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Abstract This paper presents the results of an experimental study on evolution of the nanocrystalline microstructure in a mechanically alloyed Oxide dispersion strengthened (ODS) 18Cr ferritic steel powder during densification by spark plasma sintering (SPS) in the temperature range of 1273 K (1000 °C) to 1423 K (1150 °C). Systematic Electron Back-Scatter Diffraction analysis has been carried out to study the grain size distribution and texture as a function of consolidation temperatures. Based on the kinetics of the densification process and resultant microstructure/microtexture, a sintering temperature slightly above 1323 K (1050 °C) within a range of 50 K was found to be optimum. The 18Cr-ferritic steel powder consolidated at 1323 K (1050 °C) was also studied to understand the role of dispersoids on microstructure. The dispersoids exerted a profound influence on the strength as well as toughness of the steel by restricting the grain growth at high temperatures. Further, a signature of (1 1 0) grain cluster is observed during consolidation and its preferential growth with increase in sintering temperature is noticed which lead to the alignment of the (1 1 0) plane in the direction of applied pressure. The minimum creep rate of the consolidated steel under a load of 300 MPa was found to be 5E-7 h−1 and 1E-4 h−1 at 873 and 973 K (600 and 700 °C) respectively. The apparent activation energy for creep deformation was estimated as ~ 402 kJ/mol, which is typical of lattice diffusion assisted general climb mechanism of dislocations over the barriers such as present dispersoids. [ABSTRACT FROM AUTHOR]

The study presents an EBSD based investigation on the nature of deformation occurring under different modes of cyclic loading viz. low cycle fatigue (LCF), creep-fatigue interaction (CF) and LCF-creep-HCF. Compared to LCF, cyclic life was found to decrease for CF or LCF-creep-HCF loading conditions. This was attributed to the additional damage contributions from creep in CF cycling and a combination of creep and HCF in LCF-creep-HCF loading conditions, as substantiated through a highly intergranular fracture observed in those cases. Local misorientation map derived from EBSD showed that the misorientation spread is highest for the LCF condition compared to CF and LCF-creep-HCF. This was attributed to the enhanced thermal recovery taking place during CF and LCF-creep-HCF conditions in comparison to LCF. The dislocations generated during cycling rearrange themselves into a stabilized substructure in the form of cells in the above loading conditions leading to a lower value of local misorientation in those cases. This was also accompanied by a significant decrease in number of twins in CF and LCF-creep-HCF conditions compared to LCF. The dislocation-twin interaction responsible for the process of de-twinning was found to be expedited in the former cases owing to a higher thermal recovery. [ABSTRACT FROM AUTHOR]

This paper presents the mechanical behaviour of single crystal Ta (6.5 GPa–22 GPa) 1-D shocked along the three principal crystallographic directions [100], [110] and [111]. Room temperature mechanical testing was carried out on micropillars to make possible precise load and displacement measurements. The findings exhibit a marked variation in the flow curve and strain hardening between the three orientations, and relative changes are noticed with shock loading. The yield strength (YS) increases linearly as a function of shock load. The YS was highest along [111] (282 MPa) compared with the [100] and [110] orientations (194 and 206 MPa). The strain hardening rates without pre-shock loading are found to be in the order [111] > [100] > [110] and this is not altered by pre-shock loading. Deformation was accomplished by slip on {112} planes. Further, assessments are considered to identify effective plastic strain accumulation during shock loading. The accumulation of effective plastic strain was found to be similar for all directions for shock loading up to 6.5 GPa, however, above 15 GPa the effective strain was lower for [110] compared to [100] and [111] directions. An attempt is made here to assess the flow curve hardening behaviour and variation in yield strength based on a different combination of pair orientation to throw some light into materials response at high-pressure shock loading. [ABSTRACT FROM AUTHOR]

The high-temperature deformation behaviour of Al 0.7 CoCrFeNi alloy was studied at 800–1100 °C and 0.01–10 s−1. The alloy exhibited extensive softening via recrystallization and recovery with the examined temperature and strain rate domains. The recrystallization mechanism in the FCC phase was geometric dynamic recrystallization, and that in the BCC/B2 phase was continuous dynamic recrystallization. Further, the BCC/B2 lamellae underwent fragmentation via boundary splitting and termination migration. The amount of softening and subgrain morphology were dependent on the crystallographic orientation of the grains prior to deformation. Apart from lamellar fragmentation, dynamic globularization of BCC/B2 phase was found in certain grain colonies. Compared to heat treatment, hot deformation improved the globularization kinetics. Further, BCC/B2 precipitation occurred along the grain boundaries and at the triple points of the recrystallized FCC phase. However, the overall BCC/B2 phase fraction remained constant. Post-deformed alloy was also characterized by preferential cracking along the allotriomorphic FCC phase boundaries. The difference in strength between the idiomorphic and allotriomorphic regions was attributed to this grain boundary cracking. The activation energy for deformation showed a direct dependency on temperature and strain rate but an inverse relationship with strain. • Alloy displayed extensive softening via dynamic recovery and recrystallization. • Recrystallization mechanisms in FCC and BCC were GDRX and CDRX, respectively. • Localized globularization, lamellar fragmentation and precipitation of BCC/B2. • Preferential cracking along the allotriomorphic FCC boundaries. • Variation of Q, n and m with thermo-mechanical parameters. [ABSTRACT FROM AUTHOR]

Present investigation is aimed at understanding the deformation behavior and the development of damage in a type 316 LN austenitic stainless steel (SS) weld joint (WJ) under isothermal low cycle fatigue (IF) and thermomechanical fatigue (TMF). In-phase (IP) and out-of-phase (OP) TMF tests were carried out by maintaining a phasing relation of 0° and 180° respectively, between the mechanical strain and temperature cycles. Dynamic strain ageing (DSA) was found to exert a strong influence on the cyclic stress response (CSR) of the joint under both IF and TMF cycling. The CSR was observed to be higher under TMF compared to IF cycling due to the activation of additional hardening mechanisms in the former tests. The difference in fatigue life under TMF and IF is rationalized based on the development of deformation and damage through optical and scanning electron microscopy (SEM) coupled with electron backscatter diffraction (EBSD) studies. A clear demarcation of the strain distribution in the base metal (BM) and weld metal (WM) region of the joint is achieved through detailed EBSD analysis of the tested specimens. Cyclic plastic deformation and the associated development of damage take place independently in the BM and the WM parts of the joint, competing to cause the failure. The build-up of damage under IF and TMF cycling is dictated by a combination of DSA and δ-ferrite transformation, depending on the applied strain amplitude and the strain-temperature phasing employed. The observed life variations have been rationalized in terms of the substructural evolution and fracture behavior under different testing conditions. • The CSR and fatigue life depend on the type of fatigue cycle (IF and IP/OP TMF). • Thermal cycling and imposed strain influenced the dynamic strain ageing behavior. • Failure location across the joint changes with the applied strain and type of cycle. • Localized deformation in the joint depends on the δ-ferrite transformation and DSA. [ABSTRACT FROM AUTHOR]

HCF-creep interaction in type 316LN stainless steel is investigated at 923 K by conducting HCF tests well below the endurance limit with high mean stress (high R- ratio). High mean stress inflicts creep damage at 923 K, which when imposed on a small alternating stress, leads to significant HCF-creep interaction. Depending on the magnitude of mean stress (R -ratio), three distinct regimes are identified, viz. (a) HCF regime (b) HCF-creep interaction regime and (c) creep regime. Detailed fractographic investigation revealed distinct fracture characteristics pertaining to each of the three regimes. HCF-creep interaction regime is marked by significant intergranularity at low values of mean stress which changes to predominantly dimpled fracture with increasing mean stress. The above observations are corroborated through detailed EBSD investigations which bring out the underlying deformation mechanisms responsible for failure under HCF-creep interaction. [ABSTRACT FROM AUTHOR]

This study aims to quantify the improvement in the creep crack growth (CCG) resistance in the heat-affected zone (HAZ) of boron added modified 9Cr–1Mo (P91B) steel weld at 898 K. The crack growth rates in the HAZ of P91B weld have been compared with the crack growth rates in HAZ of without boron P91 steel weld. The creep crack growth rates are estimated as per ASTM E1457 using 7 mm thick compact tension specimens with a notch in the HAZ and test duration 248h-11,000 h. The CCG rate ( a ˙) vs. C* correlation, commonly expressed as a ˙ = D (C ∗) ϕ , has been established. In comparison, the crack growth rate in the HAZ of P91 weld is found to be threefold as compared to HAZ of P91B weld; with the difference in CCG rate increasing towards a higher C* range. Creep damage assessment study has been done in the vicinity of creep crack in HAZ of a P91B weld specimen tested for 11,000 h. The extent of cavitation, cavities link-up, and micro-crack formation in HAZ of P91B weld is found to be relatively lower than HAZ of P91 weld. Most of the cavities appear to be discrete in nature. This testifies improvement in the creep crack growth resistance in the HAZ of P91B weld due to controlled boron addition of about 100 ppm to modified 9Cr–1Mo steel. [ABSTRACT FROM AUTHOR]