text] near T m. It is argued that this effect is mostly due to the generation of dumbbell (split) interstitials. The interstitial concentration c i rapidly increases upon approaching T m and becomes only 2-3 times smaller than that of vacancies just below T m. The reason for this c i -increase is conditioned by a decrease of the Gibbs free energy with temperature, which in turn originates from the high formation entropy of dumbbell interstitials and a decrease of their formation enthalpy at high c i . Special molecular dynamic simulation confirmed all basic aspects of the proposed interpretation. The results obtained (i) demonstrate the significance of interstitial concentration near T m that could lead to the revaluation of vacancy concentration at high temperatures, (ii) suggest that dumbbell interstitials play a major role in the melting mechanism of monatomic metallic crystals and (iii) support a new avenue for in-depth understanding of glassy metals.