In-situ observation of twinning and detwinning in AZ31 alloy

Twinning and detwinning behavior of a commercial AZ31 magnesium alloy during cyclic compression–tension deformation with a total strain amplitude of 4% (±2%) was evaluated using the complementary techniques of in-situ neutron diffraction, identical area electron backscatter diffraction, and transmission electron microscopy. In-situ neutron diffraction demonstrates that the compressive deformation was dominated by twin nucleation, twin growth, and basal slip, while detwinning dominated the unloading of compressive stresses and subsequent tension stage. With increasing number of cycles from one to eight: the volume fraction of twins at -2% strain gradually increased from 26.3% to 43.5%; the residual twins were present after 2% tension stage and their volume fraction increased from zero to 3.7% as well as a significant increase in their number; and the twinning spread from coarse grains to fine grains involving more grains for twinning. The increase in volume fraction and number of residual twins led to a transition from twin nucleation to twin growth, resulting in a decrease in yield strength of compression deformation with increasing cycles. A large number of -component dislocations observed in twins and the detwinned regions were attributed to the dislocation transmutation during the twinning and detwinning. The accumulation of barriers including twin boundaries and various types of dislocations enhanced the interactions of migrating twin boundary with these barriers during twinning and detwinning, which is considered to be the origin for increasing the work hardening rate in cyclic deformation of the AZ31 alloy

Load redistribution in eutectic high entropy alloy AlCoCrFeNi2.1 during high temperature deformation

The load redistribution between and within phases in eutectic high entropy alloy AlCoCrFeNi2.1 was measured using in-situ neutron diffraction during tensile deformation at 973 K. The load partitioning between phases is reversed compared to lower temperatures, with L12 becoming the stronger phase. The evolution of the orientation-specific stresses and strains in the L12 phase suggests that cube slip dominates the response. The low strength, internal load transfer and ideally plastic response of the B2 phase indicate a change in deformation mechanism compared to lower temperatures

Unexpected dynamic transformation from α phase to β phase in zirconium alloy revealed by in-situ neutron diffraction during high temperature deformation

Dynamic transformation from alpha (HCP) to beta (BCC) phase in a zirconium alloy was revealed by the use of in-situ neutron diffraction during hot compression. The dynamic transformation was unexpectedly detected during isothermal compression at temperatures of 900°C and 950°C (alpha + beta two-phase region) and strain rates of 0.01 s⁻¹ and 0.001 s⁻¹, even though equilibrium two-phase states were achieved prior to the hot compression. Dynamic transformation was accompanied by diffusion of Sn from beta to alpha phase, which resulted in changes of lattice parameters and a characteristic microstructure of alpha grains. The lattice constant of alpha phase measured by the in-situ neutron diffraction increased during the hot compression, while the lattice constant of beta phase exhibited an initial increase and subsequent decrease during the hot compression. As a result, the magnitude of lattice (elastic) strain as well as stress (elastic stress, or phase stress) in alpha phase was found to become much greater than those in beta phase. According to an atomistic simulation, the Gibbs free energy of alpha phase increased with hydrostatic compressive pressure more evidently than that of beta phase. It could be concluded from such results that the occurrence of the dynamic transformation from alpha to beta is attributed to an increase in the Gibbs free energy of alpha phase relative to beta phase owing to the difference in the phase stress; i.e., the larger lattice distortion made alpha phase thermodynamically more unstable than beta phase. The present result suggests that deformation of two-phase materials can dynamically make Gibbs free energy of plastically harder phase higher than that of the softer phase through increasing elastic energy in the harder phase, which might lead to dynamic transformation from harder phase to softer phase

Synergetic strengthening of layered steel sheet investigated using an in situ neutron diffraction tensile test

Synergetic strengthening induced by plastic strain incompatibility at the interface, and the resulting extra geometrically necessary dislocations (GNDs) generated during plastic deformation, were investigated to understand the origin of extra strength in heterogeneous structured (HS) materials. The mechanism of extra GND generation in twinning-induced plasticity (TWIP)-interstitial free (IF) steel layered sheet was quantitatively analyzed by conducting in situ neutron scattering tensile test. Load partitioning due to the different mechanical properties between the TWIP-steel core and IF-steel sheath at the TWIP/IF interface was observed during the in situ tensile testing. Because of the plastic strain incompatibility from load partitioning, extra GNDs are generated and saturate during tensile deformation. The extra GNDs can be correlated with the back-stress evolution of the HS materials, which contributes to the strength of layered materials. Because of the back-stress evolution caused by load partitioning, the strength of TWIP-IF layered steel is higher than the strength estimated by the rule-of-mixtures. This finding offers a mechanism by which extra GNDs are generated during load partitioning and shows how they contribute to the mechanical properties of HS materials.11Ysciescopu

Residual Strain Dependence on Matrix Structure in RHQ-Nb3Al Wires by Neutron Diffraction Measurement

We prepared three types of non-Cu RHQ-Nb3Al wire samples with different matrix structures: an all-Ta matrix,a composite matrix of Nb and Ta with a Ta inter filament, and an all-Nb matrix. Neutron diffraction patterns of the wire samples were measured at room temperature in J-PARC "TAKUMI". To obtain residual strains of materials, we estimated lattice constant a by multi-peak analysis in the wire. Powder sample of each wire was measured, where the powder was considered to be strain-free. The grain size of all the powder samples was below 0.02 mm. For wire sample with the all-Nb matrix, we also obtained lattice spacing d by a single-peak analysis. Residual strains of Nb3Al filament were estimated from the two analysis results and were compared. Result, residual strains obtained from the multi-peak analysis showed a good accuracy with small standard deviation. The multi-peak analysis results for the residual strains of Nb3Al filament in the three samples were all tensile residual strain in the axial direction, they are 0.12%, 0.12%, and 0.05% for the all-Ta matrix, the composite matrix, and the all-Nb matrix, respectively. Difference in the residual strain of Nb3Al filament between the composite and all-Nb matrix samples indicates that type of inter-filament materials show a great effect on the residual strain. In this paper, we report the method of measurement, method of analysis, and results for residual strain in the tree types of non-Cu RHO-Nb3Al wires.Comment: 7 pages, 8 figure

Enhancement of Uniform Elongation by Temperature Change during Tensile Deformation in a 0.2C TRIP Steel

It is important to control the deformation-induced martensitic transformation (DIMT) up to the latter part of the deformation to improve the uniform elongation (U.El) through the TRIP effect. In the present study, tensile tests with decreasing deformation temperatures were conducted to achieve continuous DIMT up to the latter part of the deformation. As a result, the U.El was improved by approximately 1.5 times compared with that in the tensile test conducted at 296 K. The enhancement of the U.El in the temperature change test was discussed with the use of neutron diffraction experiments. In the continuous DIMT behavior, a maximum transformation rate of about 0.4 was obtained at a true strain (ε) of 0.2, which was larger than that in the tensile test at 296 K. The tensile deformation behavior of ferrite (α), austenite (γ), and deformation-induced martensite (α′) phases were investigated from the viewpoint of the fraction weighted phase stress. The tensile test with a decreasing deformation temperature caused the increase of the fraction weighted phase stress of α and that of α′, which was affected by the DIMT behavior, resulting in the increase in the work hardening, and also controlled the ductility of α and α′, resulting in the enhancement of the U.El. Especially, the α phase contributed to maintaining high strength instead of α′ at a larger ε. Therefore, not only the DIMT behavior but also the deformation behavior of γ, α, and α′ are important in order to improve U.El due to the TRIP effect

Martensite phase stress and the strengthening mechanism in TRIP steel by neutron diffraction

自動車用鋼板の開発に新しい道筋 --先端鉄鋼「TRIP鋼」の引張力に対するふるまいを実験的に解明--. 京都大学プレスリリース. 2018-02-27.Two TRIP-aided multiphase steels with different carbon contents (0.2 and 0.4 mass%) were analyzed in situ during tensile deformation by time-of-flight neutron diffraction to clarify the deformation induced martensitic transformation behavior and its role on the strengthening mechanism. The difference in the carbon content affected mainly the difference in the phase fractions before deformation, where the higher carbon content increased the phase fraction of retained austenite (γ). However, the changes in the relative fraction of martensitic transformation with respect to the applied strain were found to be similar in both steels since the carbon concentrations in γ were similar regardless of different carbon contents. The phase stress of martensite was found much larger than that of γ or bainitic ferrite since the martensite was generated at the beginning of plastic deformation. Stress contributions to the flow stress were evaluated by multiplying the phase stresses and their phase fractions. The stress contribution from martensite was observed increasing during plastic deformation while that from bainitic ferrite hardly changing and that from γ decreasing