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 size effect on deformation twinning in Mg–Al–Zn alloy

Abstract

Specimens of Mg–3Al–1Zn alloy with a wide range of grain size distribution were compressed along different directions. The compressed microstructure was examined to clarify the grain size effect on deformation twinning in magnesium alloys. Small strains were used to reveal the twinning behaviour. The results show that the grain size affects the formation of deformation twins in an Mg–Al–Zn alloy. The reason for a different result being previously reported is given. This study also reports the different deformation microstructures in specimens compressed along different directions.     

Corrosion-resistant analogue of Hadfield steel

The concept of alloying austenitic steels with carbon + nitrogen is used for the development of a corrosion-resistant austenitic CrMn steel having an impact wear resistance close to that of the Hadfield steel. A higher stabilization of the austenitic phase by C + N, as compared to carbon or nitrogen alone, is substantiated by ab initio calculation of the electron structure, measurements of the concentration of free electrons and calculations of the phase equilibrium. Based on these results, the compositions (mass%) Cr18Mn18C0.34N0.61 and Cr18Mn18C0.49N0.58 were melted and tested along with Hadfield steel Mn12C1.2. Mechanical tests have shown that, as compared to the Hadfield steel, the experimental steels possess a higher strength, plasticity, hardness and the same resistance to impact wear. TEM studies of the surface layer after impact treatment revealed a mixture of the amorphous phase, nanocrystals and fine-twinned austenite. At the same time, using Mössbauer spectroscopy of conversion electrons, the ferromagnetic ordering was found in the surface layer of up to 10 μm in depth, which is the sign of the strain-induced martensitic phase. The hypothesis of a transition from the low-spin to the high-spin state of the iron atoms within the thin twins in austenite was proposed in order to interpret the discrepancy between TEM and Mössbauer studies. Potentiodynamic measurements and immersion tests show that the CrMnCN steels possess a significantly higher pitting potential and resistance to general corrosion in comparison with Hadfield steel.     

The effect of aluminum alloying on ductile-to-brittle transition in Hadfield steel single crystal

The ductile-to-brittle transition (DBT) in Fe-13Mn-1.3C (Hadfield steel, I) and Fe-13Mn-2.7 Al-1.3C (Hadfield steel, II) (wt.%) single crystals oriented along [011], [[`1]44]{[011], [{\bar{{1}}}44]}, and [[`1]11{\bar{{1}}11}] directions was investigated under tension in the temperature interval of 77 to 673 K. The DBT temperature interval was found to be independent of single crystal orientation. The DBT temperatures were estimated (1) as the mean value between the temperature corresponding to the minimum crystal ductility and the one coinciding with the onset of the plateau of the e{\varepsilon}(T)-dependence (T_DBT1); and (2) as the temperature where the volume fraction of brittle failure on the fracture surfaces was 50% (T_DBT2). The DBT temperatures estimated this way, do not coincide for both steels. Mechanical twinning has been reported as the primary reason for the occurrence of the DBT in austenitic high-carbon Hadfield steel and appears to account for the difference in DBT temperatures as well. Alloying with aluminum partially suppresses twinning in steel (II). Twinning sets in only after a certain amount of dislocation slip, but still influences the fracture mechanism of steel (II).     

Influence of carbon on development of deformation microstructures in Hadfield steels

Abstract

The microstructural development during rolling is compared in two Hadfield steels, one having low carbon content (0·65 wt-%) and the other high content (1·35 wt-%). Differences in substructure are observed which are due not to small changes in stacking fault energy, but to carbon segregation, which occurs in the low carbon steel (through vacancy diffusion) but not in the high carbon steel. This is demonstrated using Mössbauer spectroscopy and is in agreement with systematic characterisation of microstructures by optical and transmission electron microscopy. In the low carbon steel mixed microstructures are formed which contain intrinsic stacking faults, deformation twins, and brass type shear bands. In the high carbon steel mixed substructures of dislocation tangles, deformation twins, and shear bands (both copper and brass type) are found to develop. In spite of the difference of substructure development during rolling in the two steels, the difference in stacking fault energy is measured to be small (~2 mJ m^?2, i.e. <10% of the stacking fault energy).

MST/1417     

The size effect on micro deformation behaviour in micro-scale plastic deformation

When the geometry of metal deformed part is scaled down to micro-scale, the understanding and prediction of micro deformation behaviour becomes difficult. This is because the conventional material deformation models are no longer valid in micro-scale due to the size effect, which affects the deformation behaviour in micro plastic deformation, and thus leveraging the traditional knowledge of plastic deformation from macro-scale to micro-scale is not meaningful. In this paper, the size effect on micro-scale plastic deformation and frictional phenomenon are investigated via micro-cylindrical compression test, micro-ring compression test and Finite Element (FE) simulation. The experimental results show the occurrence of various size-effect related deformation phenomena, including the decrease of flow stress and the increases of: (a) irrational local deformation, (b) the amount of springback, and (c) the interfacial friction stress with the decreasing specimen size. The research further verifies that the established surface layer models, with the identified surface grain, the internal grain properties and the measured friction coefficients, are able to predict micro deformation behaviour. The research thus provides an in-depth understanding of size effect on deformation and frictional behaviours in micro-scale plastic deformation.     

A novel high manganese austenitic steel with higher work hardening capacity and much lower impact deformation than Hadfield manganese steel

To tackle the problem of poor work hardening capacity and high initial deformation under low load in Hadfield manganese steel, the deformation behavior and microstructures under tensile and impact were investigated in a new high manganese austenitic steel Fe18Mn5Si0.35C (wt.%). The results show that this new steel has higher work hardening capacity at low and high strains than Hadfield manganese steel. Its impact deformation is much lower than that of Hadfield manganese steel. The easy occurrence and rapid increase of the amount of stress-induced ε martensitic transformation account for this unique properties in Fe18Mn5Si0.35C steel. The results indirectly confirm that the formation of distorted deformation twin leads to the anomalous work hardening in Hadfield manganese steel.     

Micro-mechanism of rolling contact fatigue in Hadfield steel crossing

The formation of the fatigue cracks was due to massive vacancy clusters in the subsurface layer of Hadfield steel crossing, which are induced by the accumulated plastic deformation under the conditions of impact and contact stresses from train wheels. The high concentration layers of vacancy clusters were formed parallel to the working surface of the crossing, which caused the initial rolling contact fatigue cracks to be parallel to the working surface with a laminar distribution in the depth direction. It can be predicted that metals containing elements with larger atomic diameter should have better rolling contact fatigue and wear performances.     

Grain size effect on high-temperature fatigue properties of alloy718

High-temperature fatigue properties of Alloy718 with different grain sizes were investigated. For the coarse-grain alloy, the fatigue strength notably decreased beyond 10⁵ cycles. The fatigue fracture had originated from facets of the austenite grains. The facets caused a decreasing of fatigue strengths in a high-cycle region at high temperatures.     

Grain size effect of electro-plated tin coatings on whisker growth

Tin and tin-lead coatings electro-plated in various solutions have been observed by means of high voltage electron microscopy, and the grain size effect of the coatings on whisker growth has been examined. As a result, it was found that the tin and tin-lead coatings from which whiskers hardly grew consisted of well-polygonized grains which were a few micrometre in size, and that the tin coatings from which whiskers easily grew consisted of irregular-shaped grains which were a few tenths of a micrometre in size. The irregularshaped grains contained dislocation rings which might be formed by clustering of vacancies or interstitial atoms upon electro-plating.     

Enhanced work hardening in Hadfield steel during explosive treatment

This paper presents results concerning Hadfield steel subjected to explosive treatment and compression, respectively, with the purpose of clarifying the difference between dynamic and static deformation behaviors. A Hadfield steel sample that was deformed to a lower strain at an exceptionally high strain rate exhibited the same hardness as a sample that was deformed to a higher strain at a low strain rate. A deformation model based on in situ deformation has been developed, whereby the enhanced work hardening during explosive treatment is attributed to the deformation of the grains being mainly accommodated by the curvature of the grain boundaries and shape change of the surrounding grains in their original positions, without obvious macroscopic deformation.     

Analysis of weld cracking in shotblasting chambers made of Hadfield steel

In fillet welds made of 18Cr–9Ni–7Mn austenitic stainless steel centerline cracks were established along the connection line of two solidification fronts in the weld. On the grain boundaries oxide and carbide films were identified. Hot cracks resulted from a combined activity of a large gap between plates and the resulting high tensile stresses occurring during cooling, free root surface of the weld in the large gap, and intergranular oxide and carbide films in the weld. Since the parent metal was cut with highly oxidating plasma and atmospherically corroded surfaces was not ground prior to welding, remelted surface oxides passed into the weld pool. The increased Cr content identified in intergranular oxide films results from Cr oxidation in the filler material.     

Synthesis of high-speed tool steel surfaces on mild steel

Wear-resistant, hard surfaces of high-speed tool steel were synthesized on mild steel specimens. Discs of mild steel were subjected to carburization to a depth of 2.5 mm. Thin strips of tungsten were spot welded and the specimen was subjected to electron beam surface melting. The beam power was varied from 60 kV, 10 mA to 60 kV, 20 mA. Oscillation frequency and the specimen translation velocity were kept at 1000 Hz and 2 cm s-1, respectively. The width of the modified layer was 10 mm while the depth varied from 0.7–2.3 mm. A concentration of up to 30 wt% tungsten could be achieved in the surface layer by varying the thickness of the foil spot welded prior to electron-beam melting. Tungsten concentration was uniform along the depth. The hardness achieved in the as-solidified layer was uniform along the depth and reached 800 Hv. The reprocessing of the alloyed layer with the beam promoted fine carbide precipitation which then resulted in refinement of martensite plates.     

Grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper

This work illustrates the grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper through experimental characterisations and numerical calculations. The as-received and annealed samples with different grain sizes are prepared. Mechanical properties at high strain rates are experimentally attained by using a split Hopkinson pressure bar device. With the increase of grain size, dynamic flow stress decreases, but strain-rate dependence of flow stress increases. Johnson–Cook constitutive model is applied to carry out a numerical analysis about the grain size effect on strain-rate dependence of mechanical data and the quantificational illustrations are given.     

Characterization of different work hardening behavior in AISI 321 stainless steel and Hadfield steel

In order to distinguish the difference between AISI 321 stainless steel and Hadfield steel in work hardening behavior, both the Hollomon analysis and the differential Crussard–Jaoul analysis were used to determine the strain hardening exponent as a function of the strain. The results showed that the differential Crussard–Jaoul analysis characterized the discrepancy between AISI 321 steel and Hadfield steel in work hardening behavior more accurately than the Hollomon analysis. The work hardening of AISI 321 stainless steel resulted mainly from interactions of dislocations. When the true strain was rather low, the work hardening of Hadfield steel also resulted mainly from interactions of dislocations. At high strains, twinning would occur in Hadfield steel. It was the occurrence of twins that led to unusual work hardening at larger strains in Hadfield steel.