Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

A computational method for the analysis of phase transformation involving solidification was developed with the assumption of thermodynamic equilibria at interfaces. The region of interest was divided into finite segments, and solute diffusion across the segments was computed by the use of the direct finite difference method (FDM). Simultaneously, thermodynamic equilibrium at each interface was updated at every step of the diffusion analysis to determine the location of the interfaces. The temperature decrease and the increment of fraction solid were calculated based on thermal balance, including a heat extraction condition. Solid state transformation from δ to γ phase within each FDM segment was modeled by the use of a Clyne-Kurz (C-K) type analysis with assumptions of complete mixing of solutes in theδ phase and limited back diffusion in theγ phase. The calculation results were compared with welding solidification experiments in the iron-chromium-nickel ternary system. Good agreement was obtained with respect to solute distribution and residual fraction ofδ phase over different compositions and solidification modes of the alloys used.

The effects of microstructure, sample dimensions, and cross-sectional geometry on the deformation characteristics of electroplated nanocrystalline copper sub-micron pillars are investigated. Nanocrystalline copper pillars were produced with four types of geometry—solid core, hollow, c-shaped, and x-shaped—with outer diameters of ~1000 or 220 nm and three different average grain sizes (between 5.1 and 49.3 nm). Flow stress results from uniaxial compression tests of 1000- and 220-nm-outer-diameter pillars, with average grain sizes in the range between ~32 and 50 nm, revealed there are no observable strength dependences with the pillar cross-sectional geometries. This suggests that they behave with bulk-like character: mechanical properties independent of size and sample geometry. All of the pillar specimens examined exhibit an increase in mechanical strength with reduction of grain sizes, but soften as the crystalline dimensions are smaller than 10 to 20 nm threshold limits. Interestingly, pillars with outer diameters of 220 nm are distinctively softer than the 1000-nm-diameter samples when their grain size is at and below this threshold limit. These results indicate a strength specimen size effect exists for such fine grain copper pillars.

Part I of this study described the changes in order and of structure during cold rolling of a Ni3Al(B,Zr) alloy. The textural and microstructural changes that occur during deformation are reported in this part. In addition to other features, a high density of shear bands start forming in this alloy from a rather early stage of deformation. The cold rolling texture of the material, which is basically of pure metal type at low strain levels, changes into alloy type after rolling between 35 and 45 pct. The maximum pole density of the alloy type texture is obtained at the {168}〈211〉 location. Transmission electron microscopy (TEM) micrographs show the presence of twins in the material from a deformation level of 35 pct onward, their density increasing with increase in deformation level. As has been proposed earlier, a structural transformation from L12 to DO22 appears to take place in the γ′ phase during rolling. This will change the deformation mode from primarily slip to twinning and this could be responsible for the observed textural change with rolling. The γ phase deforms by slip in a manner similar to a fcc material with high stacking fault energy. The final texture of the material actually reflects an aggregate of the components developed in the two phases.

This study examines the sintering behavior and properties of Fe-0.8Mn-0.5C manganese powder metallurgy steels. The study focuses on the influence of mode of alloying—admixing using either high-purity electrolytic manganese or medium carbon ferromanganese as well as the fully prealloying of water-atomized powder. Three main aspects were studied during the whole sintering process—microstructure development, interparticle necks evolution, and changes in the behavior of manganese carrier particles during both heating and sintering stages. The prealloyed powder shows considerable improvement in carbon homogenization and interparticle neck development in comparison with admixed materials. The first indication of pearlite for the fully prealloyed material was registered at ~1013 K (740 °C) in comparison with ~1098 K (825 °C) in the case of the admixed systems. The negative effect of the oxidized residuals of manganese carrier particles and high microstructure inhomogeneity, which is a characteristic feature of admixed systems, is reflected in the lower values of the mechanical properties. The worst results in this respect were obtained for the system admixed with electrolytic manganese because of more intensive manganese sublimation and resulting oxidation at lower temperatures. According to the results of X-ray photoelectron spectroscopy and high-resolution scanning electron microscopy and energy dispersive X-ray analyses, the observed high brittleness of admixed materials is connected with intergranular decohesion failure associated with manganese oxide formation on the grain boundaries.

Designers of new power-generation plants are looking to make use of new and existing high-strength austenitic steels so that these plants can operate with much higher steam and, therefore, metal temperatures. However, this article shows that the Wilshire–Scharning methodology is incapable of producing accurate long-term-life predictions of these materials from short-term data. This article puts forward a modification of this approach that should enable existing and newly developed austenitic stainless steels to be brought into safe operation more cost effectively and over a shorter time span. The estimation of this model showed that the activation energy for creep was dependent on whether the test stress was above or below the yield stress. Analysis of the results from tests lasting only up to 5000 hours accurately predict the creep lives for stress-temperature conditions, causing failure in 100,000 hours or more.

In this work, two medium Mn steels (5.8 and 5.7 wt pct Mn) were subjected to a quenching and partitioning (Q&P) treatment employing a partitioning temperature which corresponded to the start of austenite reverse transformation (ART). The influence of a 1.6 wt pct Ni addition in one of the steels and cycle parameters on austenite stability and mechanical properties was also studied. High contents of retained austenite were obtained in the lower quenching temperature (QT) condition, which at the same time resulted in a finer microstructure. The addition of Ni was effective in stabilizing higher contents of austenite. The partitioning of Mn and Ni from martensite into austenite was observed by TEM–EDS. The partitioning behaviour of Mn depended on the QT condition. The lower QT condition facilitated Mn enrichment of austenite laths during partitioning and stabilization of a higher content of austenite. The medium Mn steel containing Ni showed outstanding values of the product of tensile strength (TS) and total elongation (TEL) in the lower QT condition and a higher mechanical stability of the austenite.

Surface modification by the generation of a nanostructured surface layer induced via ultrasonic impact treatment was performed at the weld toe of a welded high-strength quenched and tempered structural steel, S690QL1 (Fe-0.16C-0.2Si-0.87Mn-0.33Cr-0.21Mo (wt pct)). Such high-frequency peening techniques are known to improve the fatigue life of welded components. The nanocrystallized structure as a function of depth from the top-treated surface was characterized via a recently developed automated crystal orientation mapping in transmission electron microscopy. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation during the ultrasonic impact treatment is proposed. It involves the formation of low-angle misoriented lamellae displaying a high density of dislocations followed by the subdivision of microbands into blocks and the resulting formation of polygonal submicronic grains. These submicronic grains further breakdown into nano grains. The results show the presence of retained austenite even after severe surface plastic deformation. The average grain size of the retained austenite and martensite is $$\sim $$ 17 and $$\sim $$ 35 nm, respectively. The in-grain deformation mechanisms are different in larger and smaller grains. Larger grains show long-range lattice rotations, while smaller grains show plastic deformation through grain rotation. Also the smaller nano grains exhibit the presence of short-range disorder. Surface nanocrystallization also leads to an increased fraction of low angle and low energy coincident site lattice boundaries especially in the smaller grains ( $$D<50$$  nm).