Tensile deformation of fine-grained Mg at 4K,78K and 298K

The impact of grain size,ranging from 0.9μm to 9μm,on the mechanical properties of commercially pure Mg is investigated at temperatures of 4K,78K,and 298K.The mechanisms governing plastic flow are influenced by both grain size and temperature.At 4K and 78K,dominant deformation modes in Mg involve dislocation glide and extension twinning,regardless of grain size.The interactions between basal and non-basal dislocations and dislocations with grain boundaries promote an unusually high rate of work hardening in the plastic regime,leading to premature failure.The yield stress follows the Hall-Petch relationshipσ_(y)∼k/√d,with the slope k increasing with decreasing temperature.At 298K,in addition to dislocation glide and twinning,grain boundary sliding(GBS)becomes significant in samples with grain sizes below 3μm,considerably enhancing the material’s deformability.GBS activation provides an additional recovery mechanism for dislocations accumulating at grain boundaries,facilitating their absorption during sliding and rotation.Analysis ofσrelationship suggests that the basal slip is the dominant dislocation mode in Mg at 298K.Decreasing grain size suppresses dislocation activity and twinning and increases GBS,resulting in lowerandσvalues.Suppressing conventional deformation modes coupled with enhanced GBS yields stress softening,breaking down the Hall-Petch relationship in Mg below 3μm grain size,leading to an inverse Hall-Petch behaviour.The work reports new data on the strength,ductility,work hardening and fracture behaviour,and their variations with Mg grain size across different temperature regimes.

The dynamical complexity of work-hardening:a large-scale molecular dynamics simulation

We analyze a large-scale molecular dynamics simulation of work hardening in a model system of a ductile solid. With tensile loading, we observe emission of thousands of dislocations from two sharp cracks. The dislocations interact in a complex way, revealing three fundamental mechanisms of work-hardening in this ductile material. These are (1) dislocation cutting processes, jog formation and generation of trails of point defects; (2) activation of secondary slip systems by Frank-Read and cross-slip mechanisms; and (3) formation of sessile dislocations such as Lomer-Cottrell locks. We report the discovery of a new class of point defects referred to as trail of partial point defects, which could play an important role in situations when partial dislocations dominate plasticity. Another important result of the present work is the rediscovery of the Fleischer-mechanism of cross-slip of partial dislocations that was theoretically proposed more than 50 years ago, and is now, for the first time, confirmed by atomistic simulation. On the typical time scale of molecular dynamics simulations, the dislocations self-organize into a complex sessile defect topology. Our analysis illustrates numerous mechanisms formerly only conjectured in textbooks and observed indirectly in experiments. It is the first time that such a rich set of fundamental phenomena have been revealed in a single computer simulation, and its dynamical evolution has been studied. The present study exemplifies the simulation and analysis of the complex nonlinear dynamics of a many-particle system during failure using ultra-large scale computing.

INSTANTANEOUS THERMAL EXPANSION BEHAVIOR OF HETEROGENEOUS MATERIALS WITH A WORK-HARDENING ELASTOPLASTIC MATRIX AND ELASTIC SPHEROIDAL INHOMOGENEITIES

The instantaneous thermal expansion behavior of-two-phase heterogeneous materials subjected to a uniform temperature change is explored in the present study. The matrix phase is assumed to be a work-hardening ductile metal and the dispersive phase is assumed to consist of either aligned or randomly-oriented, elastic,, spheroidal inhomogeneities. The plastic flow and decreasing stiffness of the matrix during Eshelby's transformation strain of the equivalent inclusions are accounted for by using the deformation theory of plasticity. The explicit results of the instantaneous overall thermal expansion coefficients and the critical inelastic temperature changes are presented for aligned disc- and fiber-inclusions. For the spherical and randomly-oriented spheroidal inclusion, the present study demonstrates that when the yielding of the composites is governed by the average matrix stress, the overall response is always elastic in spite of the temperature change.

Temperature Dependent Work-hardening Behaviour in an Fe-Mn-Si-Cr-Ni Shape Memory Alloy

The work-hardening behaviour in an Fe-Mn-Si-Cr-Ni alloy has been investigated using tensile test at different temperatures and TEM observation. It was found that besides the intersection of εmartensite, the intersections of ε martensite with stacking fault and the cross-slip of dislocation which is difficult to occur in the alloy with low stacking fault energy are also important factors to the temperature dependent work-hardening behaviour.

X-RAY ANALYSIS OF WORK-HARDENING PROCESS FOR POLYCRYSTALLINE α-BRASS DURING UNIAXIAL TENSILE TESTS

X-ray diffraction Fourier profile analysis has been performed for polycrystalline α-brass plate with 31.55 wt-% Zn after a series of uniaxial tensile tests with different strain rates. Dislocation density p and configuration parameter M may be presented as a function of ap- plied stress σ_a.A mechanism of alternative long-range and short-range strain hardening has been proposed.

Mechanism of Work-hardening for Austenitic Manganese Steel under Non-severe Impact Loading Conditions

The effect of C,Mn and heat-treatment on work-hardening of austenitic Mn steel and the work-hardening mechanism have been investigated under non-severe impact loading condition.The results show that the ability of work-hardening in- creases with the increase of C and aging tempera- ture but decreases with Mn.The work-hardening with high austenitic stability results mainly from dislocations,and that with low austenitic stability results mainly from combined effects of strain-in- duced martensite and high density of dislocations under non-severe impact loading conditions.The wear resistance of medium manganese steel (Mn7) is 1.64-2.46 times that of Hadfield steel (Mnl3).

Influence of Structural Modification on Work-hardening Behaviour of Type 316 Stainless Steel

Work-hardening behaviour of type 316 austenitic stainless steel having difFerent initial dislocation structures introduced by swaging to various levels is analysed by a simplified Kock's model which takes into account the structural changes through the dislocation accumulation and annihilation process during deformation. The dislocation accumulation and annihilation factors show a temperature and structure dependence. The dislocation annihilation factor shows a plateau or decreasing tendency in the dynamic strain ageing (DSA) temperature range. This is attributed as either due to dislocation accumulation being more pronounced than dislocation annihilation or as due to precipitates being formed at DSA temperatures acting as obstacles to dislocation motion in the DSA temperature range.

圆管大直径外翻边孔加工工艺设计与翻边力计算（英文）

针对现有圆管翻边工艺无法加工与圆管直径接近的外翻边孔的缺陷,提出了一种新的加工工艺方法。考虑新工艺的变形特点、加工硬化特性以及变形区的材料厚度的变化情况,推导出针对圆管翻边的翻边力计算公式。Abaqus仿真数据表明:相对于板材翻边力计算公式,本文的翻边力计算公式更加精确。

Effect of cold rolling on the microstructural, magnetic, mechanical, and corrosion properties of AISI 316L austenitic stainless steel

This study has evaluated the effect of different levels of cold rolling（from 0 to 50%）on the microstructural,magnetic,and mechanical properties and the corrosion behavior of 316L austenitic stainless steel in Na Cl（1 mol/L）＋H_2SO_4（0.5 mol/L）solution.Microstructural examinations using optical microscopy revealed the development of a morphological texture from coaxial to elongated grains during the cold-rolling process.Phase analysis carried out on the basis of X-ray diffraction confirmed the formation of the ferromagneticα′-martensite phase under the stresses applied during cold rolling.This finding is in agreement with magnetic measurements using a vibrating sample magnetometer.Mechanical properties determined by tensile and Vickers microhardness tests demonstrated an upward trend in the hardness-to-yield strength ratio with increasing cold-rolling percentage,representing a reduction in the material’s work-hardening ability.Uniform and localized corrosion parameters were estimated via potentiodynamic polarization corrosion tests and electrochemical impedance spectroscopy.In contrast to the uniform corrosion,wherein the corrosion current density increased with increasing cold-working degree because of the high density of microstructural defects,the passive potential range and breakdown potential increased by cold working,showing greater resistance to pit nucleation.Although pits were formed,the cold-rolled material repassivation tendency decreased because of the broader hysteresis anodic loop,as confirmed experimentally by observation of the microscopic features after electrochemical cyclic polarization evaluations.

A STUDY OF DIRECTIONALITY OF LATENT HARDENING BEHAVIOR IN ALUMINIUM SINGLE CRYSTALS

By defining the yield stress in a latent hardening test as the first deviation from the elastic straight line, the yielding and hardening behavior on a latent system in the positive and negative slip direction was studied in aluminium single crystals. It is shown that the yield stresses on both the positive and negative latent systems are about equal to or a little lower than the maximum resolved shear stress in the primary test, but much higher than that of the active system. The Influence of relative orientation and prestrain on latent hardening and initial work-hardening in the secondary test was also investigated, and it was found that there is a considerable effect on initial work-hardening, but none on latent hardening. With reasonable approximation, a hardening rule for single crystal could be proposed from the experimental results, that is, except for the yield stress on the system negative to the active system that is very low, hardening on the other systems is nearly the same as self-hardening.

Influence of Oxygen Addition on Structure and Properties of Titanium Produced by Electroslag Remelting

The current work is devoted to the investigation of oxygen impact on the structure and properties of titanium. For this purpose, oxygen was introduced into titanium during chamber electro-slag remelting by three different methods: alloying by oxygen-rich residues from the Kroll process to final values between 0.053 wt.-% and 0.40 wt.-%, by reaction with the gas phase to 0.27 wt.-% and by introduction of TiO2 nanoparticles to 0.73 wt.-%. The influence of oxygen on microstructure of titanium during crystallization, heat treatment and deformation is determined as well as the effect of oxygen on the hardness and the mechanical properties of the material in different structural states. Furthermore, control methods of the structure formation process by thermal effects are proposed. Results show that the chamber electroslag remelting allows obtaining a homogeneous structure of the ingot in the investigated range of oxygen content in titanium. The hardness does not vary by more than 10 percent in longitudinal or radial direction in any of the remelted ingots.

Influence of Pre-Transformed Martensite on Work-Hardening Behavior of SUS 304 Metastable Austenitic Stainless Steel

The metastable austenite was transformed to martensite by prestrain tension of SUS304 stainless steel to study the influence of transformed martensite on its subsequent work-hardening behavior under the uniaxial tensile condition. The X-ray diffractometer （XRD） was employed to detect the transformed martensite. Results showed that the volume fraction of transformed martensite increases with increasing prestrain. The pre-transformed martensite in the microstructure remarkably affects the deformation behavior of the steel, and the strength increases and the elongation decreases. The work-hardening curve of prestrained specimens observably changes with true strain. The work-hardening exponent n of stainless steel decreases with the increase of pre-transformed martensite. The achievement is a significant contribution to the process design during pressing.

AN UPPER DISPLACEMENT BOUND IN DYNAMIC SHAKEDOWN PROBLEMS FOR ELASTOKINEMATIC WORK-HARDENING MATERIALS

A dynamic shakedown theorem for an elasto-kinematic work-hardenlng material with an arbitrary convex yield surface is reviewed. An upper displacement bound for this dynamic shakedown structure is derived. Except for the condition necessary for shakedown of the structure, no other restriction is imposed, which eliminates some of the difficulties appearing in Previous works.

Full Characterization of the Work-Hardening Behavior of Metals:An Accurate and Explicit Approach

A new approach is proposed to characterize the work-hardening behavior of metals based on the stress-strain data from uniaxial extension testing.With this new approach,the yield strength as a function of the plastic work can be determined by directly fitting a wellchosen single-variable shape function to any given uniaxial data from the initial yielding up to the strength limit,in an explicit sense with no need to carry out the usual tedious trial-and-error procedures in treating nonlinear elastoplastic rate equations toward identifying numerous unknown parameters.Numerical examples show that the simulation results with the new approach are in accurate agreement with the test data.

Cavitation Erosion Corrosion Behaviour of Manganese-nickel-aluminum Bronze in Comparison with Manganese-brass

The cavitation erosion corrosion behaviour of ZQMn12-8-3-2 manganese-nickel-aluminum bronze and ZHMn55- 3-1 manganese-brass was investigated by mass loss, electrochemical measurements （polarization curves and electrochemical impedance spectroscopy） and the cavitation damaged surfaces were observed by scanning electron microscopy （SEM）. The results showed that ZQMn12-8-3-2 had better cavitation erosion resistance than ZHMn55-3-1. After the cavitation erosion for 6 h, the cumulative mass loss of ZQMn12-8-3-2 was about 1/3 that of ZHMn55-3-1. The corrosion current density of ZQMn12-8-3-2 was less than that of ZHMn55-3-1 under both static and cavitaiton condition. The free-corrosion potentials of ZQMn12-8-3-2 and ZHMn55-3-1 were all shifted in positive direction under cavitation condition compared to static condition. In the total cumulative mass loss under cavitation condition, the pure erosion played a key role for the two tested materials （74% for ZHMn55-3-1 and 60% for ZQMn12-8-3-2）, and the total synergism between corrosion and erosion of ZQMn12-8-3-2 （39%） was larger than that of ZHMn55-3-1 （23%）. The high cavitation erosion resistance of ZQMn12-8-3-2 was mainly attributed to its lower stacking fault energy （SFE）, the higher microhardness and work-hardening ability as well as the favorable propagation of cavitation cracks for ZQMn12-8-3-2, i.e., parallel to the surface rather than perpendicular to the surface for ZHMn55-3-1.

Unexpected Effect of Nb Addition as a Microalloying Element on Mechanical Properties of δ-TRIP Steels

The concept of microalloying was applied to the δ-TRIP （transformation-induced plasticity） steel to inves- tigate the feasibility of increasing the mechanical properties and understanding the effect of microalloying on the morphology and structure of the steel. A hot rolled δ-TRIP steel with three different contents of Nb （0, 0.03, 0.07 mass%） was subjected to the microstructural and mechanical examination. The high Al and Si concentration in these steels guaranteed the presence of the considerable δ-ferrite phase in the microstructure after the casting and the subsequent hot rolling. The obtained results showed that Nb dramatically affects the microstructure the dynamic re- covery and recrystallization behavior, as well as the grain shape and thus the stability of austenite after the thermo- mechanical process of hot rolling. The results also revealed an unexpected effect of Nb on the mechanical properties. The addition of Nb to the δ-TRIP steel led to a significant decrease in the ultimate strength （from 1144 to 917 MPa） and an increase in ductility （from 24% to 28%）. These unconventional results could be explained by the change in the steel microstructure. The work-hardening＇behaviors of all samples exhibit three stages of the work-hardening rate evolution. At the stage 2, the work-hardening rate of the studied steels increased, being attributed to the TRIP effect and the transformation of austenite to martensite.

Enhancement of Strength–Ductility Balance of the Laser Melting Deposited 12CrNi2 Alloy Steel Via Multi-step Quenching Treatment

A ferrite–austenite 12CrNi2 alloy steel additively manufactured by laser melting deposition(LMD) was heat treated by direct quenching(DQ) and tempering inter-critical quenching(TIQ) at 800 ℃ for enhancing its strength–ductility balance. Both heat-treated alloy steels have the martensite–ferrite dual-phase(DP) microstructures. The volume fractions of martensite in the two treated alloy steels are nearly similar(~ 85 vol%), while the sizes of the prior austenitic grain for martensite are different. The martensite-dominated DP microstructure resulted in an obvious improvement in strength–ductility balance of the alloy steel. Compared with the DQ treatment, the multi-step TIQ treatment caused the strength–ductility balance of the alloy steel to be enhanced due to its higher total elongation. The better ductility of the TIQ-treated alloy steel can be attributed to the optimization of the microstructure. The preferred orientation of ferritic grain in the as-deposited alloy steel which was adverse to plastic deformation through dislocation slip was eliminated via the multi-step TIQ treatment. Moreover, the TIQ treatment promoted the formation of finer-grained martensite with larger areas of grain boundaries and twinning boundaries which resulted in the enhancement of the coordinated deformability of the martensite with the ferrite.

Influence of carbon addition on mechanical properties of Fe-Mn-C twinning-induced plasticity steels

Mechanical properties and microstructural evolution of Fe-22Mn-0.6C and Fe-22Mn-1.0C(wt.%)twinning-induced plasticity(TwIP)steels were investigated by monotonic,stress-relaxation and unloading-reloading tensile tests.The dynamic strain aging(DSA)effect,resulting from pinning of dislocations,effectively improved the dislocation activation volume of the two TWIP steels.In the meanwhile,DSA-facilitated twinning nucleation mechanism kept similar twinning capabilities of the two TWIP steels.With strain increasing,the formation of high-density deformation twins restricted the dislocation motion and reduced the activation volume with increasing strain.Furthermore,C addition simultaneously improved the ultimate tensile strength and uniform elongation,and significantly enhanced the friction stress,rather than back stress.The stronger short-range order effect,brought by friction stress,promotes the planar dislocation slipping,thus improving the work-hardening capability.As a result,the additional work-hardening capacity can be achieved in Fe-Mn-C with higher C addition.