Rate Dependence of Serrated Flow and Its Effect on Shear Stability of Bulk Metallic Glasses

The rate dependence of serrated flow and its effects on the stability of shear banding were systematically investigated in a prototypic bulk metallic glass.It was found that with the increase of external strain rate,the serra-ted flow is gradually suppressed and could completely disappear at a critical strain rate.The serration size,character-ized by the mean stress drop amplitude,decreases inversely with the strain rate,while the waiting time for serration decreases with the strain rate in a power-law manner.The rate dependence of the serrated flow has important effects on the dynamics and stability of shear banding process,and leads to an optimal plasticity achieved around the critical strain rate for the disappearance of serrated flow.These results are discussed and interpreted in terms of the mi-croscopic deformation theory and the stick-slip dynamics of shear banding for bulk metallic glasses.     

Direct Observation on the Evolution of Shear Banding and Buckling in Tungsten Fiber Reinforced Zr-Based Bulk Metallic Glass Composite

The evolution of micro-damage and deformation of each phase in the composite plays a pivotal role in the clarification of deformation mechanism of composite. However, limited model and mechanical experiments were conducted to reveal the evolution of the deformation of the two phases in the tungsten fiber reinforced Zr-based bulk metallic glass composite. In this study, quasi-static compressive tests were performed on this composite. For the first time, the evolution of micro-damage and deformation of the two phases in this composite, i.e., shear banding of the metallic glass matrix and buckling deformation of the tungsten fiber, were investigated systematically by controlling the loading process at different degrees of deformation. It is found that under uniaxial compression, buckling of the tungsten fiber occurs first, while the metallic glass matrix deforms homogeneously. Upon further loading, shear bands initiate from the fiber/matrix interface and propagate in the metallic glass matrix. Finally, the composite fractures in a mixed mode, with splitting in the tungsten fiber, along with shear fracture in the metallic glass matrix. Through the analysis on the stress state in the composite and resistance to shear banding of the two phases during compressive deformation, the possible deformation mechanism of the composite is unveiled. The deformation map of the composite, which covers from elastic deformation to final fracture, is obtained as well.     

Effect of pre-existing shear bands on mechanical properties and serration behaviors in bulk metallic glasses

Pre-existing (multiple) shear bands were introduced into the ductile Zr56 Co28 Al16 and Zr65 Ni10 Cu15 Al10 bulk metallic glasses (BMGs) through the lateral-deformation, respectively.It was found that the pre-exi-ting shear bands can further enhance the compressive plasticity of ductile BMGs.According to the serra-tion analysis on the plastic deformation of the as-cast and the pre-deformed samples, the serration events in the stress-strain curves during deformation display a self-organized critical (SOC) behavior.Compared with the as-cast BMGs, a larger power-law scaling exponent calculated based on serrated flow behaviors becomes larger for the pre-deformed BMGs, implying that the shear banding stability of BMGs is effec-tively enhanced.This should be caused by the pronounced interactions of shear bands during plastic de-formation for the pre-deformed BMGs.However, by introducing a large amount of multiple shear bands into the glassy matrix, it also becomes easier for shear bands to propagate along the pre-existing shear bands, leading to a lower cut-off elastic energy density for the pre-deformed BMGs.More multiple shear bands with stronger interactions for the pre-deformed BMGs could provide a larger chance to activate the shear-band cracking but less local elastic energies are remained for the subsequent crack-linking.     

Salient shear bands and second-phase addition interactions of bulk metallic glass matrix composite

This article discusses Charpy impact testing and fracture morphology of the Zr_41.25Ti_13.75Cu_12.5Ni₁₀Be_22.5 bulk metallic glass matrix composite with long tungsten fibers. Energy to failure was measured via the impact test as well as by integrating the compressive stress-strain curves, and compared for various fiber fractions. Failure energy increased with fiber volume fraction by both measures. Observation of fracture surfaces was made by using scanning electron microscopy. The results show that the fracture surface of the unreinforced bulk metallic glass (BMG) exhibits three different regions, i.e., the impact zone, the transition zone, and the ridged zone, which have different morphology. The composites present uneven or jagged morphology on macroscopic scale, while the microstructure exhibits salient shear bands and second-phase addition interactions. Bridge formation between tungsten fibers is interpreted as evidence that the shear band propagation in the matrix is suppressed by the fibers. Furthermore, shear lips were observed for the composites containing over 50 pct, fiber volume fraction, showing a great improvement in toughness.     

Serration Behavior in Zr-Cu-Al Glass-forming Systems

The metallic glass matrix composites(MGMCs)and bulk metallic glasses(BMGs)were studied by statistical analysis during plastic deformation at the strain rates of 2×10~(-2),2×10~(-3),and 2×10~(-4) s~(-1),respectively.No serration events occur in both MGMCs and BMGs during compression tests at the strain rate of 2×10~(-2) s~(-1).When deformed at the strain rate of 2×10~(-3) s~(-1),the BMG displays a larger plasticity,which is due to the larger serration events followed by a series of small serrations caused by the continuous movement of free volume.The amplitudes and elastic-energy densities increase with increasing the strain rates owing to many serrations in MGMCs.It is deduced that the Young′s modulus decreases from the normalized stress drop and fluctuations are observed on stressstrain curves,which is attributed to a lower coefficient according to the stick-slip model.     

Critical Shear Offset of Fracture in a Zr-based Metallic Glass

The nanoscale shear band operation process of Zr_(55)Pd_(10)Cu_(20)Ni_5Al_(10) metallic glass(MG)was reined in by constant force during well-designed loading-holding-unloading cyclic microcompression test.Through the test,it is revealed that the whole shear banding process involves three stages:shear band initiation,shear sliding and shear band arrest.Based on the energy balance principle,the size-affected speed of shear sliding is interpreted.The energy originated from the shear sliding leads to heat-up of the shear plane;therefore,the temperature in shear band increases with the size of shear offset caused by the energy accumulation during shear sliding.Taking the glass transition temperature as the critical temperature of fracture for the Zr-based MG,the critical shear offset is predicted to be approximately 190μm,fully in line with the experimental observation.This directly proved that the fracture of the MG is caused by the temperature rise during shear sliding.     

Plasticity improvement of a Zr-based bulk metallic glass by micro-arc oxidation

Mciro-arc oxidation (MAO) was used to coat porous films on the surface of a Zr-based bulk metallic glass sample.The compressive test results indicated that, compared with the as-cast sample, the MAO treated one exhibited higher deformation capacity, associated with multiple shear bands with higher density on the side surface and well-developed vein patterns with smaller size on the fractured surface.The pore in the MAOed film and the matrix/coating interface initiated the shear bands and impeded the rapid propagation of shear bands, thus favoring the enhanced plasticity of the MAO treated sample.The obtained results demonstrated that MAO can be considered as an effective method to finely tune the mechanical perform-ance of monolithic bulk metallic glasses.     

Stored Mechanical Work in Inhomogeneous Deformation Processes of a Pd-Based Bulk Metallic Glass

We investigated the effect of anelastic rearrangements in a Pd-based metallic glass during inhomogeneous plastic deformation producing shear bands at room temperature. Therefore, we subjected bulk metallic glasses to uniaxial stresses and characterized the influence of deformation on the global configurational energy state with ultrasonic and calorimetric methods. The results provide evidence that even in an inhomogeneous plastic deformation process at room temperature, a certain amount of energy can be stored which is thermally relaxed below the glass transition temperature.     

连续钨丝增强锆基块体金属玻璃变形过程中剪切带演化

研究体积分数为60%的连续钨丝/Zr41.2Ti13.8Cu12.5Ni10Be22.5块体金属玻璃复合材料准静态压缩变形过程中剪切带的演化过程。发现在弹性变形段不形成剪切带,剪切带是在塑性变形过程中产生并发展的,且剪切带的数量随着变形量的增加而增大,间距随着变形量的增加而减小;当间距减小到一定值时产生剪切裂纹并不断扩展,最终导致断裂破坏。     

放电等离子烧结制备铁基大块非晶材料

以惰性气体雾化的非晶铁基粉末为原料,采用放电等离子烧结技术(SPS)制备大块非晶材料.探索了SPS烧结温度对烧结体的物相、相对密度、微观结构和性能的影响.试验表明,采用优化的SPS烧结工艺,用粉末冶金的方法,可以获得致密的大块非晶材料,部分性能与铸态相当.     

Mechanical behavior of a bulk nanostructured iron alloy

Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 μm. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Combinatorial Influence of Bimodal Size of B2 TiCu Compounds on Plasticity of Ti-Cu-Ni-Zr-Sn-Si Bulk Metallic Glass Composites

We report on the formation of Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites containing bimodal size of B2 TiCu compounds. The small B2 TiCu compound with a size of 1 to 10 μm has a strong influence on the oscillation of the shear stress, thus causing wavy propagation of the shear bands. In contrast, the large B2 TiCu compound with a size of 70 to 150 μm dissipates the shear stress by branching and multiplication of the shear bands. By forming the bimodal size of B2 TiCu compound, it is possible to determine the harmonic influence to further enhance the plasticity of the Ti-Cu-Ni-Zr-Sn-Si bulk metallic glass composites.     

Effect of yttrium addition on flow behavior of Cu-Zr-Al bulk metallic glass in the supercooled liquid region

The high temperature deformation behaviors and thermal workability of Cu_(43)Zr_(48)Al_9 and(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glasses in the supercooled liquid region were investigated by the uniaxial compression tests. The results showed that the high temperature deformation behaviors were highly sensitive to strain rate and temperature, and the flow stress decreased with the increase of temperature, as well as with the decrease of strain rate. Additionally, the(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glass displayed smaller flow stress under the same condition. The flow behavior changed from Newtonian to non-Newtonian with increase of the strain rate, as well as the decrease of temperature, which could be explained by the transition state theory. We found that(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glass had better flow behavior than the Cu_(43)Zr_(48)Al_9 bulk metallic glass in the supercooled liquid region. In addition, the processing maps of the two bulk metallic glasses were constructed considering the power dissipation efficiency. The optimum domain for thermal workability of the bulk metallic glass was located using the processing map, where the power dissipation efficiency was larger than 0.8. It was shown that the(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glass, which had larger area of optimum domain, had excellent thermoplastic forming.     

大块锆基非晶合金电化学耐腐蚀行为的试验研究

利用电化学极化曲线方法研究了Zr41Ti14Cu12．5Ni5Be22．5Fe5非晶合金及成分相同的晶化合金及纯Zr在硫酸溶液中的腐蚀行为。利用减重法研究了Zr41Ti14Cu12．5Ni5Be22．5Fe5非晶合金及成份相同的晶化合金在浓硫酸溶液中的腐蚀行为。极化曲线测试结果表明，非晶合金及成分相同的晶化合金与纯金属Zr有很大差异：非晶合金过钝化电位最高，钝化区最长，而纯金属Zr过钝化电位最低；钝化区也最短。减重试验表明，非晶合金的腐蚀速率是成分相同晶化合金的1/4。以上结果表明非晶合金拥有优良的耐腐蚀性能。     

Influence of the Crystalline Phase on Strain-Rate Sensitivity of a Zr-Cu-Ni-Al Bulk Metallic Glass at Room Temperature

In this work, Zr₅₃Cu_18.7Ni₁₂Al_16.3 alloy has been cast into rod samples with different diameters. Glassy composites with various volume fractions of quenched-in crystalline are obtained. Their mechanical behaviors and fracture mechanisms have been investigated upon both quasistatic and dynamic loading. As the volume fraction of crystalline phase increases, the increase in the strain-rate sensitivity exponent could be attributed to the combination of the reduction of the shear band-related deformability and the enhancement of the dislocation-related deformability. These results may shed more insight on optimizing the microstructure and performance of bulk metallic glass composites in the future.     

Stress-State Effects on the Fracture of a Zr-Ti-Ni-Cu-Be Bulk Amorphous Alloy

The effects of wide changes in stress state on the flow and fracture behavior of a single Zr-Ti-Ni-Cu-Be bulk metallic glass (BMG) of known chemistry are summarized. Both compression and tension samples were tested with superimposed hydrostatic pressures up to 1230 MPa in addition to notched tension samples tested at 0.1 MPa as well as with 490 MPa superimposed pressure. A wider range of stress states were obtained by including compression experiments conducted in pressure/shear conditions with normal stresses up to 8.7 GPa. The critical shear stress at fracture in the present BMG chemistry and sample dimensions tested is relatively unaffected by these significant changes in stress state, again indicating the normal stress/pressure dependence of flow in this particular BMG chemistry and processing conditions is vanishingly small. The present results are compared to those obtained on other metallic glasses tested under similar conditions and those that exhibit inclusion-initiated failure that demonstrate a much different dependence for similar changes in stress state.     

Elastic deformation behavior of CuZrAlNb metallic glass matrix composites with different crystallization degrees

The room temperature brittleness has been a long standing problem in bulk metallic glasses realm.This has seriously limited the application potential of metallic glasses and their composites.The elastic deform-ation behaviors of metallic glass matrix composites are closely related to their plastic deformation states. The elastic deformation behaviors of Cu48-xZr48Al4Nbx(x=0, 3 at.%) metallic glass matrix composites (MGMCs) with different crystallization degrees were investigated using an in-situ digital image correlation (DIC) technique during tensile process.With decreasing crystallization degree, MGMC exhibits obvious elastic deformation ability and an increased tensile fracture strength.The notable tensile elasticity is attrib-uted to the larger shear strain heterogeneity emerging on the surface of the sample.This finding has impli-cations for the development of MGMCs with excellent tensile properties.     

Fatigue and fracture behavior of bulk metallic glass

Tensile, compressive, cyclic tension-tension, and cyclic compression-compression tests at room temperature were systematically applied to a Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti₅ bulk metallic glass for comprehensive understanding of its damage and fracture mechanisms. Under tensile loading, the metallic glass only displays elastic deformation followed by brittle shear fracture. Under compressive loading, after elastic deformation, obvious plasticity (0.5 to 0.8 pct) can be observed before the final shear fracture. The fracture strength under compression is slightly higher than that under tension. The shear fracture under compression and tension does not occur along the maximum shear stress plane. This indicates that the fracture behavior of the metallic glass does not follow the Tresca criterion. The fracture surfaces show remarkably different features, i.e., a uniform vein structure (compressive fracture) and round cores coexisting with the radiating veins (tensile fracture). Under cyclic tension-tension loading, fatigue cracks are first initiated along localized shear bands on the specimen surface, then propagated along a plane basically perpendicular to the stress axis. A surface damage layer exists under cyclic compression-compression loading. However, the final failure also exhibits a pure shear fracture feature as under uniaxial compression. The cyclic compression-compression fatigue life of the metallic glass is about a factor of 10 higher than the cyclic tension-tension fatigue life at the same stress ratio. Based on these results, the damage and fracture mechanisms of the metallic glass induced by uniaxial and cyclic loading are elucidated.     

Motion of Crystal Particles by the Electromagnetic Vibrations in Mg-Cu-Y Bulk Metallic Glasses

The method for producing Mg-Cu-Y and Fe-Co-B-Si-Nb bulk metallic glasses using electromagnetic vibrations is effective in forming the metallic glass phase. Disappearance or decrement of clusters by the electromagnetic vibrations applied to the liquid state is considered to cause suppression of crystal nucleation, because the electromagnetic vibrations vibrate the clusters vigorously in the melt. The purpose of this study was to investigate motion of the crystal particles by the electromagnetic vibrations in Mg-Cu-Y bulk metallic glasses. The electromagnetic vibration force vibrated the crystal particles or the clusters that become crystal nuclei in the melt, because the electric current for the electromagnetic vibrations concentrates in those. Thus, the electromagnetic vibrations were found to select vibration particles from the melt. Moreover, it was considered that composites for which second phases or other compounds are dispersed into the metallic glass phase or a nanostructure phase can be produced by the electromagnetic vibration process. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Statistical analysis on strain-rate effects during serrations in a Zr-based bulk metallic glass

By means of statistical analysis, the deformation mechanisms taking place in elastic loading and plastic shearing stages during serrated flows on the stress-strain curves for bulk metallic glasses were studied comprehensively.Normalized serration number presented a linear increasing tendency with the decrease of applied strain rates due to the reduction of free volumes.An excellent plastic deformation was illustrated from the influences of structure arrangement with activation energy.By using mean-field theory (MFT), maximum elastic-energy density at different strain rates could be predicted by MFT besides maximum stress drops during serrations.These results were helpful for understanding the serrated flow behavior or designing decent schemes to improve the plasticity of bulk metallic glasses at room temperature.