Shear band evolution and hardness change in cold-rolled bulk metallic glasses

A systematic investigation of the shear band evolution and hardness change with deformation was performed on cold-rolled Zr_64.13Cu_15.75Ni_10.12Al₁₀ and Zr_46.5Cu₄₅Al₇Ti_1.5 bulk metallic glasses. It was found that primary shear bands reach saturation after 7% in thickness reduction and only primary shear bands exist below 20% in thickness reduction, based on statistical analyses of primary shear band spacing, angle and offset. Rolling creates more free volume, and deformation-induced residual stress distribution in a heavily rolled specimen relative to the as-cast specimen has been determined. Larger tensile residual stresses are generated on the side surface as compared with those on the top surface, while compressive residual stresses in the middle are induced. Such residual stresses strongly influence the hardness measured. It is also revealed that after stress relief, the hardness does not decrease considerably in heavily rolled/annealed specimens here as compared to as-cast/annealed specimens, probably due to low shear band density.     

Constrained deformation of an Al based amorphous alloy by cold rolling

In this work, we report on the mechanisms of deformation and fracture of an Al₈₇Ni₇La₆ amorphous alloys by cold rolling. Ribbons were cold rolled at room temperature embedded in pure Al-foils (volume fraction 20%). Scanning Electron Microscopy observations of the cross section of the deformed samples revealed that, after a few rolling passes, ribbons broke into fragments and formed shear bands because of the load exerted by the ductile Al matrix starting from the roughness on the surfaces. The shear band offset was higher near the fracture surface, suggesting that a stress concentration occurs when the slip of shear band is hindered by the presence of constraint. In ribbons rolled alone elongation above 3% was achieved as well as flattening of the surface roughness.Fracture surfaces of ribbon fragments presented mainly features due to brittle shear and a limited number of veins and filaments suggesting mixed mode of fracture.No formation of crystals in the shear bands was evidenced by Transmission Electron Microscopy and Differential Scanning Calorimetry.     

Ductility and toughness of cold-rolled metallic glasses

Cold rolling of Al₈₇Ni₇La₆ amorphous ribbons was performed on ribbons embedded in pure Al foils and ribbons alone, in order to study their deformation under constraint. A change in behaviour was observed for the two series of samples due to a different load distribution, so that ribbons alone was deformed up to ɛ = 0.029 without formation of cracks while, for composite samples, small fragments were always formed. In ribbons rolled alone, along with a macroscopic strain, a plastic deformation was observed at a finer scale, in the cavities present on the ribbon surface. The locally induced strain was quantified by taking into account the size change of cavities in samples rolled to different extents. The formation of shear bands and fracture occurring during cold rolling was studied for composite samples: the shear offset, interband spacing and distribution of fracture angles were determined. A Zr₅₅Al₂₀Ni₁₂Cu₈Ti₅ amorphous ribbon was rolled with Al foils and the toughness value was estimated from the shear band offsets finding good agreement with literature data. Therefore, the toughness was estimated also for the Al-based samples obtaining the value of K_c = 28 MPa m^1/2.     

Notch effect on the tensile properties of cold-rolled Ni3Al foils

Notch effect on the tensile properties of the Ni₃Al foils has been investigated as a function of notch geometry and foil’s thickness. Tensile tests along the rolling direction (RD) and the transverse direction (TD) showed that notch weakening occurred with introduce of a notch. Plastic deformation was observed locally at notch root and the effective stress concentration factor (k_e) was much lower than the theoretical stress concentration factor (K_t). Contrary to the anisotropy in fracture stress along the RD and the TD, k_e fell on the same curve. Crack initiation mechanism was different between two tensile directions, that is, cracks initiated along the shear band in the RD tension, while cracks initiated homogenously on the slip planes in the TD tension.     

Failure of Zr61Ti2Cu25Al12 bulk metallic glass under torsional loading

The torsional properties of the Zr₆₁Ti₂Cu₂₅Al₁₂ BMG have been tested using cylinder samples, including the shear yield strength, shear elastic strain limit and shear modulus. Under torsional loading, the BMG fails via a major shear band, without obvious macroscopic plasticity on the specimen surface. The shear band maintained stable propagation by a distance of ∼300 μm (∼20% of cylinder radius) before final catastrophic failure, owing to the constraint of stress gradient along the radial direction. The combined tensile, compressive and torsional properties of the Zr₆₁Ti₂Cu₂₅Al₁₂ BMG suggest that recent ellipse criterion and eccentric ellipse criterion are more appropriate than other well-known ones in describing the yield behavior of this BMG. The cooperative shear model underestimates the shear elastic strain limit, because of its default assumption that the yielding behavior follows the Tresca yield criterion.     

Effect of rolling temperature on microstructure and texture of twin roll cast ZK60 magnesium alloy

Twin roll cast ZK60 alloy strip/sheet with final thickness of 0.5 mm was prepared,and effect of rolling temperature on microstructure and texture development was investigated using OM and XRD technique,microstructure and texture were measured on specimens subjected to rolling experiment at different rolling temperature,and macrotexture was also evaluated by X-ray diffraction method.In addition,the(1 010)and(0002) pole figures were measured,and the tensile test was performed to reveal the influence of rolling temperature on mechanical properties.The results show that the microstructure of ZK60 alloy sheet consisted of fibrous structure with elongated grains,and shear bands along the rolling direction after warm rolling.Dynamic recrystallization could be found during the warm rolling process at rolling temperature 350 °C and above.And many fine recrystallized grain could be observed in the shear bands area.It is a little difficult to see the recrystallized grain in the sheet warm rolled at 300 °C because of higher density of shear bands.The warm rolled ZK60 alloy sheet exhibited strong(0002) pole texture,the intensity of(0002) pole figure decreases with the increasing of rolling temperature and the basal pole tilted slightly to the transverse direction after warm rolling.     

Effect of cold rolling on compressive and tensile mechanical properties of Zr52.5Ti5Cu18Ni14.5Al10 bulk metallic glass

The effect of cold-rolling on structure, thermal stability, and compressive and tensile mechanical properties of the Zr_52.5Ti₅Cu₁₈Ni_14.5Al₁₀ bulk metallic glass has been investigated. The results reveal that cold-rolling does not influence the thermal stability of the material. However, the small plastic deformation induced by cold-rolling is very effective for improving the room temperature plastic strain of the glass: the compressive plastic strain increases from 1.1% for the as-cast material to 2.6% for the cold-rolled sample and the tensile ductility increases from 0% for the as-cast glass to 0.8% for the cold-rolled sample. Hardness measurements indicate that cold rolling creates a heterogeneous microstructure consisting of hard and soft regions. Most likely, the soft regions are preferred locations for shear band formation and propagation, whereas the hard regions may act as obstacles for shear band propagation, effectively limiting shear bands from propagating catastrophically and, consequently, enhancing the room temperature plastic deformation of the glass.     

Effects of hot rolled shear bands on formability and surface ridging of an ultra purified 21%Cr ferritic stainless steel

An ultra purified 21%Cr ferritic stainless steel (FSS) was hot rolled at a low finisher entry temperature (FET) of 750 °C to generate shear bands through large shear flow localizations in grain interiors. The effects of shear bands on the formability and surface ridging have been studied as compared to a conventional hot rolled band at the FET of 970 °C with few shear bands. The results showed that as compared to the final sheet produced with the FET of 970 °C, the average r-value for the final sheet with the FET of 750 °C was increased by 25%, and the surface roughness of the final sheet strained by 15% along the rolling direction was decreased by 40%. It has been observed that the existence of shear bands could enhance the nucleation for recrystallization during hot rolling and annealing for microstructure refinement, and modify the texture by intensifying the {1 1 1} textures in the final sheet. The influence of shear bands on sheet formability and resistance against surface ridging was discussed.     

Significant tensile ductility induced by cold rolling in Cu47.5Zr47.5Al5 bulk metallic glass

Significant tensile plasticity up to 0.7 ± 0.1% together with work-hardening and larger fracture strength was obtained in Cu_47.5Zr_47.5Al₅ bulk metallic glass (BMG) upon cold rolling with only 2.9 ± 0.3% thickness reduction. The good deformability could be attributed to the multiple pre-existing shear bands and structural inhomogeneity induced by rolling. The distributions of shear bands upon rolling can be predicted by a simplified rolling model. The underlying mechanism for the tensile plasticity was further discussed in the frame of potential energy landscape theory (PEL).     

Microstructure and mechanical properties of Mg-Al-Mn-Ca alloy sheet produced by twin roll casting and sequential warm rolling

Microstructural evolution and mechanical properties of twin roll cast (TRC) Mg-3.3 wt.%Al-0.8 wt.%Mn-0.2 wt.%Ca (AM31 + 0.2Ca) alloy strip during warm rolling and subsequent annealing were investigated in this paper. The as-TRC alloy strip shows columnar dendrites in surface and equiaxed dendrites in center regions, as well as finely dispersed primary Al₈Mn₅ particles on interdendritic boundaries which result in the beneficial effect on microstructural refinement of strip casting. The warm rolled sheets show intensively deformed band or shear band structures, as well as finely and homogeneously dispersed Al-Mn particles. No evident dynamic recrystallization (DRX) takes place during warm rolling process, which is more likely attributed to the finely dispersed particle and high solid solution of Al and Mn atoms in α-Mg matrix. After annealing at 350 °C for 1 h, the warm rolled TRC sheets show fine equiaxed grains around 7.8 μm in average size. It has been shown that the present TRC alloy sheet has superior tensile strength and comparative elongation compared to commercial ingot cast (IC) one, suggesting the possibility of the development of wrought magnesium alloy sheets by twin roll strip casting processing. The microstructural evolution during warm rolling and subsequent annealing as well as the resulting tensile properties were analyzed and discussed.     

The Effects of Specimen Geometry on the Plastic Deformation of AA 2219-T8 Aluminum Alloy Under Dynamic Impact Loading

The effects of specimen geometry on shear strain localization in AA 2219-T8 aluminum alloy under dynamic impact loading were investigated. The alloy was machined into cylindrical, cuboidal and conical (frustum) test specimens. Both deformed and transformed adiabatic shear bands developed in the alloy during the impact loading. The critical strain rate for formation of the deformed band was determined to be 2500 s^?1 irrespective of the specimen geometry. The critical strain rate required for formation of transformed band is higher than 3000 s^?1 depending on the specimen geometry. The critical strain rate for formation of transformed bands is lowest (3000 s^?1) in the Ø5 mm × 5 mm cylindrical specimens and highest (> 6000 s^?1) in the conical specimens. The cylindrical specimens showed the greatest tendency to form transformed bands, whereas the conical specimen showed the least tendency. The shape of the shear bands on the impacted plane was also observed to be dependent on the specimen geometry. Whereas the shear bands on the compression plane of the conical specimens formed elongated cycles, two elliptical shaped shear bands facing each other were observed on the cylindrical specimens. Two parallel shear bands were observed on the compression planes of the cuboidal specimens. The dynamic stress–strain curves vary slightly with the specimen geometry. The cuboidal specimens exhibit higher tendency for strain hardening and higher maximum flow stress than the other specimens. The microstructure evolution leading to the formation of transformed bands is also discussed in this paper.     

Size-independent strength and deformation mode in compression of a Pd-based metallic glass

We present quasi-static, room temperature compression data for Pd₄₀Ni₄₀P₂₀ metallic glasses, with specimen sizes ranging from the submicron to several millimeters in diameter. We observe no change in deformation mode over this range. At all sizes, plastic flow is localized in shear bands, which are accompanied by sudden strain bursts. This metallic glass shows only a modest increase in strength in going from bulk to micrometer-sized specimens. We show that stress gradients in tapered specimens can complicate measurement of the yield strength of metallic glasses in microcompression. Estimates of yield strength based on the minimum cross-sectional area implicitly assume that yielding is controlled by a maximum effective shear stress criterion. An alternative is the shear plane yield criterion, in which the minimum shear stress on the shear band trajectory determines yield. Application of this criterion in tapered microspecimens reinforces the notion that metallic glasses possess relatively size-independent mechanical properties.     

Superelastic properties of nanocrystalline NiTi shape memory alloy produced by thermomechanical processing

Effects of thermomechanical treatment of cold rolling followed by annealing on microstructure and superelastic behavior of the Ni₅₀Ti₅₀ shape memory alloy were studied. Several specimens were produced by copper boat vacuum induction melting. The homogenized specimens were hot rolled and annealed at 900 °C. Thereafter, annealed specimens were subjected to cold rolling with different thickness reductions up to 70%. Transmission electron microscopy revealed that the severe cold rolling led to the formation of a mixed microstructure consisting of nanocrystalline and amorphous phases in Ni₅₀Ti₅₀ alloy. After annealing at 400 °C for 1 h, the amorphous phase formed in the cold-rolled specimens was crystallized and a nanocrystalline structure formed. Results showed that with increasing thickness reduction during cold rolling, the recoverable strain of Ni₅₀Ti₅₀ alloy was increased during superelastic experiments such that the 70% cold rolled–annealed specimen exhibited about 12% of recoverable strain. Moreover, with increasing thickness reduction, the critical stress for stress-induced martensitic transformation was increased. It is noteworthy that in the 70% cold rolled–annealed specimen, the damping capacity was measured to be 28 J/cm³ that is significantly higher than that of commercial NiTi alloys.     

Al-4%Cu合金中锯齿形屈服剪切带的变形测量

通过高速CCD（1000 fps）连续记录Al—4％Cu合金材料在拉伸实验中出现的B型锯齿形屈服（PLC）剪切带发生前后一系列非相干光散斑图,结合数字散斑相关法图像分析技术,定量地给出了PLC剪切带及周围区域的位移和应变的空间分布,再现了PLC剪切带随时间演化发展的过程．实验结果清楚地证实了B型PLC剪切带的一种演化机制,即先在试样一侧成核,然后与拉伸轴成一定角度横向贯穿整个试样,最终形成非均匀塑性变形带．实验结果表明,在PLC剪切带形成瞬间变形带外存在弹性收缩变形．     

高纯铝箔在异步轧制和再结晶过程中取向的演变

采用不同速比的异步轧制技术对99.99%的高纯铝板进行不同形变量的冷轧,并对冷轧样品进行不同温度和时间的再结晶退火.利用X射线衍射技术和TEM观测探讨了异步轧制条件下高纯铝箔中变形织构和再结晶织构的演变.结果表明:高速比的异步轧制(i=1.28)在样品中产生相对较强的旋转立方织构{001}(110).异步轧制后退火的高纯铝箔样品中,立方{001}(100)织构组分的再结晶晶核的形成和长大存在一个临界转变温度,此温度与异步轧制的速比成反比.异步轧制有利于降低高纯铝箔的再结晶温度,这与异步轧制提高高纯铝箔的形变储能有关.异步轧制有利于在低温时形成强的立方{001}(100)织构组分,但此时漫散较大;随着退火温度的升高,漫散范围明显缩至8°-10°.     

Zr_(65)Al_(7.5)Ni_(10)Cu_(12.5)Ag_5非晶合金压痕下方剪切带形貌的研究

以Zr65Al7.5Ni10Cu12.5Ag5块体非晶合金作为研究对象,采用界面压痕技术和扫描电子显微镜,分别对铸态、轧制态以及轧制-退火态试样压痕下方的剪切带形貌进行了研究。实验结果表明,铸态试样以半圆形剪切带为主,同时存在少量的射线状剪切带,并且形貌比较规则;轧制态试样的剪切带形貌变得很不规则,很难分辨半圆形和射线状剪切带,并且随着变形量的增加,剪切带形貌的不规则程度增加;轧制-退火态试样中重新出现了规则的半圆形和射线状剪切带,变形量对剪切带形貌几乎没有影响。这些结果说明,轧制态试样压痕下方剪切带形貌不规则的主要原因是轧制变形使剪切带中自由体积含量显著增加。     

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.     

Effect of cold rolling on properties and microstructures of dispersion strengthened copper alloys

Mechanical properties and microstructures of unidirectionally and tandem rolled alumina dispersion strengthened copper（ADSC） alloys under different conditions were investigated by tensile test, optical microscopy（OM）, transmission electron microscopy（TEM） and scanning electron microscopy（SEM）. For unidirectionally rolled ADSC alloys, their strengths and elongations in the longitudinal direction are higher than those in the transverse direction under both cold rolling and annealing conditions. Once fracture appears in their longitudinal stress--strain curves, sudden reduction of overall stress level before complete fracture can be observed in the transverse tensile curves. The anisotropy of mechanical properties for the ADSC alloy can be greatly improved by tandem cold rolling. And no sudden reduction of overall stress level appears in the stress--strain curves for tandem rolled ADSC alloys. The differences of their microstructures and tensile fractures were analyzed. In order to compare the differences of tensile fracture mechanism in different directions, longitudinal and transverse fracture models for unidirectionally rolled ADSC alloys were also introduced.     

Mutual interaction of shear bands in metallic glasses

Shear banding is the main plastic deformation mode in metallic glasses. Even though there are many researches focused on the initiation and propagation of shear bands, the interaction among them has not been systematically studied. Using an atomic force microscope, we investigated the mutual interaction of shear bands at the surface of Cu₅₀Zr₅₀ metallic glass ribbons at the nanoscale. At the sites of the interaction, the propagation direction of one shear band can be changed by the pre-existing one, and the offset is the vector sum of the two bands. Under external stress, one shear band can be decomposed into several tiny bands and more materials could be taken into the deformation zones. Therefore, more energy can be dissipated and the deformation could be more homogeneous for the mutual interaction process. These results are useful for a mechanistic understanding of the evolution and suppression of shear band propagations, as well as the design of metallic glasses with improved plasticity.     

AZ31B镁合金轧制板材室温动态拉伸的SEM观察

利用动态拉伸台和扫描电镜(SEM)对AZ31B镁合金轧制板材进行了动态拉伸的微观形貌观察。试验证明,材料宏观织构和显微组织都会对镁合金轧制板材的室温拉伸性能产生影响;在轧制镁合金拉伸变形时滑移和孪生同时存在;裂纹的产生和扩展具有明显的方向性。轧制组织中大量的变形孪晶与应力方向的位向关系,是轧向和横向上裂纹产生及扩展机制产生较大差异的主要原因。