Lijia Zhao

Significant contribution of stacking faults to the strain hardening behavior of Cu-15%Al alloy with different grain sizes.

It is commonly accepted that twinning can induce an increase of strain-hardening rate during the tensile process of face-centered cubic (FCC) metals and alloys with low stacking fault energy (SFE). In this study, we explored the grain size effect on the strain-hardening behavior of a Cu-15u2009at.%Al alloy with low SFE. Instead of twinning, we detected a significant contribution of stacking faults (SFs) irrespective of the grain size even in the initial stage of tensile process. In contrast, twinning was more sensitive to the grain size, and the onset of deformation twins might be postponed to a higher strain with increasing the grain size. In the Cu-15u2009at.%Al alloy with a mean grain size of 47u2009μm, there was a stage where the strain-hardening rate increases with strain, and this was mainly induced by the SFs instead of twinning. Thus in parallel with the TWIP effect, we proposed that SFs also contribute significantly to the plasticity of FCC alloys with low SFE.

Optimizing strength and ductility in Cu–Al alloy with recrystallized nanostructures formed by simple cold rolling and annealing

A single-phase Cu–Al alloy with a low stacking fault energy was processed by cold rolling and subsequent annealing. Fully recrystallized microstructures composed of ultrafine grains were obtained after isothermal annealing at different temperatures. The minimum mean grain sizes achieved were below 1xa0μm. It was found that the microstructures were homogeneous after annealing at 400xa0°C, but somehow inhomogeneous after annealing at lower temperature of 300xa0°C and higher temperature of 550xa0°C. Superior strength and ductility were obtained by controlling the grain size of the microstructures. After annealing at 400xa0°C for 10xa0s, a fully and homogeneously recrystallized material with mean grain size of 770xa0nm was produced, which had a high yield strength of 524.2xa0MPa and a remarkable uniform elongation of 15.7xa0%.

The role of configurational disorder on plastic and dynamic deformation in Cu 64 Zr 36 metallic glasses: A molecular dynamics analysis

The varying degrees of configurational disorder in metallic glasses are investigated quantitatively by molecular dynamics studies. A parameter, the quasi-nearest atom, is used to characterize the configurational disorder in metallic glasses. Our observations suggest configurational disorder play a role in structural heterogeneity, plasticity and dynamic relaxations in metallic glasses. The broad configurational disorder regions distribution is the indicator of abundant potential deformation units and relaxations. Plastic flow, as well as relaxation, is believed to start at configurational disorder regions. The width of the shear bands and dynamic relaxations can then be regulated by the degree of configurational disorder regions in metallic glasses.

Novel thermomechanical processing methods for achieving ultragrain refinement of low-carbon steel without heavy plastic deformation

ABSTRACT Two novel two-step thermomechanical routes were developed to produce ultrafine-grained ferrite microstructures in a 10Ni–0.1C steel without high-strain deformation. Homogeneous ultrafine ferrite (UFF) structures having mean grain sizes down to 460u2005nm were fabricated by a total equivalent strain of 0.92 and exhibited high yield strength of 820u2005MPa with large uniform elongation of 10% and total elongation of 29%. The formation of UFF was attributed to two different phenomena, i.e. dynamic transformation from austenite to ferrite and dynamic recrystallization of ferrite. GRAPHICAL ABSTRACT IMPACT STATEMENT Ultragrain refinement and remarkable strength-ductility properties can be achieved by controlling dynamic transformation and recrystallization in a thermomechanically controlled process without high-strain deformation.

Combination of dynamic transformation and dynamic recrystallization for realizing ultrafine-grained steels with superior mechanical properties

Dynamic recrystallization (DRX) is an important grain refinement mechanism to fabricate steels with high strength and high ductility (toughness). The conventional DRX mechanism has reached the limitation of refining grains to several microns even though employing high-strain deformation. Here we show a DRX phenomenon occurring in the dynamically transformed (DT) ferrite, by which the required strain for the operation of DRX and the formation of ultrafine grains is significantly reduced. The DRX of DT ferrite shows an unconventional temperature dependence, which suggests an optimal condition for grain refinement. We further show that new strategies for ultra grain refinement can be evoked by combining DT and DRX mechanisms, based on which fully ultrafine microstructures having a mean grain size down to 0.35 microns can be obtained without high-strain deformation and exhibit superior mechanical properties. This study will open the door to achieving optimal grain refinement to nanoscale in a variety of steels requiring no high-strain deformation in practical industrial application.

Microstructural Evolution of Ferrite Grains during Dynamic Transformation in 10Ni-0.1C Steel

The effect of strain on the microstructural evolution of ferrite grains during dynamic transformation was investigated using a 10Ni-0.lC steel uniaxially compressed at a strain rate of 10−2 s−1 and temperature of 520 ℃. The deformation of the ferrite formed at relatively early stages of dynamic transformation led to the formation of subgrains inside the ferrite grains. The misorientation between subgrains changed from low angles to high angles with increasing strain. The formation of equiaxed grains surrounded by high angle boundaries was confirmed, of which volume fraction increased with increasing compression strain. The results indicated that the grain refinement in the process was not only due to dynamic transformation but also due to the deformation and dynamic recovery/recrystallization of ferrite grains transformed at relatively early stages of dynamic transformation.

Deformation-assisted diffusion for the enhanced kinetics of dynamic phase transformation

ABSTRACT Comparison on the kinetics of two different phase transformations, including phase transformation after deformation and phase transformation during deformation (i.e. dynamic transformation, DT), reveals a new discovery that the transformation kinetics can be significantly enhanced in DT even under low driving forces. DT enables continuous generation of defects (e.g. dislocations) near the phase boundary, which can act as short-circuiting diffusion paths for atoms. The diffusivity of atoms is enhanced and the activation energy for the atom jump across the phase boundary is lowered under stress during DT, resulting in more pronounced grain growth as well as accelerated transformation kinetics. GRAPHICAL ABSTRACT Impact Statement Deformation-enhanced grain growth is revealed in dynamic phase transformation, which will promote microstructure and property design of structural materials where phase transformations occur.

Microstructural evolution of metastable austenitic steel during high-pressure torsion and subsequent heat treatment

Metastable austenite in a Fe-24Ni-0.3C (wt.%) alloy was processed by high-pressure torsion and subsequently heat-treated. The HPT-processed material had lamellae structures composed of highly deformed austenite and deformation-induced martensite. The reverse transformation of the deformation-induced martensite and recovery/recrystallization of the retained austenite completed above 500 °C and resulted in fully annealed and single-phase austenite with different grain sizes. The ultrafine-grained and nanocrystalline austenite showed high yield strength and large ductility due to transformation-induced plasticity.