High-ratio Differential Speed Rolling Research Articles

Effect of Severe Plastic Deformation and Post-Deformation Heat Treatment on the Microstructure and Superelastic Properties of Ti-50.8 at.% Ni Alloy.

Severe plastic deformation via high-ratio differential speed rolling (HRDSR) was applied to the Ni-rich Ti-50.8Ni alloy. Application of HRDSR and a short annealing time of 5 min at 873 K leads to the production of a partially recrystallized microstructure with a small grain size of 5.1 μm. During the aging process for the annealed HRDSR sample at 523 K for 16 h, a high density of Ni3Ti4 particles was uniformly precipitated over the matrix, resulting in the formation of an R phase as the major phase at room temperature. The aged HRDSR sample exhibits excellent superelasticity and superelastic cyclability. This achievement can be attributed to an increase in strength through effective grain refinement and particle strengthening by Ni3Ti4 and a decrease in the critical stress for stress-induced martensite (B19') due to the presence of the R-phase instead of B2 as a major phase at room temperature. The currently proposed method for using HRDSR and post-deformation heat treatment allows for the production of Ni-rich NiTi alloys with excellent superelasticity in sheet form.

Improving Texture and Microstructure of Pure Nb by Cross Rolling Combined with High-Ratio Differential Speed Rolling

Improving Texture and Microstructure of Pure Nb by Cross Rolling Combined with High-Ratio Differential Speed Rolling

Effect of Grain Refinement and Dispersion of Particles and Reinforcements on Mechanical Properties of Metals and Metal Matrix Composites through High-Ratio Differential Speed Rolling

A differential speed rolling (DSR) technique that provides capability of producing large-scale materials with fine grains and controlled texture in a continuous manner has attracted several researchers and industries. In this study, we tried to review the articles related to DSR and especially the high-ratio DSR (HRDSR) technique that is associated with a high speed ratio between the upper and lower rolls (≥2) and compare the change in microstructure and mechanical properties after HRDSR with the results obtained by using other severe plastic deformation (SPD) techniques to see the potential of the HRDSR technique in enhancing the mechanical properties of metals and metal matrix composites. The reviewed results show that HRDSR is an important technique that can effectively refine the grains to micro or nano sizes and uniformly disperse the particles or reinforcement throughout the matrix, which helps extensively in improving ambient and superplastic mechanical properties of various metals and alloys.

Microstructure tailoring of Al0.5CoCrFeMnNi to achieve high strength and high uniform strain using severe plastic deformation and an annealing treatment

Ultrafine-grained alloys fabricated by severe plastic deformation (SPD) have high strength but often poor uniform ductility. SPD via high-ratio differential speed rolling (HRDSR) followed by an annealing treatment was applied to Al0.5CoCrFeMnNi to design the microstructure from which both high strength and high uniform strain can be achieved. The optimized microstructure was composed of an ultrafine-grained FCC matrix (1.7–2 μm) with a high fraction of high-angle grain boundaries (61 %–66 %) and ultrafine BCC particles (with a size of 0.6–1 μm and a volume fraction of 8 %–9.3 %) distributed uniformly at the grain boundaries of the FCC matrix. In the severely plastically deformed microstructure, the nucleation kinetics of the BCC phase was accelerated. Continuous static recrystallization (CSRX) took place during the annealing process at 1273 K. Precipitation of the BCC phase particles occurring concurrently with CSRX effectively retarded the grain growth of the FCC grains . The precipitation of the hard and brittle σ phase was, however, suppressed. The annealed sample processed by HRDSR with the optimized microstructure exhibited a high tensile strength of over 1 GPa with a good uniform elongation of 14 %–20 %. These tensile properties are comparable to those of transformation-induced plasticity steel. Strengthening mechanisms of the severely plastically deformed alloy before and after annealing were identified, and each strengthening mechanism contribution was estimated. The calculated results matched well with the experimental results.

Achievement of nearly fully amorphous structure from NiTi alloys via differential speed rolling at 268 K and effect of annealing on superelasticity

High-ratio differential speed rolling (HRDSR) technique was applied to a Ni-rich NiTi alloy at a subzero temperature of 268 K and a nearly fully amorphous microstructure was obtained after a single HRDSR pass with a thickness reduction of 50%. This result was attributed to the severe plastic deformation under low roll pressure at low temperatures where high-density of dislocations and vacancies were rapidly generated while dynamic recovery was suppressed. Subsequent annealing at 673 K for 2 min resulted in the formation of homogeneous nanograins with sizes of 20–30 nm. With increasing annealing time, grain size continued to increase and the volume fraction of remaining amorphous phase continued to decrease. Compared to that for the sample with the ultrafine grains of 100–200 nm, the alloy with the nanograins of 20–30 nm exhibited the superelastic strain larger by a factor of 35 (3.6% vs. 0.1%), indicating that the nanograin refinement through amorphization and subsequent annealing is an effective method of improving superelasticity of NiTi alloys. The homogeneous refinement to nanosized grains contributes to enhancement of superelasticity by considerably increasing the resistance of austenite matrix to plastic deformation. The presence of some amount of amorphous phase that remained after heat treatment may also contribute to superelasticity through the load transfer effect or/and particle strengthening effect. The currently proposed technique provides a solution for the easy and continuous production of (nearly) amorphous NiTi alloys in sheet form.

Microstructure and tensile properties of magnesium nanocomposites fabricated using magnesium chips and carbon black

In this study, carbon black (0, 0.01, 0.03 and 0.08 wt%) and AZ31 (Mg–3Al–1Zn) magnesium chips were used to fabricate carbon black-reinforced magnesium matrix composites with extrusion or a combination of extrusion and high-ratio differential speed rolling. After hot pressing at 693 K and extrusion at 623 K with an extrusion ratio of 22, the magnesium chips coated with carbon black were soundly bonded into a bulk composite material. The grain sizes of the extruded materials were similar with a size of 48.2–51.5 µm despite the difference in the amount of carbon black. The yield strength and ultimate tensile strength increased from 177 to 191 MPa and from 240 to 265 MPa, respectively, as a result of the addition of 0.01% carbon black; however, a further increase in the strength was marginal with additional carbon black. The same trend was observed in the strain hardening behavior. The tensile elongation increased by to the addition of 0.01% carbon black (from 15.8% to 17.4%) due to the increased work hardening effect, but decreased with additional carbon black due to its agglomeration and poor dispersion at higher concentration. After high-ratio differential speed rolling (HRDSR) on the extruded materials and subsequent annealing, the AZ31 and AZ31 composites had a similar fine grain size of 16.3–17.9 µm. The annealed HRDSR composites showed the best mechanical properties at a higher content of carbon black (0.03%) compared to that (0.01%) for the extruded composites. This resulted from the enhanced dispersion effect of the carbon black due to the high shear flow induced during the HRDSR process. The extruded composites exhibited the three distinct hardening stages (stage II, stage III and stage IV), while the annealed HRDSR composites mainly displayed the stage III hardening. The addition of carbon black increased the strain hardening rate at all the strain hardening stages in both of the extruded and annealed HRDSR materials. At the initial hardening stage, the strain hardening rates of the extruded composites were higher than those of the annealed HRDSR composites, but this became reversed at the later stage of hardening. Possible explanations for this observation were discussed. The strength analysis suggests that dislocation-carbon black interaction by Orowan strengthening and dislocation generation due to a difference in thermal expansion between matrix and carbon black are the major strengthening mechanisms.

Successful transition from low-temperature superplasticity to high-strain-rate superplasticity with increasing temperature in an ultrafine-grained Mg–Y–Zn–Zr alloy

Simultaneous achievement of low-temperature superplasticity (LTSP) below 0.5Tm (Tm: melting temperature) and high-strain-rate superplasticity (HSRS) above 0.5Tm from the identically processed material is advantageous for industrial applications. Severe plastic deformation by high-ratio differential speed rolling (HRDSR) with different roll speed ratios (2 and 3) was applied to cast Mg–Y–Zn–Zr alloys with the eutectic icosahedral phase (I-phase) structure. Two types of microstructures were obtained, depending on the roll speed ratio. The alloy processed at a speed ratio of 3 (HRDSR3) had an ultrafine-grained microstructure (∼1 μm) and finely broken eutectic I-phase particles. The alloy processed at a speed ratio of 2 (HRDSR2) also had an ultrafine grained microstructure, but with a higher fraction of non-fully fragmented original grains and a lower density of fine I-phase particles (< 1 μm in size). In both alloys, many I-phase particles were agglomerated. Below 523 K, both materials had good LTSP but HRDSR3 showed a better LTSP than HRDSR2. This was because HRDSR3 had a higher fraction of ultrafine grained area participating in grain boundary sliding and a higher density of fine I-phase particles contributing to the pinning effect on grain growth. Above 573 K, however, HRDSR2 showed HSRS but HRDSR3 failed to show HSRS. This resulted because thermal stability of ultrafine grained structure of HRDSR3 became poorer than that of HRDSR2 as the temperature increased above 0.5Tm. When solute solubility increased with temperature, the density of the small I-phase particles (<1 μm) exerting the effective pinning effect on grain boundary migration decreased at a more rapid rate in HRDSR3 than in HRDSR2 during the sample heating and holding duration. This was because a more significant reduction in particle size by higher shear resulted in a higher rate of dissolution of particles into matrix upon temperature increase. Retension of fine grains during the sample heating and holding duration was important in achieving HSRS because it allowed for active occurrence of grain boundardy sliding during deformation. Grain boundary sliding promoted progressive dispersion of agglomerated I-phase particles into the matrix, which was equally important in achieving HSRS because as the I-phase particles were more uniformly dispersed during deformation, the pinning effect of the particles increased, leading to a more effective suppression of dynamic grain growth of the fine grains. The obtained results suggest the importance of control on the size of I-phase particles and grains in achieving LTSP and HSRS from the identically processed Mg alloys in the low and high temperature ranges, respectively. The deformation behaviors of HRDSR2 and HRDSR3 as well as the condition for the successful transition from LSPT to HSRS with increasing temperature could be explained by using the model that assumes that the two deformation mechanisms of grain-size sensitive grain boundary sliding and grain-size insensitive dislocation climb creep compete each other during plastic deformation.

Microstructural Evolution and Electrochemical Properties of HRDSR AZ61-X (X = Ca, Ti) Alloys.

The microstructure and corrosion properties of as-cast AZ61 (Mg-6%Al-1%Zn) and AZ61 alloys doped with titanium and calcium and subjected to high ratio differential speed rolling were investigated. Addition of the alloying elements to the AZ61 alloy resulted in remarkable modification of the morphology and the amount of continuous β (Mg17Al12)-phase. Addition of Ti to the as-cast AZ61 alloy causes a decrease in the volume fraction (or discontinuity of the β-phase), leading to strong anodic dissolution. In contrast, addition of Ca to the as-cast AZ61 alloy is rather effective for preventing pitting corrosion. This is attributed to the formation of a semi-continuous network β-structure. The (Mg, Al)4Ca phases dispersed between the β (Mg17Al12)-phases led to continuity in the AZ61 alloy with Ca. The AZ61 and AZ61-X(Ca, Ti) alloys subjected to severe plastic deformation via high-ratio differential speed rolling possessed a nano-composite-like microstructure in which the α-Mg matrix with an ultra-fine grain was surrounded by a large number of fine β particles. These particles were either dynamically precipitated or broken at the grain boundaries, as well as in the grain interiors, by the high ratio differential speed rolling process. The corrosion resistance of the AZ61 and AZ61-X (X = Ca, Ti) alloys subjected to high ratio differential speed rolling was largely improved by the microstructural modification. The high ratio differential speed rolling process greatly influenced the texture of the Mg alloys, which significantly affected their corrosion behavior.

Shape memory and superelasticity of nanograined Ti-51.2 at.% Ni alloy processed by severe plastic deformation via high-ratio differential speed rolling

A novel method of producing nanograined Ni-rich superelastic NiTi alloys in sheet form was proposed using a combination of severe plastic deformation via high-ratio differential speed rolling (HRDSR) and post-deformation annealing. The HRDSR-processed microstructure was composed of heavily deformed austenite and martensite grains, and amorphous phases. After annealing at 673 K, the severely deformed microstructure with no functional properties evolved to the nanograined structure (20–70 nm) composed of austenite and martensite nanograins and sub-nanograins through static recovery or continuous static recrystallization process. The nanograined microstructure had a high resistance to martensitic transformation upon cooling and slip deformation during straining. As a result, the HRDSR-processed alloy annealed at 673K exhibited superior superelasticity compared to the alloys with coarse grains. At the higher annealing temperature of 873 K, the micron-sized recrystallized grains with low dislocations developed through discontinuous static recrystallization process. In this case, deformation during straining was governed by the detwinning of twinned martensite, and as a result, shape memory effect was more significantly pronounced than superelasticity.

Microstructures and mechanical properties of the non-equiatomic FeMnNiCoCr high entropy alloy processed by differential speed rolling

High-ratio differential speed rolling was applied to a non-equiatomic Fe41Mn25Ni24Co8Cr2 high entropy alloy. After subsequent annealing, the processed alloy exhibited a superior combination of high strength and high uniform elongation compared with the one processed by conventional rolling, which was achieved through the development of a bimodal grain size distribution, composed of recrystallized fine grains and unrecrystallized nano (sub)grains.

Effect of Post-annealing and Strong Deformation Process on the Mechanical and Corrosion Properties of a Mg-Mn alloy for Biomedical Application

In this study, the influence of an annealing treatment and strong deformation process on the mechanical and corrosion properties of a Mg alloy for biomedical applications was investigated. The mechanical and corrosion properties in a Mg-1.6wt% Mn (M1) alloy as functions of the twin density and grain refinement were compared. The value of the corrosion current (Icorr) depended linearly on the area fraction of the twin in a coarse and twinned microstructure. The grain refinement via high-ratio differential speed rolling (HRDSR) processing was rather effective for obtaining remarkable increase in hardness and an improvement in the corrosion characteristic compared to a reduction of the twin density in as cast M1 alloy.

Characterization of the microstructures and the shape memory properties of the Fe-Mn-Si-Cr-Ni-C shape memory alloy after severe plastic deformation by differential speed rolling and subsequent annealing

The Fe-13.51Mn-4.82Si-8.32Cr-3.49Ni-0.15C shape memory alloy was subjected to severe plastic deformation by high-ratio differential speed rolling and subsequent annealing (between 873 and 1373K) to find the factors that affect recovery stress and recovery strain. As the annealing temperature increased (i. e., as grain size increased), the recovery strain increased. This was attributed to decrease in the density of grain boundaries and the density of twins with increasing grain size. The recovery stress, however, showed a different behavior. It increased with annealing temperature up to 973K, at which an ultrafine grain size near 1.6μm was obtained, and then decreased with further increase in the temperature. The difference between the recovery strain and the plastic strain induced by recovery stress under constrained displacement during cooling was proposed to determine the amount of recovery stress. There was an optimum grain size for the largest difference between the recovery strain and the plastic strain. The current study shows that grain-size reduction is not good for having a high recovery strain but it is beneficial for having a high recovery stress in the Fe-Mn-Si shape memory alloys.

Superplastic behavior of an ultrafine-grained Mg-13Zn-1.55Y alloy with a high volume fraction of icosahedral phases prepared by high-ratio differential speed rolling

An ultrafine-grained (UFG) Mg-13Zn-1.55Y alloy (ZW132) with a high volume fraction (7.4%) of icosahedral phase (I-phase, Mg3Zn6Y) particles was prepared by applying high-ratio differential speed rolling (HRDSR) on the cast microstructure following homogenization. The alloy exhibited excellent superplasticity at low temperatures (tensile elongations of 455% and 1021% 473K- 10−3s−1 and 523K- 10−3s−1, respectively). Compared with UFG Mg-9.25Zn-1.66Y alloy (ZW92) with a lower volume fraction of I-phase particles (4.1%), which was prepared using the same processing routes, the UFG ZW132 alloy exhibited a higher thermal stability of grain size. Rapid grain coarsening, however, occurred at temperatures beyond 523K, leading to a loss of superplasticity. The high-temperature deformation behavior of the HRDSR-processed ZW132 alloy could be well described assuming that the mechanisms of grain boundary sliding and dislocation climb creep competed with each other and considering that the grain-size was largely increased by accelerated grain growth at the temperatures beyond 523K.

Characteristics and interrelation of recovery stress and recovery strain of an ultrafine-grained Ni-50.2Ti alloy processed by high-ratio differential speed rolling

The characteristics of the recovery stress and strain of an ultrafine-grained Ni-50.2 at% Ti alloy prepared by high-ratio differential speed rolling (HRDSR) were examined, and the factors that influence the recovery stress and strain and the relation between the two were studied. After HRDSR, both the recovery stress and strain were enhanced compared to the initial condition. The subsequent annealing treatment at 673 K, however, reduced the shape recovery properties. The constitutive equation showing that the maximum recovery stress is a sole function of the recovery strain was developed. The recovery strain increased as the yield stress increased. Thus, the maximum recovery stress increased with an increase in yield stress. The recovery stress measured at room temperature (i.e., residual recovery stress) was, on the other hand, affected by the yield stress as well as the austenite-to-martensite transformation temperature. As the yield stress increased and as the martensitic transformation temperature decreased, the residual recovery stress increased.

The effect of volume fraction and dispersion of icosahedral phase particles on the strength and work hardening of Mg-Zn-Y alloys

The cast microstructure of a Mg-13Zn-1.55Y alloy (ZW132) with a high volume fraction of I-phase (7.4%) was refined considerably by severe plastic deformation via high-ratio differential speed rolling (HRDSR). Ultrafine grains (0.7–1.3µm) with high angle boundary fractions of 0.48–0.50 were obtained after HRDSR with speed ratios of 2 or 3. The alloy processed at a speed ratio of 3 exhibited high strength and high ductility, with a yield stress of 332MPa and a tensile elongation of 16.3%. The ductility of the rolled ZW alloy was controlled by the work hardening rate, which increased as the amount of I-phase, the degree of refinement of the eutectic I-phase pockets, the degree of dispersion of the broken I-phase particles over the matrix, and the size of the resultant grains increased. A model that considers the contribution of these factors to the work hardening rate was proposed. Grain-size reduction was found to be a major strengthening mechanism. Dispersion of the I-phase particles broken from the eutectic I-phase pockets did not contribute significantly to the material's strength.

Finite Element Simulation of Shear Strain during High-Ratio Differential Speed Rolling of Aluminum Alloy 5083

High-ratio differential speed rolling (HRDSR) is a process of severe plastic deformation (SPD) that can be used to improve the structure and properties of aluminum alloys. The mechanism of SPD during HRDSR comes from its large equivalent strain, which is composed of compressive strain and additional shear strain. Plastic strain control of aluminum alloys are of importance for improvement of sheet microstructure and properties. This paper presents the results of the finite element simulation of shear strain during high-ratio differential speed rolling of aluminum alloy 5083. Four deformation routes UD, TD, RD and ND were simulated. By the route UD the sheet was not rotated between two deformation steps, while by the other three cases it was rotated with 180° degrees. By the route RD the rotation axis was the rolling direction, by the route TD the transverse direction and by the route ND the normal direction. The effect of rolls velocity ratio, friction coefficient and deformation route on the shear strain and the effective strain of Al 5083 was found. The results of investigation can be used to optimize the high-ratio differential speed rolling process to improve microstructure and mechanical properties of aluminum sheets.

Microstructure and superplasticity of the as-cast Mg–9Al–1Zn magnesium alloy after high-ratio differential speed rolling

The direct application of high-ratio differential speed rolling (HRDSR) on the cast microstructure of Mg–9Al–1Zn (AZ91) alloy resulted in significant refinement of grain size and divorced eutectic β-phase with semi-network structure. Nanosized β-phase particles (50–100nm) precipitated at grain boundaries and within grains from the Al-rich α-phase, which formed as a result of fragmentation and decomposition of the divorced eutectic β-phase during severe plastic deformation by HRDSR. The HRDSR-processed AZ91 alloy exhibited excellent superplasticity at low temperatures below 573K. The optimum superplasticity (with a maximum tensile elongation of 830%) was achieved at 573K at 1 × 10−3s−1. Beyond 573K, serious deterioration in superplasticity occurred due to a loss of thermal stability of grain size caused by the dissolution of the fine β-phase particles into the matrix. Models that can explain the high-temperature flow behavior of the ultrafine grained AZ91 alloy were proposed.

Enhancement of recovery stresses of the Ni-50.2Ti alloy by severe plastic deformation using a high-ratio differential speed rolling technique

The Ni-50.2Ti alloy was severely plastically deformed by differential speed rolling with high speed ratios in range of 2–4.2. The maximum recovery and residual recovery stresses, which are important for applications in civil engineering, were improved after rolling. This result was attributed to the significant increase in the strength of the martensite phase and the large decrease in martensitic transformation finish temperature (Mf) by microstructural refinement. Post-deformation annealing at 673K deteriorated the shape memory properties, which was attributed to the formation of a martensitic R-phase in the alloy and an increase in the Mf temperature.

Modeling of the roll wear and material damage during high-ratio differential speed rolling of aluminium alloy 7075

The technology of high-ratio differential speed rolling (HRDSR) with high contact friction can be used for production of aluminium alloys with UFG structure. But a high friction coefficient is not desirable because of the increasing of the material damage. The paper presents a numerical analysis of the rolls wear and material damage during HRDSR of aluminium alloy 7075. The FEM was used to calculate the rolls wear by Archard’s model and material damage by normalized Cockroft-Latham model.

Stress corrosion cracking of high-strength AZ31 processed by high-ratio differential speed rolling

Stress corrosion cracking (SCC) in distilled water was studied for AZ31, processed by differential-speed-rolling to different strengths, using Linear Increasing Stress Tests (LISTs). The stress corrosion crack velocity was 5.0 +/- 2.5 x 10(-9) m s(-1), independent of applied stress rate and independent of material strength. SCC susceptibility was greater at lower applied stress rates manifest most importantly as a lower threshold stress for stress corrosion crack initiation. SCC susceptibility could be characterised by the ratio of threshold stress to yield stress, which was dependent on processing details and was as low as 0.3. (C) 2015 Production and hosting by Elsevier B.V. on behalf of Chongqing University.