Strain rate and cold rolling dependence of tensile strength and ductility in high nitrogen nickel-free austenitic stainless steel

The tensile strength and ductility of a high nitrogen nickel-free austenitic stainless steel with solution and cold rolling treatment were investigated by performing tensile tests at different strain rates and at room temperature. The tensile tests demonstrated that this steel exhibits a significant strain rate and cold rolling dependence of the tensile strength and ductility.With the increase of the strain rate from 10~(-4)s~(-1)to 1 s~(-1), the yield strength and ultimate tensile strength increase and the uniform elongation and total elongation decrease. The analysis of the double logarithmic stress–strain curves showed that this steel exhibits a two-stage strain hardening behavior, which can be well examined and analyzed by using the Ludwigson equation. The strain hardening exponents at low and high strain regions(n_2and n_1) and the transition strain(εL) decrease with increasing strain rate and the increase of cold rolling RA. Based on the analysis results of the stress–strain curves, the transmission electron microscopy characterization of the microstructure and the scanning electron microscopy observation of the deformation surfaces, the significant strain rate and cold rolling dependence of the strength and ductility of this steel were discussed and connected with the variation in the work hardening and dislocation activity with strain rate and cold rolling.     

On the generation of nanograins in pure copper through uniaxial single compression

Attempts at generating nanograins through uniaxial single compression were made by deforming copper samples at 298 K and 77 K. At 298 K, dynamically-deformed samples (DDS) become softer, in contrast to quasi-statically deformed samples (QDS), which show a hardness close to the saturation value. The microstructure of DDS is characterised by deformation twins and equiaxed micron-sized grains, and the observed softening is due to the occurrence of recrystallisation (RX). At a reduced temperature of 77 K, nanograins are generated in DDS, whereas QDS show forest dislocations and twins. The generation of nanograins, which evolve through rotational DRX, is associated with the formation of shear bands with an amorphous structure. Compared with twinning, it appears that amorphisation plays a more pronounced role in high strain rate deformation at reduced temperatures (77 K). The hardness of DDS, obtained from compression at 77 K, exceeds the saturation value by 16%, whereas that of QDS corresponds approximately to saturation.     

Dislocation-mediated creep process in nanocrystalline Cu

Nanocrystalline Cu with average grain sizes ranging from ～ 24.4 to 131.3 nm were prepared by the electric brushplating technique.Nanoindentation tests were performed within a wide strain rate range,and the creep process of nanocrystalline Cu during the holding period and its relationship to dislocation and twin structures were examined.It was demonstrated that creep strain and creep strain rate are considerably significant for smaller grain sizes and higher loading strain rates,and are far higher than those predicted by the models of Cobble creep and grain boundary sliding.The analysis based on the calculations and experiments reveals that the significant creep deformation arises from the rapid absorption of high density dislocations stored in the loading regime.Our experiments imply that stored dislocations during loading are highly unstable and dislocation activity can proceed and lead to significant post-loading plasticity.     

A model of grain refinement and strengthening of Al alloys due to cold severe plastic deformation

This paper presents a model which quantitatively predicts grain refinement and strength/hardness of Al alloys after very high levels of cold deformation through processes including cold rolling, equal channel angular pressing (ECAP), multiple forging (MF), accumulative roll bonding (ARB) and embossing. The model deals with materials in which plastic deformation is exclusively due to dislocation movement within grains, which is in good approximation the case for many metallic alloys at low temperature, for instance aluminium alloys. In the early stages of deformation, the generated dislocations are stored in grains and contribute to overall strength. With increase in strain, excess dislocations form and/or move to new cell walls/grain boundaries and grains are refined. We examine this model using both our own data as well as the data in the literature. It is shown that grain size and strength/hardness are predicted to a good accuracy.     

Deformation-induced martensite and nanotwins by cryogenic laser shock peening of AISI 304 stainless steel and the effects on mechanical properties

Laser shock peening (LSP) of stainless steel 304 was carried out at room and cryogenic temperature (liquid nitrogen temperature). It was found that the deformation-induced martensite was generated by LSP only when the laser-generated plasma pressure is sufficiently high. Compared to room temperature laser shock peening (RT-LSP), cryogenic laser shock peening (CLSP) generates a higher volume fraction of martensite at the same laser intensity. This is due to the increase in the density of potential embryos (deformation bands) for martensite nucleation by deformation at cryogenic temperature. In addition, CLSP generates a high density of deformation twins and stacking faults. After CLSP, an innovative microstructure, characterised by networks of deformation twins, stacking faults and composite structure (martensite and austenite phases), contributes to material strength and microstructure stability improvement. The combined effect of higher surface hardness and a more stabilised microstructure results in greater fatigue performance improvement of the CLSP samples compared to that of the RT-LSP samples.     

Strength and plastic deformation of polycrystalline diamond composites

ABSTRACT

The use of nanopolycrystalline diamond has allowed a systematic study on deformation of polycrystalline diamond composites (PCDCs). Bulk PCDCs samples containing either Co or SiC as a binding agent were deformed under high pressure and temperature to strains up to 18% at strain rates ～10^?5?s^?1. All samples exhibit strong work hardening. The strength of PCDCs depends on the amount and type of binding agents and is consistently weaker than that of diamond single crystals. The weakening may be due to the binder materials, which play an important role in affecting grain boundary structures. In SiC-based PCDC, significant grain fragmentation occurs. Nearly all grain boundaries are wetted by SiC after large deformation, resulting in lower strength. In Co-based PCDC, the microstructure is dominated by dislocations, deformation twins, and separated grain boundaries. The density of deformation twins increases significantly with strain, with the twin domain width reaching as low as 10–20?nm at 14% strain.     

Phase transformations and mechanical properties of stainless steel in the nanostructural state

The peculiar features of structurization and changes in mechanical properties are studied during equichannel-angular (ECA) pressing treatment and cold deformation (strain) through rolling of metastable austenitic steel up to a rolling rate of e = 2.1. It is shown that the ECA pressing treatment of austenitic steel resulted in enhancement of its strength characteristics and conserved its high plasticity values, exceeding by an order of magnitude, these characteristics after cold plastic strain, which is associated with the optimization of the kinetics of γ → α transformation.     

Deformation and annealing behaviour of a low carbon high Mn TWIP steel microalloyed with Ti

ABSTRACT

A low carbon high Mn, Ti microalloyed dual phase TWIP steel has been processed through cold rolling and annealing. X-ray diffraction reveals the maximum austenite (≈92%) in HRACST sample whereas, the 50CD sample shows 29% ferrite. The microstructure of HRAC and HRACST samples reveal austenite grains with annealing twins and deformation induced ferrite (DIF). The higher amount of DIF along with deformation twins form during cold deformation. Annealing at 500°C shows recovery, whereas at 700°C shows partial recrystallisation and at 900°C reveals almost full recrystallisation. TEM microstructures of the 900°C for 30?min samples reveal annealing twins with TiC particle. Strong Brass {110}<112> and Goss{110}<001> texture components are observed in HRAC, HRACST and 50CD samples. Goss Twin (GT) {113}<332> and Copper Twin (Cu-T) {552}<115> components are observed in 50CD sample. Addition of Ti results in an average grain size of 20?μm. Maximum YS (1176?MPa) and UTS (1283?MPa) values with the lowest ductility of 11% have been obtained for the 50CD sample which is related to the formation of extensive deformation twin and a higher fraction of DIF. 700°C-30?min and 700°C-60?min samples show an increase in ductility (23% and 34%, respectively) with a marginal decrease in tensile strength (1054?MPa). Annealing at 900°C shows ductility restoration up to 60% with higher tensile strength compared to HRACST sample. Ductile fracture of HRAC and HRACST samples transform to brittle fracture in the 50CD sample. Annealing at 900°C for 30?min shows ductile fracture with some (Fe, Mn)S and TiC particles.     

Grain size effects on microstructural stability and creep behaviour of nanotwinned Ni free-standing foils at room temperature

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses (<800 MPa) and grain refinement via the detwinning mechanism at high applied stresses (>800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process accelerates the creep strain rate in the studied NT-Ni foils. A modified phase-mixture model combined with Arrhenius laws is put forward to predict the scaling behaviour between the creep strain rate and the applied stress, which also predicts the transition from grain growth-reduced to grain refinement-enhanced steady-state creep strain rate at a critical applied stress. Our findings not only provide deeper insights into the grain size effect on the mechanical behaviour of nanostructured metals with high SFE, but also benefit the microstructure sensitive design of NT metallic materials.     

High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling

Creep tests were performed on MC2 single crystal superalloy at 950°C/200?MPa and 1150°C/80?MPa under isothermal and thermal cycling conditions with a tensile axis along the [0?0?1] direction. It was found that the thermal cycles strongly affect the creep behavior at 1150°C but not at 950°C. This was related to the repetitive precipitation and dissolution of small γ′ rafts at the higher temperature, as revealed by quantitative characterization of the γ/γ′ microstructure. The dislocation microstructure exhibits similar trends in all the tested conditions, with a very high activity of a[1?0?0]-type dislocations climbing through the rafts. Such climbing dislocations constitute a recovery process for the deformation active system. It appears that the density of a[1?0?0] dislocations, and not their climb velocity or diffusion rate, is the key parameter for the control of creep rate. The thermal cycles, which imply the creation and subsequent dissolution of rafts, provided new dislocations, which explains the acceleration of creep observed under such conditions.     

Mechanisms of high-temperature creep of nickel-based superalloys under low applied stresses

[001] single-crystal specimens of the superalloys CMSX-4 and CMSX-10 were tested for creep at 1100°C under tensile stresses between 105 and 135?MPa, where they show pronounced steady creep. The deformed superalloys were analysed by density measurements, scanning electron microscopy and transmission electron microscopy which supplied information about porosity growth, evolution of the γ–γ′ microstructure, dislocation mobility and reactions during creep deformation. It is shown that, under the testing conditions used, steady creep strain mostly results from transverse glide–climb of (a/2) ?011? interfacial dislocations. A by-product of the interfacial glide–climb are vacancies which diffuse along the interfaces to growing pores or to a ?100? edge dislocations climbing in the γ′ phase. Climb of a ?100? dislocations in the γ′ phase is a recovery mechanism which reduces the constraining of the γ phase by the γ′ phase, thus enabling further glide of (a/2) ?011? dislocations in the matrix. Moreover the γ′ dislocations act as vacancy sinks facilitating interfacial glide–climb. The creep rate increases when the γ–γ′ microstructure becomes topologically inverted; connection of the γ′ rafts results in extensive transverse climb and an increase of the number of a?100? dislocation segments in the γ′ phase.     

Effect of temperature on the anisotropy of AZ31 magnesium alloy rolling sheet under high strain rate deformation

In order to investigate the effect of temperature on the anisotropic behaviour of AZ31 magnesium alloy rolling sheet under high strain rate deformation, the Split Hopkinson Pressure Bar was used to analyse the dynamic mechanical properties of AZ31 magnesium alloy rolling sheet in three directions, rolling direction(RD), transverse direction (TD) and normal direction (ND). The texture of the rolling sheet was characterised by X-ray analysis and the microstructure prior and after high strain rate deformation was observed by optical microscope (OM). The results demonstrated that AZ31magnesium alloy rolling sheet has strong initial {0?0?0?2} texture, which resulted at the obvious anisotropy in high strain rate deformation at 20 °C. The anisotropy reflected in stress–strain curve, yield stress, peak stress and microstructure. The anisotropy became much weaker when the deformation temperature increased up to 250 °C. Continuing to increase the deformation temperature to 350 °C the anisotropy of AZ31 rolling sheet essentially disappeared. The decreasing tendency of anisotropy with increasing temperature was due to the fact that when the deformation temperature increased, the critical resolved shear stress (CRSS) for pyramidal 〈c + a〉 slip, which was the predominant slip mechanism for ND, decreased close to that of twinning, which was the predominant deformation mechanism for RD and TD. The deformation mechanism at different directions and temperatures and the Schmid factor (SF) at different directions were discussed in the present paper.     

Effect of deformation and annealing on the microstructure and magnetic properties of grain-oriented electrical steels

The effect of plastic deformation and subsequent annealing on the magnetic properties and microstructure of a grain-oriented (GO) electrical steel has been studied. True strain (ε) from 0.002 to 0.23 was applied by rolling in two directions, rolling (RD) and transverse (TD). The deterioration of power losses varies according to the direction of deformation. Annealing the strained material—at 800 °C/2 h—leads to a recrystallization and restored magnetic properties. The main components of annealed-textures are around 15–35° from those of deformed-textures for both RD and TD. Rolling along {1 1 0} 〈0 0 1〉 direction leads to the development of deformation twins.     

Effect of thermomechanical processing on microstructural evolution and deformation behaviour of polycrystalline magnesium under quasi-static and high strain rate conditions

Abstract

The effect of thermomechanical processing on microstructure evolution and room temperature flow behaviour of polycrystalline magnesium in compression at strain rates of ~10^?2 and ~10³ s^?1 was investigated. Different initial microstructures were produced by optimising rolling and annealing cycles. Prior to annealing for 1 h at 350 °C, Mg samples were processed by two different treatments such as (i) hot rolling at 350 °C and (ii) hot rolling at 350 °C plus cold rolling at room temperature. Introduction of cold working step led to an increased fraction of hard oriented grains with a marginal grain size difference in post-annealed samples. A profound effect of thermomechanical processing on strain hardening rate as well as rate-sensitive flow behaviour of Mg was observed. The influence of prior processing history and strain rate on flow behaviour of Mg was clearly reflected in terms of texture strengthening/weakening phenomena and formation of microstructural deformation bands.     

Effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo0.15 high-entropy alloy

Abstract

The effects of torsional deformation on the microstructures and mechanical properties of a CoCrFeNiMo_0.15 high-entropy alloy have been investigated. The torsional deformation generates a gradient microstructure distribution due to the gradient torsional strain. Both dislocation activity and deformation twinning dominated the torsional deformation process. With increasing the torsional equivalent strain, the microstructural evolution can be described as follows: (1) formation of pile-up dislocations parallel to the trace of {1 1 1}-type slip planes; (2) formation of Taylor lattices; (3) formation of highly dense dislocation walls; (3) formation of microbands and deformation twins. The extremely high deformation strain (strained to fracture) results in the activation of wavy slip. The tensile strength is very sensitive to the torsional deformation, and increases significantly with increasing the torsional angle.     

Role of interfaces in shock-induced plasticity in Cu/Nb nanolaminates

We investigate deformation of pure Cu, pure Nb and 30?nm Cu/30?nm Nb nanolaminates induced by high strain rate shock loading. Abundant dislocation activities are observed in shocked pure Cu and Nb. In addition, a few deformation twins are found in the shocked pure Cu. In contrast, in shocked Cu/Nb nanolaminates, abundant deformation twins are found in the Cu layers, but only dislocations in the Nb layers. High resolution transmission electron microscopy reveals that the deformation twins in the Cu layers preferentially nucleate from the Cu(112)//Nb(112) interface habit planes rather than the predominant Cu(111)//Nb(110) interface planes. Our comparative study on the shock-induced plastic deformation of the pure metals (Cu and Nb) and the Cu/Nb nanolaminates underscores the critical role of heterogeneous phase interfaces in the dynamic deformation of multilayer materials.     

Intermediate temperature creep mechanisms in Ni 3 Al

The intermediate-temperature creep response of single-crystal Ni 3 Al(Ta) has been investigated along both [ ] and [001] axial orientations. The effect of the existing deformation structure (i.e. pre-straining) on the [ ] creep response was reported. The creep responses of virgin specimens and specimens prestrained at room temperature (RT) and 520°C are compared. In order to compare the dislocation structures prior to creep, the microstructure of specimens which had been deformed at a constant strain rate at RT and 520°C, but not subjected to creep, was also examined. Creep curves show that the temperature of pre-strain influences the subsequent creep properties. The primary creep response, like the yielding response, appears to be controlled by the kink size distribution, while the secondary creep response is thought to be controlled by the kink separation (or the length of the Kear-Wilsdorf locks). Specimens crept along [ ] display steady state creep properties and rectangularly oriented [ ](010) dislocations, while a virgin specimen crept along [001] displays an increasing secondary creep rate (inverse creep) and d110 ¢{100}-type dislocations. Inverse creep along [001] is thought to be the result of an increasing density of edge kink octahedral sources where there is little resolved shear stress on the cube planes.     

Development of new-generation low-carbon steel: Part II-wear behaviour

A new-generation unalloyed low-carbon steel (containing 0.1?wt.% C) has been recently developed by the research group of the present corresponding author through incomplete austenitisation-based cyclic ice-brine quenching possessing an exceptionally high strength (UTS?=?1.7?GPa) along with elimination of a yield point phenomenon. This is attributed to the evolution of a novel microstructure that consists of fine plate martensite crystals with a dispersion of nano-sized cementite particles and clusters. The present research work is conceived as the Part II of this investigation to establish this new-generation ultrahigh strength low-carbon steel as a unique wear-resistant steel substituting the conventional dual-phase steel along with the readily awaited in-depth correlation between wear mechanism and structural evolution. The wear behaviour of heat-treated steels is investigated against an alumina disc using a pin-on-disc tribometer. The steel subjected to incomplete austenitisation-based cyclic ice-brine quenching exhibits much better wear resistance than conventional dual-phase steel. Dominant microcutting and microploughing abrasion aggravate wear loss, especially at higher load, in dual-phase steel that inherently possesses lower matrix hardness. But, very high-surface hardness is attained in the incomplete austenitisation-based cyclic ice-brine quenched steel by virtue of a significant strain hardening of martensite matrix in between hard nano-sized cementite particles. Besides, the wear rate is not allowed to shoot up even at the highest load through the generation of hard abrasion-resistant tribo-oxide layer of Al₂FeO₄. This envisages an advent of novel wear-resistant steel as a better substitution for the dual-phase steel.     

Experimental study of room-temperature indentation viscoplastic ‘creep’ in zirconium

In this paper we have studied the mechanisms of so-called ‘indentation creep’ in a zirconium alloy. Nanoindentation was used to obtain strain rate data as the sample was indented at room temperature, at a homologous temperature below that for which creep behaviour would be expected for this material. A high value of strain rate was obtained, consistent with previous work on indentation creep. In order to elucidate the mechanism of time-dependent deformation, a load relaxation experiment was performed by uniaxial loading of a sample of the same alloy. By allowing relaxation of the sample from a peak load in the tensile test machine, a similar stress exponent was obtained to that seen in the nanoindentation creep test. We conclude that for metals, at temperatures below that at which conventional creep will occur, nanoindentation ‘creep’ proceeds through deformation on active slip systems that were initiated by prior loading beyond the plastic limit. It is therefore more appropriate to describe it as a viscoplastic process, and not as creep deformation.     

Effect of thermomechanical processing on the creep behaviour of Udimet alloy 188

Udimet alloy 188 was subjected to grain-boundary engineering involving thermomechanical processing in an attempt to improve the creep performance and determine the effects on creep deformation processes. The as-received sheet was cold-rolled to either 10, 25 or 35% reduction per pass followed by a solution treatment at 1191°C for 1 h plus air cooling. This sequence was repeated four times and the resultant microstructure and grain-boundary character distribution were described using electron backscatter diffraction. The fraction of general high-angle grain boundaries tended to increase with increased cold rolling. The 10 and 25% cold-rolled materials exhibited lower creep rates than the 35% cold-rolled material. The measured creep stress exponents and activation energies suggested that dislocation creep with lattice self-diffusion was dominant at 760°C for stresses ranging between 100 and 220 MPa. A transition in the creep exponent below the applied stresses of 100 MPa indicated that a different secondary creep mechanism was rate-controlling at low stresses. A significant amount of grain-boundary cracking was observed both on the surface and subsurface of deformed samples, but surface cracks were greater in number and size than those within the bulk. The cracking behaviour was similar in both vacuum and air environments, indicating that grain-boundary cracking was not caused by environment. To assess the mechanisms of crack nucleation, in situ scanning electron microscopy was performed during elevated-temperature (T ≤ 760°C) tensile-creep deformation. Sequential secondary electron imaging and electron backscatter diffraction orientation mapping were performed in situ to allow the evolution of crack nucleation and linkage to be followed. Cracking occurred preferentially along general high-angle grain boundaries and less than 15% of the cracks were found on low-angle grain boundaries and coincident site lattice boundaries. A fracture initiation parameter analysis was performed to identify the role of slip system interactions at the boundaries and their impact on crack nucleation. The parameter was successful in separating the population of intact and cracked general high-angle boundaries at lower levels of strain, but not after crack coalescence dominated the fracture process. The findings of this work have significant implications regarding grain-boundary engineering of this alloy and potentially for other alloy systems.