Role of stress-assisted martensite in the design of strong ultrafine-grained duplex steels

This work explains the occurrence of transformation-induced plasticity via stress-assisted martensite, when designing ultrafine-grained duplex steels. It is found that, when the austenite is reduced to a fine scale of about 300 nm, the initial deformation-induced microstructure can be dominated by parallel lamellae of epsilon martensite or mechanical twinning, which cannot efficiently provide nucleation sites for strain-induced martensite. Hence, alpha martensite nucleation occurs independently by a stress-assisted process that enhances transformation-induced plasticity in ultrafine-grained austenite. This metallurgical principle was validated experimentally by using a combination of transmission Kikuchi diffraction mapping, transmission electron microscopy, and atom probe microscopy, and demonstrated theoretically by the thermodynamics model of stress-assisted martensite.The authors acknowledge the facilities, and the scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility (ammrf.org.au) node at Sydney Microscopy & Microanalysis, at the University of Sydney.This is the accepted manuscript. The final version is available from Elsevier at http://www.sciencedirect.com/science/article/pii/S1359645414006958

The role of graphite addition on spark plasma sintered titanium nitride

Abstract: tComposites of titanium nitride reinforced with graphite were synthesized using sparkplasma sintering at 2000◦C. The effects of graphite addition on the microstructure, relativedensity, and mechanical properties of TiN ceramics matrix were examined. The investiga-tion was performed on TiN powder with varying graphite content (1–5 wt.%) for 8 h using anenergy ball milling equipment. Results show that TiN without and with graphite (TiN + 1 wt.%graphite) sintered at 2000◦C recorded sintered relative density of 96.7% and 97% respec-tively. Additionally, TiN with 3 wt.% graphite had a relative density of 98%. However, theshrinkage of TiN + 3 wt.% graphite was observed to be the lowest compared to other com-posites at the same sintering conditions. Microstructural analysis indicates that the grainof titanium nitride in the composite was very fine and continuous. Subsequently, a bimodalparticle sizes were observed when 5 wt.% graphite was dispersed in TiN. The highest Vickersmicrohardness of 23.5 GPa and fracture toughness of 6.5 MPa m1/2were achieved with com-posites reinforced with 3 wt.% graphite at milling period of 8 h. The combination of TEM/EDSand HRTEM/FFT show a single pattern of diffraction and consistency in interplanar distanceobtained from X-ray diffractometry of the milled sample. There is a clear coherence interfacebetween the phases

Design of Advanced High Strength Steels

A new advanced high strength steels (AHSS) is designed based on Fe-C-Mn-Al composition. Martensitic steel is processed in intercritical region to achieve an ultrafine-grained duplex γ–(α + α') microstructure. The focus was on tuning the degree of austenite plasticity via controlling its stability, called austenite engineering. Interest in austenite engineering stems from transformation-induced plasticity (TRIP) effect, which is known to enhance ductility. The thermodynamic and kinetic analyses were used to optimize the annealing condition. The evolution of microstructure and mechanical properties was studied using different techniques. Due to high heating rate, the austenite reversion occurred before recrystallization of the ferrite. The final microstructure was duplex steel with globular-shaped grains. High volume fraction of the austenite phase was obtained (f_γ>40%) in very short time annealing. By increasing annealing temperature and time, austenite fraction and grain size increased. However, due to dilution of the austenite from stabilizers elements, the stability of the austenite dropped and transformed into martensite during quenching. This led in variety of austenite stabilities that resulted in different combination of mechanical properties. The critical factors influencing the onset of TRIP effect is studied and it was found that both early and delayed onset of the TRIP effect will lead to worse ductility. Hence, to achieve ultrahigh strength and excellent ductility, austenite stability shall be controlled to precisely trigger out TRIP. This study find out that discontinuous yielding or Lüders bands phenomenon can be used in ultrafine duplex steels to improve ductility. The results showed that superb combination of strength (σ_YS>1.0GPa and σ_UTS>1.4GPa) and ductility (ε_t≥20%) could be achieved in short time annealing of less than 10 minutes. This work evidence that tuning the austenite to a marginal stability enables us to design strong and ductile steels