B.B. He

High dislocation density–induced large ductility in deformed and partitioned steels

A ductile steel shows its strength Many industrial applications require materials to have high strength while remaining pliable, or ductile. However, the microstructure that increases strength tends to reduce ductility. He et al. used a processing mechanism to create a “forest” of line defects in manganese steel. This deformed and partitioned steel was produced by cold-rolling and low-temperature annealing and contained a dislocation network that improved both strength and ductility. Science, this issue p. 1029 Deformation and low-temperature annealing creates a high-strength steel with large ductility. A wide variety of industrial applications require materials with high strength and ductility. Unfortunately, the strategies for increasing material strength, such as processing to create line defects (dislocations), tend to decrease ductility. We developed a strategy to circumvent this in inexpensive, medium manganese steel. Cold rolling followed by low-temperature tempering developed steel with metastable austenite grains embedded in a highly dislocated martensite matrix. This deformed and partitioned (D and P) process produced dislocation hardening but retained high ductility, both through the glide of intensive mobile dislocations and by allowing us to control martensitic transformation. The D and P strategy should apply to any other alloy with deformation-induced martensitic transformation and provides a pathway for the development of high-strength, high-ductility materials.

On the nanoindentation behaviour of complex ferritic phases

Systematic nanohardness measurements on martensite, lower bainite, upper bainite, granular bainite and ferrite were carried out using nanoindentation technique. The variation of nanohardness among these ferritic phases is ascribed to the different capacity of strengthening factors in confining the geometrically necessary dislocation within indentation plastic zone. The large scatter of nanohardness distribution in lath martensite may be due to the uneven block boundary strengthening, inhomogeneous dislocation forest strengthening and the different extent of auto-tempering.

Effect of ausforming temperature and strain on the bainitic transformation kinetics of a low carbon boron steel

The effect of ausforming temperature and strain on the bainitic transformation kinetics was investigated in a low carbon boron steel. A new mechanism, which is based on the competition between the increase in nucleation rate and the decrease in average volume of bainite sheaf after deformation, is proposed. The increase in nucleation rate is due to the decrease in boron concentration at the grain boundaries after small deformation and the formation of sub-grain boundaries at the grain interior after large deformation. The decrease in average volume of bainite sheaf is ascribed to the frequent impingement of bainite sub-units after deformation. The increase in nucleation rate after deformation results in the decrease in incubation time, which is confirmed from the experiment. The increase in nucleation rate overcomes the decrease in average volume of bainite sheaf, resulting in the increase in transformation velocity and volume fraction after small deformation. On the contrary, the decrease in the average volume of bainite sheaf overcomes the increase in nucleation rate after large deformation, leading to the decrease in transformation velocity and volume fraction of bainite.

On the Mechanical Stability of Austenite Matrix After Martensite Formation in a Medium Mn Steel

The present work employs the nanoindentation technique to investigate the effect of prior martensite formation on the mechanical stability of a retained austenite matrix. It is found that the small austenite grains that were surrounded by martensite laths have higher mechanical stability than the large austenite grains that were free of martensite laths. The higher mechanical stability of small austenite grains is due to its higher amount of defects resulting from the prior martensite formation. These defects act as barriers for the later martensite formation and therefore contribute to the higher mechanical stability of small austenite grains. As a result, the present work suggests that the formation of martensite tends to stabilize the surrounding austenite matrix. Therefore, it may explain the lower transformed amount of martensite after quenching as compared to the theoretical calculation using the Koistinen and Marburger (K–M) equation.

Effect of Free Surface on the Stability of Individual Retained Austenite Grains in a Duplex Stainless Steel

The present work explored the effect of free surface on the stability of individual austenite grains in a duplex stainless steel. It was found that martensitic transformation took place automatically in the retained austenite grain when a free surface was introduced. This is due to the fact that the martensite nucleation energy barrier can be lowered to a thermally surmountable value as the strain energy induced by martensitic transformation is largely lowered when the matrix constraints were removed.

Revealing heterogeneous C partitioning in a medium Mn steel by nanoindentation

The present work employs nano-indentation technique to investigate the C partitioning in a medium Mn steel subjected to quenching and tempering process. It is found that the C partitioning between martensite and austenite is inhomogeneous. Particularly, the large lenticular martensite has negligible C partitioning into the austenitic matrix as it maintains an ultra-high nanohardness comparable to the one without tempering. Strategies to suppress the formation of large martensite are discussed.

Martensitic Transformation in Micron-Sized Fcc Single Crystals

Detailed transmission electron microscopy examinations verified that α′-martensite formed in micron-sized pillars is nearly dislocation free, surprisingly different than its counterpart in bulk samples, which usually contains a high dislocation density. Furthermore, the martensite was found to nucleate at the intersection between two packets of stacking faults in this low stacking fault energy material. A corresponding mechanism for the nucleation and growth of martensite in micron-sized pillars was proposed.

Simultaneous Increase of Both Strength and Ductility of Medium Mn Transformation-Induced Plasticity Steel by Vanadium Alloying

In general, microalloying such as vanadium (V) is considered to deteriorate the mechanical stability of austenite grains in medium Mn steel due to the consumption of C content. In this paper, we show that the mechanical stability of austenite grains could be optimized by V-alloying. This is because the V-alloying will refine the austenite grain size and tends to stabilize the austenite grains. The competition between the grain refinement and the reduced C content results in proper mechanical stability of austenite grains, providing continuous transformation-induced plasticity (TRIP) effect. In addition, the V-alloying suppresses the formation of intergranular cracks, leading to a ductile fracture morphology and a large non-uniform elongation.

Growth Mechanism of Primary and Eutectic TiB2 Particles in a Hypereutectic Steel Matrix Composite

The growth mechanism of primary and eutectic TiB2 particles in a hypereutectic steel matrix composite (SMC) has been investigated by combining microstructure and crystallographic analysis in the present work. It is found that the TiB2 particles in the as-cast microstructure have complex morphologies including two kinds of primary particles and several categories of eutectic particles. Twin-induced dendritic growth of primary TiB2 particles and epitaxial growth of eutectic fibers are found in the present SMC by detailed crystallography analysis. Furthermore, we demonstrate that the crystallographic features strongly affect the solidification process and the final microstructures. Finally, several alloying strategies are proposed to control the solidification microstructure.