M.X. Huang

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.

Effect of chemical composition on work hardening of Fe-Mn-C TWIP steels

The work hardening behaviour of Fe-Mn-C twinning induced plasticity (TWIP) steels with a wide compositional range has been investigated. Based on the consideration that twinning provides a dynamic composite effect resulting in high work hardening rate in TWIP steels, the present work proposes a model to describe such behaviour as a function of chemical composition. The model predictions are in good agreement with experimental observations.

Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence

Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97 Eu0.03 Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm-2 ) are particularly effective at reducing κL . When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.

Irreversible thermodynamics modelling of plastic deformation of metals

Abstract Irreversible thermodynamics is employed to describe plastic deformation of metallic single crystals and coarse grained polycrystals. Dislocations are assumed to increase the crystal entropy via processes of dislocation generation, glide and annihilation. It is postulated that the entropy progresses according to the relationship dS/dγ=κ(C/T)dτ/dγ, where S is the entropy of the deformed metal, γ is the shear strain, κ is a scaling factor measuring the average distance between dislocations, C is a material dependent constant, T is the absolute temperature and τ is the shear stress. A succinct expression for dislocation evolution is obtained; it is qualitatively similar to that proposed by Kocks and Mecking (Prog. Mater. Sci., 2003, 48, 171–273) on a phenomenological basis. The model is applied to the description of the deformation behaviour of Cu and Al with good results.

Modelling strength and ductility of ultrafine grained BCC and FCC alloys using irreversible thermodynamics

Abstract A novel grain size dependent strain hardening model is derived from the theory of irreversible thermodynamics. The model yields the evolution of the dislocation densities in the grain interior and at the grain boundary, as well as their contributions to the flow stress. It is found that submicron grain sizes have a lower dislocation density in the grain interior, causing ductility to decrease greatly. The predicted stress–strain curve shapes, uniform elongation and ultimate tensile strength values for interstitial free steels (body centred cubic) and aluminium alloys (AA1100, face centred cubic) show good agreement with experimental observations.

Modelling steady state deformation of fcc metals by non-equilibrium thermodynamics

Abstract The steady state of plastic deformation is modelled by non-equilibrium thermodynamics theory. Based on energy conservation and constant entropy requirements at the steady state, the saturation dislocation density ρ is found to be determined by ρ=λ ϵ/(bv c), where λ is a constant that depends on the material properties, ϵ is the strain rate, b is the magnitude of Burgers vector and v c is the dislocation climb velocity along the dislocation line. Then, by employing the Taylor relation, the saturation flow stress is obtained. The model is applied to four pure fcc single crystals under tensile testing and polycrystalline Al at steady state creep. The predictions are in good agreement with the experimental observations.

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.

Critical Assessment 15: Science of deformation and failure mechanisms in twinning induced plasticity steels

Abstract Manganese rich austenitic twinning induced plasticity steels with high strength and high ductility have been developed in 1990s as promising candidates for automotive applications. Tremendous efforts have therefore been made to explore the unusual deformation and failure mechanisms of these alloys. We provide here a critical assessment of the recent progress in understanding their deformation and failure mechanisms and discuss some scientific challenges that remain unresolved, for example, a physically based twinning kinetics model.

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.