Showing papers on "Grain boundary strengthening published in 2010"

The effect of grain size and grain orientation on deformation twinning in a Fe-22 wt.% Mn-0.6 wt.% C TWIP steel

Abstract: We investigate the effect of grain size and grain orientation on deformation twinning in a Fe–22 wt.% Mn–0.6 wt.% C TWIP steel using microstructure observations by electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). Samples with average grain sizes of 3 μm and 50 μm were deformed in tension at room temperature to different strains. The onset of twinning concurs in both materials with yielding which leads us to propose a Hall–Petch-type relation for the twinning stress using the same Hall–Petch constant for twinning as that for glide. The influence of grain orientation on the twinning stress is more complicated. At low strain, a strong influence of grain orientation on deformation twinning is observed which fully complies with Schmid's law under the assumption that slip and twinning have equal critical resolved shear stresses. Deformation twinning occurs in grains oriented close to 〈1 1 1〉//tensile axis directions where the twinning stress is larger than the slip stress. At high strains (0.3 logarithmic strain), a strong deviation from Schmid's law is observed. Deformation twins are now also observed in grains unfavourably oriented for twinning according to Schmid's law. We explain this deviation in terms of local grain-scale stress variations. The local stress state controlling deformation twinning is modified by local stress concentrations at grain boundaries originating, for instance, from incoming bundles of deformation twins in neighboring grains.

Saturation of Fragmentation During Severe Plastic Deformation

Abstract: In this review, we focus on the saturation microstructure that evolves during severe plastic deformation (SPD). These nanocrystalline or ultrafinegrained microstructures consist predominantly of high-angle boundaries, although low-angle boundaries are also present. Deformation temperature, alloying, and strain path are the dominant factors controlling the saturation grain size in single-phase materials. The saturation grain size decreases significantly with decreasing deformation temperature, although the dependency is stronger at medium homologous temperatures and less in the lowtemperature regime. The saturation microstructure is sensitive to strain rate at medium temperatures and less so at low temperatures. The addition of alloying elements to pure metals also reduces the saturation grain size. The results indicate that grain boundary migration is the dominant process responsible for the limitation in refinement by SPD. Therefore, second-phase particles of the nanometer scale can stabilize even finer microstructures. This mechanism of stabilization of the microstructure is an effective tool for overcoming the limit in refinement of single-phase materials by SPD. The improved thermal stability of the obtained nanostructures is another benefit of the introduction of second-phase particles.

Statistical analyses of deformation twinning in magnesium

Abstract: To extract quantitative and meaningful relationships between material microstructure and deformation twinning in magnesium, we conduct a statistical analysis on large data sets generated by electron backscattering diffraction (EBSD). The analyses show that not all grains of similar orientation and grain size form twins, and twinning does not occur exclusively in grains with high twin Schmid factors or in the relatively large grains of the sample. The number of twins per twinned grain increases with grain area, but twin thickness and the fraction of grains with at least one visible twin are independent of grain area. On the other hand, an analysis of twin pairs joined at a boundary indicates that grain boundary misorientation angle strongly influences twin nucleation and growth. These results question the use of deterministic rules for twin nucleation and Hall–Petch laws for size effects on twinning. Instead, they encourage an examination of the defect structures of grain boundaries and their role in twin ...

Grain Boundary Segregation in Metals

Abstract: I Introduction II Grain boundaries: structure, description and thermodynamics III Approaches to Study Grain Boundary Segregation IV Models of Equilibrium Grain Boundary Segregation V Effect of Variables on Equilibrium Grain Boundary Segregation VI Principles of Non-Equilibrium Segregation VII Grain Boundary Segregation and Related Phenomena and metallurgical phenomena VIII Concluding remarks

Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels

Abstract: Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4m) and ultrafine grained (1.2m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4m) reference material. Both yield strength and tensile strength follow a Hall–Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

Strengthening mechanisms in an Al–Mg alloy

Abstract: Recent improvements in strength and ductility of 5083 aluminum alloys have been obtained through the development of complex microstructures containing either reduced grain sizes (ultra-fine and nano-grained materials), grain size distributions (bimodal microstructures), particle reinforcements, or combinations of the above. Optimization of such microstructures requires an understanding of the conventional, coarse-grained basis alloy. We present here a complete experimental data set for conventional Al-5083 H-131, with primary alloying element Mg (4.77 wt%) and secondary element Mn (0.68 wt%) in compression over a range of strain rates (10−4–6000 s−1) at room temperature. The various strengthening mechanisms in Al-5083 are explored, including solute strengthening, precipitate hardening, strain hardening, strain rate hardening, and strengthening due to dislocation sub-structures. Previous experiments found in the literature on Al–Mg binary alloys allow us to calculate the solute strengthening due to Mg in solid solution, and TEM analysis provides information about precipitate hardening and dislocation cell structures. A basic strength model including these strengthening mechanisms is suggested.

Flow stress analysis of TWIP steel via the XRD measurement of dislocation density

Abstract: In this study, the rate of dislocation accumulation in the tensile strained twinning induced plasticity (TWIP) steel was calculated via the X-ray diffraction (XRD) measurements and compared with other fcc metals and alloys. The results indicated that the XRD technique is an alternative method to estimate the dislocation density. Moreover, flow stress analysis of Fe–31Mn–3Al–3Si TWIP steel with the grain size of about 18 μm indicated that, beside a direct effect of the dislocation interactions on the flow stress, another strengthening mechanism is also required to describe the flow behavior. For this reason, the strengthening contribution due to the formation of mechanical twins was considered as a reduction of dislocation mean free path. Interestingly, the estimated flow stress equation consisting of the strengthening effects of both dislocation interactions and dynamic microstructure refinement due to mechanical twinning (i.e., the dynamic Hall–Petch effect) are in good agreement with the experimental data and equation proposed by Ludwigson for low SFE materials.

Coarsening in Sintering: Grain Shape Distribution, Grain Size Distribution, and Grain Growth Kinetics in Solid-Pore Systems

Abstract: Sintering occurs when packed particles are heated to a temperature where there is sufficient atomic motion to grow bonds between the particles The conditions that induce sintering depend on the material, its melting temperature, particle size, and a host of processing variables It is common for sintering to produce a dimensional change, typically shrinkage, where the powder compact densifies, leading to significant strengthening Microstructure coarsening is inherent to sintering, most evident as grain growth, but it is common for pore growth to occur as density increases During coarsening, the grain structure converges to a self-similar character seen in both the grain shape distribution and grain size distribution Coarsening behavior during sintering conforms to classic grain growth kinetics, modified to reflect the evolving microstructure These modifications involve the grain boundary coverage due to pores, liquid films, or second phases and the altered grain boundary mobility due to these phases

How Grain Growth Stops: A Mechanism for Grain-Growth Stagnation in Pure Materials

Abstract: The thermodynamic equilibrium state of crystalline materials is a single crystal; however, polycrystalline grain growth almost always stops before this state is reached. Although typically attributed to solute drag, grain-growth stagnation occurs, even in high-purity materials. Recent studies indicate that grain boundaries undergo thermal roughening associated with an abrupt mobility change, so that at typical annealing temperatures, polycrystals will contain both smooth (slow) and rough (fast) boundaries. Mesoscale grain-growth models, validated by large-scale polycrystalline molecular dynamics simulations, show that even small fractions of smooth, slow boundaries can stop grain growth. We conclude that grain-boundary roughening provides an alternate stagnation mechanism that applies even to high-purity materials.

Grain refinement and mechanical behavior of a magnesium alloy processed by ECAP

Abstract: Equal-channel angular pressing (ECAP) is an effective tool for refining the grain structure of magnesium alloys and improving the ductility at moderate temperatures. However, grain refinement in these alloys differs from other metals because new grains are formed along the boundaries of the initial structure and these newly formed grains slowly spread to consume the interiors of the larger grains in subsequent passes. A model is presented for grain refinement in magnesium alloys processed by ECAP based on the principles of dynamic recrystallization where new fine grains are formed along the initial boundaries and along twin boundaries. This model provides an explanation for a wide range of experimental data and introduces the concept of grain size engineering for achieving selected material properties in magnesium alloys.

A new perspective on paleopiezometry: Dynamically recrystallized grain size distributions indicate mechanism changes

Abstract: The dynamically recrystallized grain size is a material parameter associated with dislocation creep of crystalline solids that is especially important as a flow stress indicator via piezometer calibrations. Grain sizes have been measured in many studies of deformed rocks as well as metals and ceramics, but global analyses of the frequency distribution of dynamically recrystallized grain sizes are lacking. Here we present the first systematic investigation of the recrystallized grain size distribution, for quartz. The grain diameters, compiled from 555 samples of 31 studies of quartz mylonites deformed over a wide range of conditions, extend from ∼3 μm to 3 mm, with distinct maxima at 10–20 μm and 70–80 μm, and minima at 35–40 μm and ∼120 μm. The frequency maxima correlate with distinct microstructures and the minima with the transitions between these microstructures, which we interpret to result from the dominance of the recrystallization mechanisms of bulging, subgrain rotation, and grain boundary migration recrystallization. These results demonstrate the necessity of distinct piezometer calibrations for different recrystallization mechanisms and highlight the importance of the recrystallized grain size for theoretical models of dynamic recrystallization.

On the Hall–Petch relationship in a nanostructured Al–Cu alloy

Abstract: Mechanical properties of bulk nanocrystalline Al–4Cu alloys with grain sizes from 47 to 105 nm, synthesized by mechanically alloying followed by vacuum hot pressing at different temperatures, were analysed through Hall–Petch relation. Hall–Petch analysis revealed a high frictional stress (170 MPa) and a high positive slope ( 0.13 MPa m ) as compared to pure Al, which has a frictional stress (15–30 MPa) and a slope ( 0.06 – 0.09 MPa m ). From a detailed evaluation of different strengthening mechanisms it is inferred that the Al 2 Cu precipitates and oxide particles are the likely reason for such high values of frictional stress and slope.

Evaluation of the block boundary and sub-block boundary strengths of ferrous lath martensite using a micro-bending test

Abstract: We report our investigation of the block boundary and sub-block boundary strengths of lath martensite evaluated through a micro-bending test. The sub-block boundaries contribute very little to the macroscopic strength of the lath martensite. In contrast, the presence of a block boundary in the specimen greatly increased the strength. In addition, the block boundary induced a serrated flow and load drop after yielding in the load–displacement curve. The load drop and serrated flow were attributable to dislocation pile-up and subsequent propagation of dislocations across the block boundary. In a microstructural observation of specimens after deformation, we found that a block boundary significantly restricts the motion of dislocations, while a sub-block boundary does not. We concluded that the block boundary is the most effective grain boundary for strength in lath martensite.

The role of stacking faults and twin boundaries in grain refinement of a Cu-Zn alloy processed by high-pressure torsion

Abstract: A recent model developed to predict the smallest grain sizes obtainable by severe plastic deformation has worked well for materials with medium to high stacking fault energies (SFEs) but not for those with low SFEs. To probe this issue, experiments were conducted using a Cu–30 wt.% Zn alloy with a very low SFE of 7 mJ/m2 as the model material. High-pressure torsion was used as the grain refinement technique. The results indicate that stacking faults and twin boundaries play a key role in the grain refinement process such that the smallest achievable grain size is determined by the highest stacking fault and twin density that the system is able to produce. An amorphization of grain boundaries was also observed in the final structure. These observations are very different from those reported for materials having medium to high SFEs and they confirm the operation of a different grain refinement mechanism.

Modeling of grain size effect on micro deformation behavior in micro-forming of pure copper

Abstract: In micro-scaled plastic deformation process such as micro-forming, material grain size effect is difficult to reveal and investigate using conventional material models. Finding a way to study and model the grain size effect on micro-scaled deformation behavior is a non-trivial issue that needs to be addressed in greater depth. In this study, the grain size effect is investigated through micro-compression of pure copper. The deformation behaviors, including inhomogeneous material flow and the decrease of flow stress with the increase of grain size for the same size of specimens, are studied. It is revealed that when the specimen is composed of only a few grains, the grains with different sizes, shapes and orientations are unevenly distributed in the specimen and each grain plays a significant role in micro-scaled plastic deformation and leads to inhomogeneous deformation and the scatter of experimental data. Furthermore, it is found that the decrease of flow stress is caused by the dwindling of grain boundary strengthening effect when the grain size is increased. Based on the experiment results and the proposed composite model, the methodologies to estimate grain properties and model grain size effect are developed. Through Finite Element (FE) simulation, the grain size effect on deformation behavior and the scatter of flow stress are modeled. The results of the physical experiment and the proposed modeling methodologies provide a basis for understanding and further exploration of micro-scaled plastic deformation behavior in micro-forming process.

In situ TEM study of grain growth in nanocrystalline copper thin films.

Abstract: Nanocrystalline metals demonstrate a range of fascinating properties, including high levels of mechanical strength. However, as these materials are exposed to high temperatures, it is critical to determine the grain size evolution, as this process can drastically change the mechanical properties. In this work, nanocrystalline sputtered Cu thin films with 43 ± 2 nm grain size were produced by dc-magnetron sputtering. Specimens were subsequently annealed in situ in a transmission electron microscope at 100, 300 and 500 °C. Not only was grain growth more evident at 500 °C but also the fraction of twins found. An analysis of grain growth kinetics revealed a time exponent of 3 and activation energy of 35 kJ mol − 1. This value is explained by the high energy stored in the form of dislocation, grain boundaries and twin boundaries existing in nanocrystalline copper, as well as the high probability for atoms to move across grains in nanocrystalline materials.

Unusual super-ductility at room temperature in an ultrafine-grained aluminum alloy

Abstract: Processing by severe plastic deformation (SPD) typically increases the strength of metals and alloys drastically by decreasing their grain size into the submicrometer or nanometer range but the ductility of such materials remains typically low. This report describes the first demonstration that it is possible to increase the room temperature ductility of aluminum-based alloys processed by SPD and to attain elongations to failure of >150% while retaining the enhanced strength. This unique combination of properties is due to the occurrence of grain boundary sliding at room temperature. The sliding was obviously achieved by introducing a grain boundary wetting of the aluminum/aluminum grain boundaries.

Deformation behavior of magnesium in the grain size spectrum from nano- to micrometer

Abstract: This study investigates the deformation behavior of magnesium produced by hot extrusion of ball-milled powders in grains ranging from 120 μm down to 60 nm in size. For microcrystalline magnesium, lattice dislocation interactions with grain boundaries and/or twin boundaries provide a Hall–Petch relationship between the flow stress and the grain size. The Hall–Petch slope is negatively deviated as the grain size is reduced below 1 μm since twinning offers an additional deformation mode. As the grain size is further reduced below 100 nm, twinning is significantly suppressed and a portion of grain boundary sliding for plastic deformation increases, providing an inverse Hall–Petch relationship. Microstructure observation, a negligible strain hardening rate, a relatively high index of strain rate sensitivity, and a low activation volume in compression tests also demonstrate the particular deformation behavior of nanocrystalline magnesium.

Sensitisation behaviour of grain boundary engineered austenitic stainless steel

Abstract: Thermo-mechanical processes involving both single and multiple cycles of low level (5%) strain and annealing were applied to specimens of a type 304 austenitic stainless steel in order to encourage grain boundary engineering (GBE). As a result of the GBE processing the total length proportion of Σ3n coincidence site lattice (CSL) boundaries was increased from 43% up to a maximum of 75% in conjunction with moderate grain growth. The increases in Σ3 and Σ9 boundaries resulted in significant decreases in the degree of sensitisation following exposure at 650 °C for up to 4 h and assessment through Double Loop-Electrochemical Potentiokinetic Reactivation (DL-EPR) tests. Over 97% of Σ3 boundaries were immune to sensitisation and approximately 80% of Σ9 boundaries were either immune or partially resistant to sensitisation, whereas all other CSL boundaries and general boundaries did not resist sensitisation. Therefore, only Σ3 and Σ9 boundaries were ‘special’. Deformation applied by cold rolling was more effective than tensile deformation in bringing about GBE. In summary, the results presented here show that increasing the fraction of Σ3 and Σ9 boundaries through GBE processing, accompanied by only moderate grain growth, provides an effective route to protection from sensitisation and intergranular corrosion.