Cold-worked Metals Research Articles

Anomalous growth of dislocation density in titanium during recovery

It is generally believed that the dislocation density in cold-worked metals decreases during recovery. In this study, we used in-situ synchrotron X-ray diffraction (SXRD) to demonstrate that the dislocation density in α-titanium increases at the initial stages of heating. This phenomenon was clearly reproduced by classical molecular dynamics (MD) simulation. It was concluded, that under conditions of low dislocation mobility, the generation of new dislocations can reduce the overall level of stresses and potential energy during heating of the deformed material. The paper describes the dynamics of dislocations, as well as the dislocation reactions at different stages of heating of a cold-worked titanium sample.

Assessing the thermal stability of laser powder bed fused AlSi10Mg by short-period thermal exposure

ABSTRACT Laser powder bed fused (LPBFed) AlSi10Mg is recognised for its superior mechanical properties. However, its thermal stability has never been justified. Herein, we exposed as-built AlSi10Mg to different temperatures (200–500°C) for only 3 min to evaluate its thermal stability. Results showed that LPBFed AlSi10Mg had relatively low thermal stability. Only 3 min of thermal exposure at 200°C would deteriorate its tensile strength dramatically. Microstructural analysis revealed that with increasing thermal input, as-built AlSi10Mg exhibited a microstructural evolution similar to annealing of cold-worked metals, namely recovery, recrystallisation followed by grain-growth. The excessive energy stored in as-built microstructure due to fast cooling during LPBF was deduced as the driving force for this phenomenon. Therefore, such microstructural change was at the expense of dislocations stored in the as-built material, which in turn caused deterioration in tensile strength. The present findings may provide guidance for the application of LPBFed AlSi10Mg.

Anomalous Growth of Dislocation Density in Titanium During Recovery

It is generally believed, that the dislocation density in cold-worked metals decreases during recovery. Here, we used in-situ synchrotron X-ray diffraction to show that the dislocation density in [[EQUATION]] -titanium increases at the initial stages of heating. Under conditions of low dislocation mobility, the generation of new dislocation decreases the overall level of stresses in the deformed material. This phenomenon was clearly confirmed by classical molecular dynamics simulations. The dislocation reactions and dislocations dynamics were considered in detail.

Critical Assessment 39: Effects of applied fields during annealing of metals – a review

There have been many, sometimes conflicting, reports over the years as to how the annealing of cold-worked metals may be affected in the presence of applied fields. The annealing phenomena considered here are recovery, recrystallisation and grain growth. Practical significance relates to the kinetics of these processes and the microstructures and textures that result after these heat treatments. The externally applied ‘fields’ considered here are magnetic fields, electric fields, electric currents and mechanical (elastic) loading. The literature considered relates specifically to metals, both ferrous and non-ferrous. Attempts are made to identify consistent behaviours. In some cases, the variability of observations does not permit this. However, reasonably firm conclusions are possible in some circumstances.

Elucidating the dual role of grain boundaries as dislocation sources and obstacles and its impact on toughness and brittle-to-ductile transition

In this paper, we resolve the role of grain boundaries on toughness and the brittle-to-ductile transition. On the one hand, grain boundaries are obstacles for dislocation glide. On the other hand, the intersection points of grain boundaries with the crack front are assumed to be preferred dislocation nucleation sites. Here, we will show that the single contributions of grain boundaries (obstacles vs. source) on toughness and the brittle-to-ductile transition are contradicting, and we will weight the single contributions by performing carefully designed numerical experiments by means of two-dimensional discrete dislocation dynamics modelling. In our parameter studies, we vary the following parameters: (i) the mean free path for dislocation glide, δ, combined with (ii) the (obstacle) force of the grain boundary, ϕ, and (iii) the dislocation source spacing along the crack front, λ. Our results show that for materials or microstructures for which the mean distance of the intersection points of grain boundaries with the crack front is the relevant measure for λ, a decrease of grain size results in an increase of toughness. The positive impact of grain boundaries outweighs the negative consequences of dislocation blocking. Furthermore, our results explain the evolving anisotropy of toughness in cold-worked metals and give further insight into the question of why the grain-size-dependent fracture toughness passes through a minimum (and the brittle-to-ductile transition temperature passes through a maximum) at an intermediate grain size. Finally, a relation of the grain-size-dependence of fracture toughness in the form of K(dδ, dλ) = KIC + kdδ0.5/dλ is deduced.

Nonadditive strengthening functions for cold-worked cubic metals: Experiments and constitutive modeling

Strong metals are greatly desired for lightweight and energy-efficient industrial design. The strengthening of metals is traditionally accomplished by the additive contributions of various obstacle families (e.g., solid solutions, particles, and grain boundaries) and dislocation self-interactions that impede dislocation motion. In the present work, unlike a traditional additive understanding, a distinctive nonadditive strengthening mixture rule for obstacles and dislocations were validated based on experimental and modeling analyses in numerous cold-worked steels and aluminum alloys. Concretely, numerous well-annealed body-centered cubic steels and face-centered cubic aluminum alloys were prepared, in which the hierarchical strength levels of solid solutions, grain boundaries, and/or particles were estimated. The above specimens were then cold rolled to various strain levels. Dislocation densities were quantified by utilizing X-ray diffraction line-profile-analysis, and the dislocation density was found to increase faster with an increasing strain level when a high strength was presented in well-annealed specimens. When plotting the yield stresses as a function of the square roots of the dislocation densities in massive distorted samples based on the Taylor hardening law, it is interesting to note that an approximate single linearity was obtained. Individual dislocation strengthening was found to respond to the total strength in the deformed specimens, which indicated that the full nonadditive strengthening mixture rule was employed between the obstacle families and the dislocation contributions. The mechanisms of the observed nonadditive strengthening were also discussed by implementing additional experiments and transmission electron microscopy observations. Then, the modified constitutive models based on both one and two internal parameters’ Knocks-Mecking models were developed respectively, which excellently captured the effects of the various obstacle families on dislocation storage processes. The developed models also rationalized the observed nonadditive strengthening mixture rule.

History of the Recrystallisation of Metals: A Summary of Ideas and Findings until the 1950s

Abstract The paper reviews the events that led to the understanding of the recrystallisation of metals throughout history, covering the timeline from 3500 BC until the 1950s and dealing mainly with the misconceptions related to the “recrystallisation” and “amorphisation”. This history begins with the thermomechanical processing of native metals in pre-Columbian America (3,000 BC), and the production of brass coins in Rome (100 BC). The analysis of the fracture surfaces of metals, commonly used between the 16th and 18th centuries in Europe for the quality control of metallic products, is briefly discussed. The observation of crystalline and fibrous fractures lead to the common misconception of the 19th century that “recrystallisation” takes place during the heating of “amorphous” cold-worked metals. The use of reflecting optical microscopy in the late 19th century indicated, however, that the plastic deformation only promotes a morphological change in the microstructure of the metals without any “amorphisation”. In the early 20th century, the plastic deformation of metals by crystal plane slipping and twinning was well documented by metallographic observation during tensile testing. Nevertheless, in 1915, Prof. Rosenhain advocated that the “crystal” inside the slip bands of a plastically deformed metal was “amorphous” and responsible for the work hardening. The discovery of the X-ray diffraction technique, in 1913, allowed to resolve the ongoing dispute of the “amorphisation” of metals during plastic deformation. Between the 1920s and 1950s, X-ray diffraction was practically the only means of studying the internal structure of metals to explain the nature of their plasticity, confirming many aspects of the dislocation theory. This technique was crucially important to understand the atomic crystal structure of metals in the annealed and cold-work states. Our history finishes in the 1950s when the transmission electron microscopy finally revealed the first images of dislocation in metals. Up to that date, most of the understanding of the dislocations’ distribution and organisation in cold-work and annealed states had already been derived from X-ray diffractograms, which gave birth to the modern physical metallurgy.

The Role of Hydrogen in Hydrogen Embrittlement of Metals: The Case of Stainless Steel

Hydrogen embrittlement (HE) of metals has remained a mystery in materials science for more than a century. To try to clarify this mystery, tensile tests were conducted at room temperature (RT) on a 316 stainless steel (SS) in air and hydrogen of 70 MPa. With an aim to directly observe the effect of hydrogen on ordering of 316 SS during deformation, electron diffraction patterns and images were obtained from thin foils made by a focused ion beam from the fracture surfaces of the tensile specimens. To prove lattice contraction by ordering, a 40% CW 316 SS specimen was thermally aged at 400 °C to incur ordering and its lattice contraction by ordering was determined using neutron diffraction by measuring its lattice parameters before and after aging. We demonstrate that atomic ordering is promoted by hydrogen, leading to formation of short-range order and a high number of planar dislocations in the 316 SS, and causing its anisotropic lattice contraction. Hence, hydrogen embrittlement of metals is controlled by hydrogen-enhanced ordering during RT deformation in hydrogen. Hydrogen-enhanced ordering will cause the ordered metals to be more resistant to HE than the disordered ones, which is evidenced by the previous observations where furnace-cooled metals with order are more resistant to HE than water-quenched or cold worked metals with disorder. This finding strongly supports our proposal that strain-induced martensite is a disordered phase.

Elastic and transport properties in polycrystals of cracked grains: Cross-property relations and microstructure

Some arguments of Bristow (1960) concerning the effects of cracks on elastic and transport (i.e., electrical or thermal conduction) properties of cold-worked metals are reexamined. The discussion is posed in terms of a modern understanding of bounds and estimates for physical properties of polycrystals – in contrast to Bristow’s approach using simple mixture theory. One type of specialized result emphasized here is the cross-property estimates and bounds that can be obtained using the methods presented. Our results ultimately agree with those of Bristow, i.e., confirming that microcracking is not likely to be the main cause of the observed elastic behavior of cold-worked metals. However, it also becomes clear that the mixture theory approach to the analysis is too simple and that crack–crack interactions are necessary for proper quantitative study of Bristow’s problem.

Evolution of dislocation cell structures in plastically deformed metals

We use a recently developed microstructure evolution model with stochastic dynamics to study the evolution of ordered dislocation patterns. Dislocations are described as groups arranged into arrays, with an initially random distribution of orientations, moving in an isotropic viscous medium. Stochastic displacements of dislocation walls describe the rearrangement of small groups of dislocations to form minimum-dissipation cells structures while lowering the overall misorientation. The result of large mechanical deformation, as could occur in cold-working, is simulated by introducing a preferential direction along which misorientation gradients of various kind are superimposed. The simulations show the emergence of ordered microstructure patterns, with high-density dislocation boundaries running perpendicular to the initial gradient, and a low-density cellular band structure in between. The results are compared to experimental data in heavily cold-worked metals.

Disclinations in Plastically Deformed Metallic Materials

Cold-working of metallic materials up to large strains is usually characterised by simultaneous substructure evolution on different length scales and accompanied by the formation of significant lattice rotations. A promising tool for the description of such microstructure development is the concept of partial disclinations. Transmission electron microscopy (TEM) studies illustrate clearly, that defects of this kind are frequently existent in cold-worked metals and have to be accepted as an important defect entity in the substructure evolution at larger strains. Moreover, it is shown that substructure modelling on the base of a coupled dislocation-disclination dynamics leads to satisfying correspondence of calculated substructure characteristics with experimental results obtained by TEM, X-ray diffractometry, and EBSD (electron backscattering diffraction), and to a satisfying prediction of the macroscopic deformation behaviour, i.e., especially the transition from stage III to stage IV of crystal plasticity.

Development of constitutive models for metal forming with cyclic strain softening

Abstract Many cold and hot worked metals undergo strain hardening and softening when subjected to cyclic plastic straining. The degree of strain softening depends on the amount of prior cold work or heat treatment introduced into the material and upon the magnitude and the number of cycles of applied cyclic plastic strain. Cyclic strain softening leads to a lower flow stress, higher ductility, reduced tensile strength upon subsequent straining and lower energy consumption during material processing. In order to optimise metal forming processes, the work hardening and softening should be analysed to include the influence of deformation conditions within the constitutive model. This paper developed a constitutive model based on the Estrin and Mecking phenomenological model for cyclic hardening without prestraining whilst using a combined Estrin and Mecking model and Avrami equation for cyclic softening with prestraining. As cyclic softening is largely dependent on the work hardening previously achieved and on deformation conditions, the constitutive model takes account of these conditions in addition to those used in conventional constitutive models.

Simulation of primary recrystallization using a modified three-dimensional cellular automaton

A modified three-dimensional cellular automaton algorithm has been developed to simulate primary static recrystallization of cold-worked metals. The driving force for the nucleation and growth of recrystallized grains is the volume free energy due to the stored dislocation density of the deformed matrix. Growth terminates upon impingement. The code allows the introduction of specific conditions for recovery, nucleation and grain boundary motion. The model is capable of simulating kinetics, microstructure and texture development during recrystallization. The modification of the cellular automaton approach allows for the simulation of an orientation dependent growth rate. A novel algorithm minimizes the computational load and computer memory requirement.

An experimental investigation on the effect of h/D ratio on dynamic form — cladding of domes

Explosive ‘form-cladding’ is a solid state welding process, accomplished alongwith the plastic deformation of two sheet metals in a single detonation of a chemical explosive. The advantageous features include (I) the significant decrease in the total cycle time (ii) the ‘form-cladding’ of dissimilar metals (iii) the ‘form-cladding’ of heat treated and cold worked metals together, without changes in their mechanical properties (iv) the absence of ‘heat affected zones’ in the resultant ‘form-clads’. Though the influencing parameters of explosive forming and explosive cladding significantly affect explosive ‘form-cladding’, the influence of h/D (depth of the die/Diameter of the die) ratio is unexplored. The effect of h/D ratio on the ‘form-clads’ is reported. The energy released by a nitroglycerine explosive, with 2500 m/s detonation velocity, was utilized to perform contact, open air ‘form-cladding’ explosions.

A theory of the formation of slip channels in cold-worked bcc metals

Abstract The paper presents a theoretical investigation of microstructural instabilities in bcc metals that are plastically deformed after previous work hardening at low temperatures. Deformation of such materials is, under certain conditions, accompanied by a pronounced localization of plastic strain in narrow slip bands. On the microstructural level, these bands correspond to slip channels in which the dislocations caused by predeformation have disappeared almost completely. It is the aim of the present work to formulate the processes which make slip channels emerge from an initially homogeneous dislocation microstructure. To this end, the interplay between collective dislocation motion and strain softening due to dislocation annihilation is treated in terms of a stochastic dislocation dynamics. The development of slip channels is proposed to correspond to a fluctuation-induced phase transition. Critical conditions for this transition are determined, and the results are compared with experimental findings.

Spectral series of internal friction peaks in cold-worked metals

We suggest that both the Bordoni and the Hasiguti peaks (of the type P1 and P3) be addressed in terms of one basic microscopic mechanism; the nucleation of multikink–antikink pairs in pinned dislocation segments. Correspondingly, the relevant activation energies are multiples of one quantum of energy, the single kink (antikink) rest energy, whence the possibility of ordering these internal friction peaks according to simple spectral series. Such a conclusion appears to be consistent with the experimental data.

Internal friction by pinned dislocations: Theory of the Bordoni peak.

The two most prominent internal friction peaks in cold worked metals at low temperature are shown to be related manifestations of the same nucleation mechanism of kink-antikink pairs in pinned dislocation segments depending, respectively, on the absence (Bordoni peak) or the presence (Niblett-Wilks peak) of geometrical kinks (antikinks). Correspondingly, diffusion of the nucleated thermal pairs accounts for the observed background internal friction. Peak and background components are both computed analytically and compared with experimental data.

Behavior of the bordoni peak in zone-refined Cu during annealing at high temperatures

The Bordoni peak is thrown as a relaxation peak which appears at around 50K (for measuring frequency f {approximately} 1Hz) in various cold worked metals with the fcc structure. Seeger has proposed that this relaxation is associated with a kink pair formation (KPF) on dislocations. Since all the characteristics of the Bordoni peak so far observed experimentally are consistent with the KPF model, this model was widely accepted as the origin of the Bordoni peak. However, the flow stress at low temperatures expected from the temperature of the Bordoni peak is about two orders of magnitude larger than that observed in the macroscopic deformation experiments. If the Bordoni peak is attributed to the KPF model, the authors have to postulate the presence of preferential deformation sites on dislocations (deformation short-circuits) to explain the very low flow stresses in fcc metals. Possible candidates for such sites might be sites at bent corners of extended dislocations, and lubrication sites. They have, however, no definite model at present. In the present work, the behavior of the Bordoni peak in zone-refined Cu was studied during annealing after cold work. In contrast to the results on the ordinary pure Cu, the Bordoni peak was foundmore » to grow during annealing even after 800 C. The results will be shown to be in excellent agreement with the KPF model.« less

Mechanism of Snoek-Köster relaxation in body-centered-cubic metals.

The importance of the effects of mutual interactions between the interstitial solute atoms in the Cottrell atmosphere on the Snoek-K\"oster relaxation in cold-worked bcc metals is emphasized in a theoretical approach based on the coupling model. It is shown that the large activation enthalpy of the relaxation time and the breadth of the internal-friction peak associated with the Snoek-K\"oster relaxation can be explained quantitatively by this model for various foreign atoms in several bcc metals.

Grain boundary resistance to fatigue crack growth: Chen, Q. and Liu, H.W. Scr. Metall. Mater. (1 Apr. 1993) 28 (7), 849–852

Many cold and hot worked metals undergo strain hardening and softening when subjected to cyclic plastic straining. The degree of strain softening depends on the amount of prior cold work or heat treatment introduced into the material and upon the magnitude and the number of cycles of applied cyclic plastic strain. Cyclic strain softening leads to a lower flow stress, higher ductility, reduced tensile strength upon subsequent straining and lower energy consumption during material processing. In order to optimise metal forming processes, the work hardening and softening should be analysed to include the influence of deformation conditions within the constitutive model. This paper developed a constitutive model based on the Estrin and Mecking phenomenological model for cyclic hardening without prestraining whilst using a combined Estrin and Mecking model and Avrami equation for cyclic softening with prestraining. As cyclic softening is largely dependent on the work hardening previously achieved and on deformation conditions, the constitutive model takes account of these conditions in addition to those used in conventional constitutive models.