Electron back scattered diffraction (EBSD) characterization of warm rolled and accumulative roll bonding (ARB) processed ferrite

An interstitial free (IF) steel was severely deformed using accumulative roll bonding (ARB) process and warm rolling. The maximum equivalent strains for ARB and warm rolling were 4.8 and 4.0, respectively. The microstructure and micro-texture were studied using optical microscopy and scanning electron microscopy equipped with electron back scattered diffraction (EBSD). The grain size and misorientation obtained by both methods are in the same range. The microstructure in the ARB samples after 6 cycles is homogeneous, although a grain size gradient is observed at the layers close to the surface. The through thickness texture gradient in the ARB samples is different from the warm rolled samples. While a shear texture (〈1 1 0〉//rolling plane normal direction (ND)) at the surface and rolling texture at the center region is developed in the ARB samples, the overall texture is weak. The warm rolled samples display a sharp rolling texture through the thickness with increasing the sharpness toward the center. These differences are attributed to the fact that the central region of ARB strip is comprised of material that was once at the surface. The ARB process can suppress the formation of shear bands which are conventional at warm rolled IF steels. EBSD study on the sample with 6th cycle of ARB following the annealing at 750 °C verified a texture gradient through the thickness of the sheet. The shear orientations at the surface and at the quarter thickness layers can be identified even after annealing. The overall weak texture and existence of shear orientations make ARB processed samples unfavorable for sheet metal forming in compare with warm rolled samples.     

累积叠轧技术的研究现状与展望

对采用累积叠轧工艺制备超细晶组织的技术进行详细的综述,介绍了累积叠轧技术的原理、ARB材料的组织与力学性能特征,并对ARB变形过程中的剪切变形、晶粒细化机制和强化机制进行分析。采用ARB技术可以制备大尺寸的超细晶组织材料,其室温抗拉强度通常比粗晶材料的高2～4倍。ARB材料的强化源于晶粒细化、位错强化、在大变形轧制时形成的稳定基面织构、表面的氧化膜以及内生原有夹杂物在强烈塑性变形情况下的破碎与弥散分布。分析了ARB技术的优越性,对其在制备超细晶材料领域的应用进行了展望。     

Role of shear strain in ultragrain refinement by accumulative roll-bonding (ARB) process

The role of shear strain on ultragrain refinement of aluminum during accumulative roll-bonding (ARB) process was studied. The complicated distribution of redundant shear strain through thickness of the ARB processed sheets was quantitatively shown first, and good correspondence between the grain size and the amount of shear strain was found.     

Cyclic deformation of ultrafine-grained aluminum

Cyclic deformation of ultrafine-grained (UFG) Al with different grain sizes has been studied. It was found that UFG Al had shorter fatigue life than its coarse-grained counterparts. For UFG Al, the fatigue life decreases with decreasing grain size. Shear bands (SBs) shorten fatigue life. SBs are always inclined at 45° to the loading axis, and extend across the whole specimen. A SB is a thin sheet of tangled dislocations that have different Burgers vectors; its thickness is much less than the grain size. The strain–stress field inside a SB is very high. SBs produce shear steps, but not surface extrusions/intrusions, on the specimen surface. Thick shear bands (TSBs), about 200–300 μm, were found in the 6.36 μm grain size specimens, which also inclined 45° to the loading axis. TSBs consist of dislocation cells. The formation of TSBs does not reduce the fatigue life.     

Bulk mechanical alloying of Cu–Ag and Cu/Zr two-phase microstructures by accumulative roll-bonding process

Cu–Ag eutectic alloy and multilayered stacked sheets of Cu/Zr were repeatedly cold-rolled by the accumulative roll-bonding (ARB) process as attempts of bulk mechanical alloying. Both ARB-processed two-phase microstructures were deformed into a very fine lamellar structure with a spacing of several nm. The Cu–Ag sheets were deformed by a number of multiple shear bands after two cycles of ARB. Intermixing of Cu and Ag occurred within the fine Cu lamellae and the shear bands. On the other hand, the Cu/Zr sheets were deformed inhomogeneously accompanied by the formation of multiple fracturing and necking of the coarse Zr layers. The Zr layers were fragmented into fine nodules after five or six cycles. The ARB processed specimen with an equivalent strain of 14.1 showed that amorphization starts from the Cu/Zr interfaces. The microstructural changes are discussed from the Gibbs free energy diagrams.     

Observation and modeling of the through-thickness texture gradient in commercial-purity aluminum sheets processed by accumulative roll-bonding

The texture evolution in commercial-purity aluminum (AA1070) processed by accumulative roll-bonding (ARB) is investigated with the aid of X-ray diffraction and crystal plasticity modeling. The experimental results indicate strong texture gradients through the sheet thickness, from rolling-type textures with orthorhombic symmetry at the center to shear-type textures with monoclinic symmetry near the surface. The experimental textures are reproduced well by polycrystal plasticity modeling carried out with deformation histories from finite element simulations. The observations of a relatively strong {4 4 11}〈11 11 8〉 component at the center and a {0 0 1}〈1 1 0〉 component at the surface are attributed to their higher orientation stability than the other rolling- and shear-type orientations. Examination of the average through-thickness textures suggests that the ARB technique may not be an effective means to develop apparent {1 1 1}〈u v w〉 components and thus to enhance the normal anisotropy of plasticity of the bulk sheet materials.     

Deformation behavior of an ultrafine-grained Al–Mg alloy produced by equal-channel angular pressing

Presented here is the deformation behavior of Al–1.5 wt% Mg alloy severely plastically deformed to equivalent pre-strains of 8 and 13 using the equal-channel angular pressing technique. The average subgrain size after severe plastic deformation was 280 and 230 nm respectively. Strain rate change and stress relaxation tests in the range 10⁻⁴–10⁻² s⁻¹ and 298–523 K were performed. The strain rate sensitivity of ultrafine-grained (UFG) Al–1.5Mg was enhanced and the peak strain rate sensitivity shifted to lower temperatures as compared with the coarse-grained (CG) alloy. The increased strain rate sensitivity is a direct consequence of the reduced activation volume. The increase in pre-strain from 8 to 13 has a small effect on both the microstructural refinement and the subsequent deformation behavior. With increasing temperature the UFG material softens compared with the CG material. This demarcation has been clearly shown on a strain rate by temperature plot. Refinement of grain results in an enhanced solute drag regime, primarily due to the decreased activation energy of diffusion.     

High-strain-rate response of ultra-fine-grained copper

The high-strain-rate response of ultra-fine-grained (UFG) copper processed by equal channel angular pressing (ECAP) was characterized by three different dynamic testing methods: reverse Taylor impact, cylindrical compression specimens, and hat-shaped specimens in Hopkinson bar experiments. Upon recovery after impact, the specimens were found to undergo dynamic recrystallization at a calculated temperature of 360 K, indicating that the UFG copper is thermally unstable. Reverse Taylor tests were conducted on as-received oxygen-free high-conductivity copper rod and ECAP specimens with 2 and 8 sequential deformation passes. The dynamic deformation of the samples was modeled using AUTODYN-2D, and a modified Johnson–Cook constitutive equation was found to closely capture the dynamic response. Both the dynamic experiments and analysis from the reverse Taylor tests indicate enhanced strain-rate sensitivity in comparison with conventional polycrystalline copper, in agreement with predictions of reduced activation volume. The shear band thickness, as obtained in forced localization tests, showed a marked decrease, in comparison to conventional polycrystalline copper; this effect is interpreted as due to an accelerated thermal softening and inherent instability exhibited by the UFG structure.     

累积叠轧焊AZ31镁合金微观组织和织构演变的EBSD研究

对AZ31镁合金热轧板在350℃进行了累积叠轧焊(ARB)变形,采用EBSD技术研究了AZ31镁合金的微观组织和织构演变.结果表明,ARB可以显著细化AZ31镁合金的晶粒组织,经过3道次变形后平均晶粒尺寸为2.18μm,后续的ARB变形使AZ31镁合金的微观组织更均匀,但晶粒不会再显著细化,说明存在临界ARB变形道次,使晶粒细化和晶粒长大之间达到动态平衡.AZ31镁合金在ARB变形过程中的晶粒细化机制为连续动态再结晶,尤其还观察到了旋转动态再结晶.动态再结晶的形变储存能来源于多道次累积的剧烈应变和沿厚度方向分布复杂的剪切变形.ARB变形过程中旋转动态再结晶和剪切变形使新晶粒c轴发生旋转,导致基面织构弱化.     

Microstructure and tensile behavior of a friction stir processed magnesium alloy

In this work the effect of multi-pass friction stir processing (FSP) followed by warm pressing on an as-extruded ZK60 Mg plate was investigated. The microstructure, texture and resulting mechanical properties are reported here. Multi-pass FSP to partial depths on the top and bottom plate surfaces produced a novel, layered structure with three distinct microstructural zones associated with stirred, transition and core regions. In the stirred zone, FSP, followed by pressing at 200 °C, created a 0.8 μm ultrafine grain size which accounts for ～55 vol.% of the material. The transition region (～10 vol.%), showed extensively sheared coarse grains distributed in a matrix of finer grains. However, the core region (～35 vol.%) showed extensive twinning inside coarse grains in an overall bimodal microstructure reminiscent of extrusion. The processed Mg with a strong basal texture exhibited high yield strength (>300 MPa) and retention of adequate tensile ductility (>10%). The enhancement in mechanical properties of processed Mg is found to be highly influenced by the layered microstructure: UFG grained stirred zone, finer precipitates and strong basal texture.     

Microstructure–mechanical property relationships in ultrafine-grained NbZr

The present paper reports on the microstructure–mechanical property relationships in an ultrafine-grained (UFG) niobium–1 wt.% zirconium (NbZr) alloy, a potential biomedical material, severe plastically deformed at room temperature utilizing equal channel angular extrusion (ECAE). Monotonic tensile and low-cycle fatigue (LCF) experiments were carried out on the NbZr samples processed along ECAE routes 8B_C and 16E, along with extensive microstructure analysis. The important finding is that the NbZr alloy processed along ECAE routes that lead to a higher volume fraction of high-angle grain boundaries (HAGBs) exhibits a stable cyclic deformation response in the LCF regime. This stands in good agreement with prior studies on other materials, such as UFG interstitial-free steel, in which the stable fatigue behavior was associated with the dominance of HAGBs. The current results provide a venue for utilizing the UFG NbZr alloy in biomedical applications that require a combination of long-term durability, high strength and very good biocompatibility, where the latter is not altered by ECAE processing. Furthermore, for the first time, we present guidelines for optimizing processing parameters that define the microstructure–cyclic stability relationship in UFG alloys.     

Mechanical anisotropy and deep drawing behaviour of AZ31 and ZE10 magnesium alloy sheets

The influence of the initial microstructure on the deep drawability and the associated microstructural evolution in two different magnesium alloy sheets, AZ31 and ZE10, has been examined. Tensile testing at room temperature shows that the AZ31 sheet has high plastic strain ratios, r = 2–3, which are caused by strong basal-type texture. The ZE10 sheet shows lower r values, r ～ 1, as a result of its weak texture. Deep drawing experiments carried out over the temperature range 100–300 °C revealed that the ZE10 sheet can be successfully deep-drawn at lower temperatures than the AZ31 sheet. The ZE10 cups show earing despite the weak texture and low normal anisotropy, while earing of the AZ31 cups is negligible. In the ZE10 cups, deformation is accommodated mainly by 〈a〉 slips and by compression as well as secondary twinning. The occurrence of dynamic recrystallization is observed in successfully deep-drawn AZ31 cups.     

Microstructural evolution during deformation of tin dioxide nanoparticles in a comminution process

Nanoparticles can be produced by wet grinding in stirred media mills if agglomeration is prevented by stabilization of the particles. Since the fracture mechanisms at the lower nanoscale are not yet understood, we studied the evolution of the microstructure within tin dioxide particles. Electrostatic stabilization allows the formation of tin dioxide with a mean particle size of 25 nm as measured by dynamic light scattering. High-resolution transmission electron microscopy (HRTEM) images show particles well below 10 nm and mean crystallite sizes of 9 nm were obtained from X-ray diffraction by applying the Rietveld refinement method. Additionally, TEM and HRTEM analyses were conducted to gain detailed insight into the microstructural effects governing the grinding process. Microscopy revealed surprisingly rich phenomena including the formation of shear bands, twinning and stacking faults that directly affect the grinding behavior. Interestingly the ceramic nanoparticles showed not only fracture patterns expected from brittle fracture but also many traces of plastic deformation. For comparison the uniaxial compression of particles up to 30 nm in diameter was simulated using molecular dynamics. The simulated particles shared microstructural details with the real samples, most importantly the shear bands which lead to significant plastic deformation. The internal microstructure produced during multiple particle stressing events in the mill and also observed in the simulations is directly linked to the fracture mechanism and the experimentally observed grinding limit.     

叠加层数对工业纯铝板材累积叠轧焊合的影响

在室温、无润滑及总压下量相同的条件下,分别采用每道次两层叠轧和每道次三层叠轧的工艺对退火态AA1070工业纯铝板材进行了累积叠轧焊合试验,比较了最终板材界面焊合、织构和力学性能的差别。结果表明,与两层叠轧工艺相比,通过三层叠轧工艺所制备的板材金属层之间焊合状况更好,具有较强的剪切织构和轧制织构,表现出较高的伸长率和较低的抗拉强度,具有较好的综合力学性能。这些差别主要归结于三层叠轧条件下较大的道次压下量和有效剪切变形。     

Mechanical behaviour and in situ observation of shear bands in ultrafine grained Pd and Pd–Ag alloys

An in situ tensile test with grey scale correlation has been performed to study the deformation process in ultrafine grained (UFG) Pd and Pd–x at.% Ag (x = 5 or 20) alloys produced by high-pressure torsion. Shear band nucleation and propagation was found to be an important deformation mechanism after strain localization during the tensile test. The underlying microscopic mechanism is related to cooperative grain boundary sliding. Moreover, the additional influence of stacking fault energy was found to change the nature of the deformation mechanism from localized strain in Pd to more homogeneous deformation in Pd–20% Ag. In situ analysis and the findings are new and give innovative insight into the basics of deformation in UFG face-centred cubic metals.     

Texture,microstructure and mechanical properties of equiaxed ultrafine-grained Zr fabricated by accumulative roll bonding

The texture, microstructure and mechanical behavior of bulk ultrafine-grained (ufg) Zr fabricated by accumulative roll bonding (ARB) is investigated by electron backscatter diffraction, transmission electron microscopy and mechanical testing. A reasonably homogeneous and equiaxed ufg structure, with a large fraction of high angle boundaries (HABs, ∼70%), can be obtained in Zr after only two ARB cycles. The average grain size, counting only HABs (θ > 15°), is 400 nm. (Sub)grain size is equal to 320 nm. The yield stress and UTS values are nearly double those from conventionally processed Zr with only a slight loss of ductility. Optimum processing conditions include large thickness reductions per pass (ε ∼ 75%), which enhance grain refinement, and a rolling temperature (T ∼ 0.3T_m) at which a sufficient number of slip modes are activated, with an absence of significant grain growth. Grain refinement takes place by geometrical thinning and grain subdivision by the formation of geometrically necessary boundaries. The formation of equiaxed grains by geometric dynamic recrystallization is facilitated by enhanced diffusion due to adiabatic heating.     

Evolution of microstructural parameters and flow stresses toward limits in nickel deformed to ultra-high strains

A quantitative analysis of microstructure and strength as a function of strain is presented for polycrystalline nickel (99.5%) deformed by high-pressure torsion in the strain range 1–300 (ε_VM, von Mises strain). Typical lamellar structures consisting of extended boundaries and short interconnecting boundaries have been found, with additional features at large strains which are equiaxed regions, small equiaxed subgrains and deformation twins. The evolution of microstructure and microstructural parameters falls in stages: (i) the first stage at ε_VM = 1–12; (ii) a transition stage at ε_VM = 12–34; and (iii) a saturation stage at ε_VM ⩾ 34. A scaling analysis of spacing between boundaries shows a universal behavior up to ε_VM = 300, indicating that the predominant deformation mechanism is dislocation glide whereas twin formation is of minor importance. A clear link is observed between the evolution in structure and flow stress, which can guide the development of strong metals with a structural scale extending below 50–100 nm.     

Effect of equal-channel angular pressing routes on high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy

The effect of equal-channel angular pressing (ECAP) route on the high-strain-rate deformation behavior of ultra-fine-grained aluminum alloy was investigated. The 8-pass ECAPed specimens deformed via three different routes consisted of ultra-fine grains 0.5 μm in size, and contained a considerable amount of second-phase particles, which were fragmented and distributed in the matrix. In the torsion tests, the maximum shear stress significantly increased with increasing number of ECAP passes, while the maximum shear stress and fracture shear strain were lowest in the specimen deformed via route A among the three 8-pass ECAPed specimens. Observation of the deformed area beneath the fractured surface revealed the adiabatic shear bands of 100 μm in width in the specimen deformed via route A, which minimized the maximum shear stress and fracture shear strain, whereas they were hardly formed in the specimens deformed via route B or C. The formation of adiabatic shear bands was explained in terms of critical shear strain, deformation energy required for void initiation, and microstructural homogeneity related to ECAP routes.     

Processing of AZ31 magnesium alloy by accumulative roll-bonding at gradient temperature

In the present investigation a wrought magnesium alloy AZ31 was successfully processed by the accumulative roll-bonding (ARB) at gradient temperature up to six cycles with the lowest temperature of 250 °C. This is performed through different thermomechanical processing routes (different ARB cycles at different temperatures of 350-200 °C). The microstructures and mechanical properties were investigated. The results indicate that significant grain refinement is observed after the first two cycles at the highest ARB temperature as a result of dynamic recrystallization, which is necessary for the subsequently ARB cycles at relatively lower temperature with the aim to restrict grain growth. No significant finer grain size was observed through the fifth and sixth cycles while the microstructure homogeneity is further improved. The grain structure can be effectively refined at lower ARB processing temperature and higher cycles. The resulting material exhibited high strength and relatively high ductility at ambient temperature when ARB deformed above 250 °C. The mechanical properties of the ARB deformed materials are strongly dependent on several main factors: the amount and the homogeneity of strain achieved, grain size and microstructure homogeneity, textures developed during ARB and interface bonding quality.     

Hot rolling of gamma titanium aluminide foil

Metal flow and microstructure evolution during the thermomechanical processing of thin-gage foil of a near-gamma titanium aluminide alloy, Ti–45.5Al–2Cr–2Nb, with an equiaxed-gamma microstructure was investigated experimentally and theoretically. Foils of thickness of 200–250 μm were fabricated via hot rolling of sheet in a can of proprietary design. The variation in gage of the rolled foils was ±15 μm except in very sporadic (local) areas, with variations of approximately 60 μm relative to the mean. Metallography revealed that the larger thickness variations were associated with large remnant colonies lying in a hard orientation for deformation. To rationalize these observations, a self-consistent model was used to estimate the strain partitioning between the softer (equiaxed-gamma) matrix and the remnant colonies. Furthermore, the efficacy of pre- or post-rolling heat treatment in eliminating remnant colonies was demonstrated and quantified using a static-spheroidization model. The elimination of remnant colonies via spheroidization prior to foil rolling gave rise to improved gage control.