Grain size effect on deformation twinning and detwinning

This article systematically overviews the grain size effect on deformation twinning and detwinning in face-centered cubic (fcc) metals. With decreasing grain size, coarse-grained fcc metals become more difficult to deform by twinning, whereas nanocrystalline (nc) fcc metals first become easier to deform by twinning and then become more difficult, exhibiting an optimum grain size for twinning. The transition in twinning behavior from coarse-grained to nc fcc metals is caused by the change in deformation mechanisms. An analytical model based on observed deformation physics in nc metals, i.e., grain boundary emission of dislocations, provides an explanation of the observed optimum grain size for twinning in nc fcc metals. The detwinning process is caused by the interaction between dislocations and twin boundaries. Under a certain deformation condition, there exists a grain size range where the twinning process dominates over the detwinning process to produce the highest density of twins.     

Grain boundary engineering: an overview after 25 years

Abstract

In 1984, 'grain boundary design', later known as 'grain boundary engineering (GBE)', was proposed. The central premise of GBE is that specific thermomechanical treatments, mainly on face centred cubic materials which readily form annealing twins, can be used to improve resistance to various forms of intergranular degradation such as corrosion, cracking or embrittlement. Engagement with the concept has accelerated in recent years. This overview charts the progress of GBE from its inception 25 years ago to the present day, including suggestions of key topics for ongoing or future research. These topics comprise confirmation of which boundaries are 'special' in terms of crystallography and properties, optimisation of processing regimes, new approaches to GBE in systems without annealing twinning and incorporation of connectivity metrics, especially in three dimensions.     

Modelling the influence of microstructural morphology and triple junctions on hydrogen transport in nanopolycrystalline nickel

Hydrogen transport in nanopolycrystalline (NPC) face centred cubic (FCC) nickel has received considerable attention as a result of the material's unique structural embrittlement behaviour. Triple junctions, where three grain boundaries meet, play an important role in hydrogen diffusion. Experiments have indicated that hydrogen transport at a triple junction (TJ) is orders of magnitude greater. In this contribution, a multiphase NPC model is proposed and used to investigate the influence on hydrogen transport of TJs within the surface of the NPC nickel using finite nanostructural element analyses. This 2D multiphase NPC model increases the density of triple junctions as the grain size reduces. The multiphase NPC model consists of two phases comprising nano grain interiors (GI) and intergranular phases. The intergranular (Ig) phase is divided into grain boundary affected zones (GBAZ) regions and TJ regions. The results of this finite nanostructural analysis show that hydrogen transport is enhanced at TJs and the bulk diffusion of hydrogen in NPC material is faster as the volume fraction of TJ increases and nano grain size decreases. The accumulation of hydrogen in three phase (GI, GBAZ, and TJ) microstructures is higher than the two phase (GI and Ig) microstructure case. The accumulation of hydrogen in TJ and Ig are heterogeneous in NPC nickel. The importance of the microstructural morphology in terms of the presence of pores, fine grains in TJ, changes in the shape of TJ with changes in the density of TJ and a TJ effect related to hydrogen transport in NPC nickel is all evidenced. This means that the TJ and microstructural morphology cannot be neglected when predicting hydrogen transport in a NPC nickel.     

Grain boundary engineering of titanium-stabilized 321 austenitic stainless steel

Grain boundary engineering (GBE) primarily aims to prevent the initiation and propagation of intergranular degradation along grain boundaries by frequent introduction of coincidence site lattice (CSL) boundaries into the grain boundary networks in materials. It has been reported that GBE is effective to prevent intergranular corrosion due to sensitization in unstabilized 304 and 316 austenitic stainless steels, but the effect of GBE on intergranular corrosion in stabilized austenitic stainless steels has not been clarified. In this study, a twin-induced GBE utilizing optimized thermomechanical processing with small pre-strain and subsequent annealing was applied to introduce very high frequencies of CSL boundaries into a titanium-stabilized 321 austenitic stainless steel. The resulting steel showed much higher resistance to intergranular corrosion after sensitization subsequent to carbon re-dissolution heat treatment during the ferric sulfate–sulfuric acid test than the as-received one. The high CSL frequency resulted in a very low percolation probability of random boundary networks in the over-threshold region and remarkable suppression of intergranular corrosion during GBE.     

Grain boundary reorientation in copper

The present route to grain boundary engineering (GBE) is usually based on multiple annealing twinning which can only be applied to a certain subset of materials, namely those that twin prolifically. A more general approach has been highlighted recently, following experimental evidence that certain boundary planes in iron bicrystals are ‘special’, and that this classification is not based on misorientation. It was suggested that, under suitable conditions, individual interfaces could reorient the most energetically advantageous orientations. This approach concurs with a similar concept of ‘grain boundary plane engineering’, proposed previously. In the present article we explore this concept and report the effect of long duration, low temperature annealing on the distribution of boundary misorientation and planes in copper. The new findings give support to the possibility of grain boundary structure optimisation via controlled annealing. To have established that grain boundary plane reorientation is feasible opens up new avenues and challenges in the field of grain boundary research. This could have significant impact both scientifically in terms of understanding grain boundary structure and technologically in the field of GBE.     

Effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation in polycrystalline aluminum

The effects of grain boundary- and triple junction-character on intergranular fatigue crack nucleation were studied in coarse-grained polycrystalline aluminum specimens whose grain boundary microstructures were analyzed by SEM-EBSD/OIM technique. Fatigue crack nucleation occurred mainly along grain boundaries and depended strongly on both the grain boundary character and grain boundary configuration with respect to the persistent slip bands. However, it was little dependent on the geometrical arrangements between the grain boundary plane and the stress axis. Particularly, random boundaries become preferential sites for fatigue crack nucleation. The fatigue cracks were also observed at CSL boundaries when the grain-boundary trace on the specimen surface was parallel to persistent slip bands. On the other hand, no intergranular fatigue cracks were observed at low-angle boundaries. The fatigue cracks were observed at triple junctions as well as grain boundaries. Their nucleation considerably occurred at triple junctions where random boundaries were interconnected. The grain boundary engineering for improvement in fatigue property was discussed on the basis of the results of the structure-dependent intergranular and triple junction fatigue crack nucleation.     

Current Progress of Mechanical Properties of Metals with Nano-scale Twins

Focus on face-centered cubic （fcc） metals with nano-scale twins lamellar structure, this paper presents a brief overview of the recent progress made in improving mechanical properties, including strength, ductility, work hardening, strain rate sensitivities, and in mechanistically understanding the underling deformation mechanisms. Significant developments have been achieved in nano-twinned fcc metals with a combination of high strength and considerable ductility at the same time, enhanced work hardening ability and enhanced rate sensitivity. The findings elucidate the role of interactions between dislocations and twin boundaries （TBs） and their contribution to the origin of outstanding properties. The computer simulation analysis accounts for high plastic anisotropy and rate sensitivity anisotropy by treating TBs as internal interfaces and allowing special slip geometry arrangements that involve soft and hard modes of deformation. Parallel to the novel mechanical behaviors of the nano-twinned materials, the investigation and developments of nanocrystalline materials are also discussed in this overview for comparing the contribution of grain boundaries/TBs and grain size/twin lamellar spacing to the properties. The recent advances in the experimental and computational studies of plastic deformation of the fcc metals with nano-scale twin lamellar structures provide insights into the possible means of optimizing comprehensive mechanical properties through interfacial engineering.     

The Institute of Metals: 1987 Awards

Abstract

In the present study, an Al-Cu based metallic matrix was reinforced with graphite particles using an innovative partial liquid phase casting (rheocasting) technique. The results of microstructural characterisation studies revealed the presence of columnar-equiaxed grain morphology, afinite amount of porosity, interdendritic/intercellular Al-Cu phase, and predominantly grain boundary segregated distribution of graphite particles. Heat treatment studies conducted on rheocast composite specimens revealed accelerated ((Al-4.5Cu)/6.8C) and retarded ((Al-4.5Cu)/4.2C) aging kinetics when compared with unreinforced specimens. Ambient temperature tensile testing results, contrary to those reported by other investigators, revealed an increase in 0.2% yield stress and ultimate tensile strength of graphite reinforced specimens when compared with unreinforced specimens. The enhanced tensile properties realised by the graphite reinforced specimens are correlated with the processing influenced microstructural characteristics of these specimens.     

Grain boundary engineering for improving stress corrosion cracking of 304 stainless steel

Grain boundary engineering (GBE) via low strain tension and annealing was used to enhance the resistance to stress corrosion cracking of a 304 stainless steel. Electron backscattered diffraction (EBSD) analysis exhibited that the GBE steel had a higher fraction of low-∑ coincidence site lattice (CSL) boundaries, larger grain-clusters, longer twin boundary chains, and fewer paths of connected non-twin boundaries with a more zigzag shape. Slow strain rate tests in high-temperature water showed that the GBE steel performed better plasticity, higher tensile strength, and similar yield strength compared to conventional steel. The low fraction of random boundaries in GBE steel resulted in a lower frequency of intergranular crack initiation, and the zigzag paths of non-twin boundaries made the intergranular crack propagation more difficult.     

First-principles calculation of grain boundary energy and grain boundary excess free volume in aluminum: role of grain boundary elastic energy

We examined the grain boundary energy (GBE) and grain boundary excess free volume (BFV) by applying the first-principles calculation for six [110] symmetric tilt grain boundaries in aluminum to clarify the origin of GBE. The GBE increased linearly as BFV increased. The elastic energy associated with BFV, namely the grain boundary elastic energy, was estimated as a function of BFV and the shear modulus. The grain boundary elastic energies were close in value to the GBEs. The charge density distributions indicated that the bonding in the grain boundary region is significantly different from the bonding in the bulk. The grain boundary elastic energies were 15–32% higher than the GBEs. This overestimation of the grain boundary elastic energy is caused by the characteristics of the electronic bonding at the grain boundary, which is different from bonding in the bulk. We have concluded that GBE results mainly from the grain boundary elastic energy.     

Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

Effect of variation in physical properties of the metallic matrix on the microstructural characteristics and the ageing behaviour of Al-Cu/SiC metal matrix composites

In this study, aluminium-based metallic matrices with varying amount of copper (1 wt% Cu and 4.5 wt% Cu) were reinforced with SiC particulates using a partial liquid phase casting technique. The results of the present investigation showed smaller sized and higher weight percent of SiC particulates being successfully incorporated with a decrease in the weight percent of copper in the matrix. Microstructural characterisation studies conducted on the composite samples revealed an increase in uniformity of distribution of SiC particulates, improved SiC/Al interfacial integrity and smaller grain size of the metallic matrices with decreasing weight percent of copper. Results of the microstructural characterisation studies also exhibited the presence of solute rich zone in the near vicinity of SiC particulates and the nucleation of secondary phases both at and in near vicinity of SiC particulates. The result of the ageing studies revealed an accelerated ageing kinetics for the Al-1%Cu/SiC composite when compared to the Al-4.5%Cu/SiC composite samples. The results of accelerated ageing kinetics were rationalised in terms of the effect of variation in the physical properties of the metallic matrix and the ensuing microstructural characteristics due to variation in the amount of copper in the matrix.     

Atomistic simulations of metallic microstructures

This paper reviews recent results in the simulation of the mechanical response of metallic microstructures at the atomic level. The role of the grain boundary network in deformation process is the concentration of this paper as studied by virtual tensile and nanoindentation tests. The grain boundary network is found to contribute to plastic deformation through the process of dislocation nucleation, absorption and transmission, as well as grain boundary accommodation mechanisms such as grain boundary sliding and migration. The microstructural grain boundary network is also critical to the nucleation and propagation of cracks. The challenges and opportunities in this area are discussed.     

Observation of the morphology of thin films of solid helium deposited from the fluid onto sapphire

Thin layers of solid helium were grown on sapphire single-crystal substrates at pressures from about 500 bar to 9 kbar. Grain boundaries can be observed in these layer crystals. The morphology of the grains depends on the crystal modification. In the hcp phase (below about 1.13 kbar) a system of parallel bands is observed, probably due to slip and twinning. In the fcc phase (above 1.13 kbar) a polygonal structure similar to a helium froth is found. Melting of this froth in the fcc phase shows grain boundary melting; fluid helium is wetting the fcc grains. Grain boundaries in the hcp phase are, in contrast, not wetted by fluid helium. Near the triple point at 1.13 kbar and 15.0 K one can deposit both crystalline phases side by side. In such structures, the transition fcc hcp⁴He can be observed during isothermal holding. The transition proceeds by the parallel motion of low-energy grain boundaries.     

Universal features of grain boundary networks in FCC materials

Grain boundary character distributions and triple junction distributions have been determined for 70 experimental microstructures, comprising aluminum-, copper-, austenitic iron- and nickel-based alloys in a wide variety of processed states. In these FCC metals, the fraction of coincidence site lattice (CSL) boundaries ranges from about 12% (as for a random Mackenzie distribution) to values as high as 75%. Despite wide variations in composition, processing, and grain size, we find that the grain boundary character distribution and triple junction distributions of these materials have striking similarities, and can be described by just a few parameters. This universality arises due to the highly non-random laws that govern the assembly of the grain boundary network, and due to the kinematic limitation that CSL boundaries arise primarily through twinning.     

The assisting and stabilizing role played by ω phase during the {112} <111>β twinning in Ti-Mo alloys:A first-principles insight

The ω phase is commonly observed in β-Ti alloys and plays a significant role on various properties of β-Ti alloys.Although many results about the role ofω phase on mechanical properties of β-Ti alloys have been derived from theoretical and experimental studies,the role ofω phase on deformation mechanism hitherto remains elusive and deserves to be further studied.In this work,the role played by ω phase during the {112 } <111>β twinning in Ti-Mo alloys were investigated by first-principles calculations at atomic scale.In the energy favorable interface of(112)β/(10(1)0)ω,we found that partial dislocations slipping on the successive (10(1)0)ω planes ofω phase can lead to the formation of { 112} <111>β twin nucleus.And the twin nucleus grows inwards ω grain interior through atomic shuffle.Thus,a new twinning mechanism of {112 } <111>β assisted by ω phase was proposed.Furthermore,our calculations indicated that the Pearance of ITB (interfacial twin boundary) ω phase can improve the stability of the symmetrical 12 } <111 >β twin boundary (TB),which can well explain the experimental phenomenon that the ITB ω phase always accompanies the formation of {112 } <111>β twin.Finally,a probable microstructure evolution sequence was suggested,namely β matrix → β matrix + athermal ω phase → (112)[11(1)]twin → (112)[11(1)]β twin + ITB ω phase.Our calculations provide new insights on the role played by ω phase during the twinning process of {112} <111>β,which can deepen the understanding on the deformation behaviors of β-Ti alloys.     

Tensile deformation behavior of high manganese austenitic steel: The role of grain size

The tensile deformation behavior and microstructural evolutions of twinning induced plasticity (TWIP) steel with the chemical composition of Fe–31Mn–3Al–3Si and average grain sizes in the range of 2.1–72.6 μm have been analyzed. For each grain size, the Hollomon analysis and also the Crussard–Jaoul (C–J) analysis as an alternative method to describe the work hardening behavior were investigated. The results indicated that the optimum mechanical properties as a function of work hardening capacity can be obtained by changing the grain size. The microstructural observations showed that the pile-ups of planar dislocations are necessary for triggering the mechanical twinning and grain refinement suppresses the mechanical twinning in TWIP steel. Furthermore, the mechanical twinning increases with increasing applied strain. As a result, a high instantaneous work hardening due to the mechanical twin boundaries enhances the uniform elongation. The contribution from the strain of twinning and hardening due to an increase in the hardness of the twinned regions (i.e., the Basinski mechanism) may be also useful in achieving the high strength–ductility in TWIP steels.     

Challenges in Continuum Modelling of Intergranular Fracture

Abstract: Intergranular fracture in polycrystals is often simulated by finite elements coupled to a cohesive zone model for the interfaces, requiring cohesive laws for grain boundaries as a function of their geometry. We discuss three challenges in understanding intergranular fracture in polycrystals. First, 3D grain boundary geometries comprise a five‐dimensional space. Second, the energy and peak stress of grain boundaries have singularities for all commensurate grain boundaries, especially those with short repeat distances. Thirdly, fracture nucleation and growth depend not only upon the properties of grain boundaries, but also in crucial ways on edges, corners and triple junctions of even greater geometrical complexity. To address the first two challenges, we explore the physical underpinnings for creating functional forms to capture the hierarchical commensurability structure in the grain boundary properties. To address the last challenge, we demonstrate a method for atomistically extracting the fracture properties of geometrically complex local regions on the fly from within a finite element simulation.     

Synthesis and recyclability of a magnesium based composite using an innovative disintegrated melt deposition technique

Abstract

This study has addressed the feasibility of synthesising and recycling a silicon carbide reinforced magnesium composite using an innovative disintegrated melt deposition technique with the aim of improving the mechanical properties. Microstructural characterisation studies revealed a marginal decrease in porosity and reinforcement content, and no change in grain morphology, reinforcement distribution pattern, and interfacial integrity between matrix and reinforcement following recycling. Results of physical and mechanical property characterisation revealed increases in 0.2% yield strength, ultimate tensile strength, ductility, and coefficient of thermal expansion of the recycled specimens when compared with the parent composite. These properties have been rationalised in terms of the microstructural characteristics associated with the disintegrated melt deposited composite specimens. Particular emphasis was placed on studying the effect of recycling on the microstructure and properties of the composite.     

Grain boundary engineering of 304 austenitic stainless steel by laser surface melting and annealing

In order to improve the intergranular corrosion resistance of 304 stainless steel, laser surface remelting experiments were conducted using a 2 kW continuous wave Nd: YAG laser. The grain boundary character distribution (GBCD) and microstructures of the materials were analyzed using EBSD, SEM and OM. The experimental results showed that combination of laser surface melting and annealing on 304 stainless steel resulted in a high frequency of twin boundaries and consequent discontinuity of random boundary network in the materials, which led to an improvement of resistance to intergranular corrosion. The maximum CSL density could reach 88.6% under optimal processing conditions: 1220 K and 28 h.