Mg-based bulk metallic glasses:A review of recent developments

Metallic biomaterials have been widely used in the field of medical implants for replacement purposes and/or for regeneration of tissue.Metals such as stainless steel(316 L),cobalt-chromium alloys and titanium alloys(Ti-6Al-4 V)are widely used as metallic implants today.However,they often exhibit unsatisfactory results such as stress shielding,the release of toxic ions and are often permanent and invasive–where a second surgery is required to remove the implant once the bone is fully healed.Magnesium as a biomaterial have attracted much attention recently due to its excellent biocompatibility,similar mechanical properties to bone and biodegradability.Unlike other metals and bio ceramics,the ability for magnesium alloys to undergo biodegradation eliminates the requirement for a second surgery to remove the implant.Additionally,the degradation of magnesium releases Mg2+ions,which stimulates metabolism as they are a cofactor in numerous numbers of enzymes.Despite the advantages of magnesium alloys,the rapid degradation of magnesium proved to be challenging as the implant is unable to retain its structural integrity sufficiently enough to act as an implant.To improve the corrosion resistance of magnesium alloys,researchers have been working on the synthesis and characterization of Mg-based bulk metallic glasses,which can significantly improve the corrosion resistance of Mg-based alloys.This paper is a comprehensive review that compiles,analyzes and critically discusses the recent literature on the latest understanding of the processing,mechanical and biological characteristics of Mg-based bulk metallic glasses.

Crystallization of Mg-based bulk metallic glass

Mg-based bulk metallic glass fabricated by conventional copper mould method was aged at different temperatures. X-ray diffractometry(XRD), scanning electron microscopy(SEM), atomic force microscopy(AFM) and focused ion beam(FIB) miller were employed to examine specimens obtained under different conditions. The crystallization of Mg-based bulk metallic glass depends upon both the aging temperature and the aging time. As temperature increases or the holding time increases, the microstructure of the aged specimen varies from glassy one to crystalline one plus glassy phase and then to absolutely multiphase crystalline one. From the FIB images, it is clear that Mg-based bulk metallic glass could not only crystallize completely but also display dendrite-like growth style. From the AFM images, there are not only significant variations of microstructures but also surface morphology of specimens obtained under different conditions. It is proposed that the surface morphology varies as the treating temperature increases. The Vickers hardness of different specimens increases as the fraction of crystalline phase (s) increases.

Casting effect on compressive brittleness of bulk metallic glass

An interesting phenomenon of cooling-rate induced brittleness in Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass （BMG） was reported. It was found that the as-cast BMG specimens exhibited a brittle-ductile transition when the larger specimens were machined into smaller specimens through removing the cast-softening surface layer by layer. After compression tests, the as-machined small specimens, owing to the absence of the cast-softening surface, displayed highly dense and intersecting shear bands, and extensive plastic deformation. This is in contrast to the catastrophic failure and low deformability in the as-cast large specimens. More free volume was detected in the smaller as-fractured specimens, by differential scanning calorimetry, which may be attributed to the occurrence of strain softening and increased plasticity. Compared with the relatively smooth fracture surface in the smaller specimens, the larger specimens showed more diverse features on the fracture surface due to their graded structures.

Effect of Ti on microstructure,mechanical and corrosion properties of (Zr_(0.55)Al_(0.1)Ni_(0.05)Cu_(0.3))_(100-x)Ti_x bulk metallic glasses

The（Zr_（0.55）Al_（0.1）Ni_（0.05）Cu_（0.3））_（100-x）Ti_x alloys were prepared using an in-situ suck-casting method in a copper mold. The effects of Ti addition on the microstructure, mechanical and corrosion properties of the（Zr_（0.55）Al_（0.1）Ni_（0.05）Cu_（0.3））_（100-x）Ti_x alloys were investigated by X-ray diffraction, scanning electron microscopy, compressive tests and corrosion tests. It has been found that the addition of Ti higher than 4%（mole fraction） causes the formation of many crystalline phases in the alloy. The alloys with 1%-3% Ti display an obvious yield stage on their compressive stress-strain curves. An appropriate addition of Ti can improve the strength and ductility of the alloys. All the alloys have high corrosion resistance in 1 mol/L Na OH solution, and are corroded in 1 mol/L HCl solution. However, the appropriate addition of Ti can significantly improve the corrosion resistance of the alloys in HCl solution.

Enhanced plasticity of Fe-based bulk metallic glass by tailoring microstructure

The room temperature compressive plasticity of Fe75MosP10Cs.3B1.7 bulk metallic glass （BMG） was improved from 0.5% to 1.8% by increasing the sample diameter from 1.5 mm to 2.0 mm. With increasing the sample diameter to 2.0 mm, a heterogeneous microstructure with in-situ formed a-Fe dendrite sparsely distributed in the amorphous matrix can be attained. This heterogeneous mierostructure is conceived to be highly responsible for the enhanced global plasticity in this marginal Fe-based BMG.

Preparation and properties of novel Cu-based bulk metallic glasses Cu_(55-x)Zr_(37)Ti_8In_x

The glassy rods were successfully fabricated in the Cu-Zr-Ti-In alloy system by casting into a copper mold. The value of ATx reaches a maximum of 66 K for the BMG CusoZraTTi8In5 alloy. The reasons for enhancing glass forming ability of Cu-based BMGs with the addition of indium were discussed from atomic size and thermodynamics. Alternatively, the BMG Cu52Zra7Ti8In3 exhibits the highest compressive strength （1981 MPa） and the best plasticity among glassy Cu55-xZra7TisInx （x_〈5）. The total plastic deformation of Cu52Zr37TisIn3 before fracture approaches 1.2%.

Effects of Y addition on structural and mechanical properties of CuZrAl bulk metallic glass

The effects of Y addition on the structural and mechanical properties of CuZrAl bulk metallic glass（BMG） were studied.The results show that the glass forming ability of CuZrAl system is improved by the addition of Y and the fracture strength decreases with Y addition due to the reduction of binding energy induced by Y.The fracture surface is dominated by vein-like patterns in Cu45Zr48Al7 bulk metallic glass,and changes to smooth regions in Cu46Zr42Al7Y5 BMG.TEM observation shows that Cu45Zr48Al7 BMG has a composite microstructure of nanocrystalline phases dispersed in amorphous matrix.However,the Cu46Zr42Al7Y5 BMG shows a fully amorphous structure.

Electrochemical corrosion properties of Zr- and Ti-based bulk metallic glasses

The corrosion behaviors of Ti-based and Zr-based amorphous alloys and their corresponding crystallized alloys were studied by electrochemical methods. It is found that the corrosion potentials of Zr-based amorphous alloy and its corresponding crystalline counterpart are both lower than those of the Ti-based amorphous alloy in the 1 mol/L H2SO4 solution. In the 3.5% NaCl solution,Zr-based crystallized alloy exhibits the lowest corrosion potential among the experimental samples. No passivation is observed in the corrosion process for the Zr-based crystalline alloy. However, Zr- and Ti-based amorphous alloys both exhibit passivation characteristics. EIS measurements indicate the amorphous alloys exhibit better corrosion resistance than the crystallized one in the NaCl solution. Surface analysis shows that both amorphous alloys in the NaCl solution are eroded by pitting corrosion. In the H2SO4 solution, all the alloys display similar behaviors and their surfaces can mostly keep intact except for some cracks on the corroded surface at local region.

Growth kinetics for intermetallic compound layer between molten In-Sn alloy and CuZr-based bulk metallic glass

The growth kinetics of intermetallic compound layer between molten In-Sn alloy and Cu40Zr44Al8Ag8 bulk metallic glass substrate was examined by solid state isothermal aging at the temperature range between 333 and 393 K.The aged samples were characterized by scanning electron microscopy and energy dispersive spectrometry.It is found that the intermetallic compound layer is composed of Zr,Cu and Sn.The layer growth of the intermetallic compound is mainly controlled by a diffusion mechanism over the temperature range and the value of the time exponent is approximately 0.5.The apparent activation energy for the growth of total intermetallic compound layers is 98.35 kJ /mol calculated by the Arrhenius equation.

Effect of surface roughness on plasticity of Zr_(52.5)Cu_(17.9)Ni_(14.6)Al_(10)Ti_5 bulk metallic glass

Zr52.5Cu17.9Ni14.6Al10Ti5 bulk metallic glass （BMG） rods were abraded to different surface roughnesses using different types of waterproof abrasive papers and sometimes polishing pastes, and the compressive deformation behavior was examined. The results show that the yield strength of the BMG is hardly affected while the compressive plasticity increases from 2.3% to 4.5% with decreasing the surface roughness. Observation of the fractured samples under a scanning electron microscope indicates that the rise in plasticity is accompanied with an increase in shear band density. The results suggest that it is necessary to reduce the surface roughness of BMGs for achieving a large plasticity.

Dry sliding tribological behavior of Zr-based bulk metallic glass

The tribological behavior of a Zr-based bulk metallic glass（BMG） was investigated using pin-on-disk sliding measurements in two different environments,i.e.,air and argon,against an yttria-stabilized zirconia counterface.It was found that the wear of the Zr-based BMG was reduced by more than 45% due to the removal of oxygen from the test environment at two different loads,i.e.,16 N and 23 N.The wear pins were examined using X-ray diffractometry,differential scanning calorimetry,scanning electron microscopy and optical surface profilometry.A number of abrasive particles and grooves presented on the worn surface of the pin tested in air,while a relatively smooth worn surface was observed in the specimens tested in argon.The wear mechanism of the pin worn in air was dominated by abrasive wear compared with an adhesive wear controlled process in the tests performed in argon.

Effect of minor Fe addition on glass forming ability and mechanical properties of Zr_(55)Al_(10)Ni_5Cu_(30) bulk metallic glass

A series of rod samples with diameter of 3 mm（Zr0.55Al0.10Ni0.05Cu0.30）100-xFex（x=0,1,2,3,4） were prepared by magnetic suspend melting and copper mold suction casting method.The effects of a small amount of Fe on glass forming ability（GFA） and mechanical properties of Zr55Al10Ni5Cu30 bulk metallic glass（BMG） were investigated.The results show that the addition of an appropriate amount（less than 3%,mole fraction） of Fe enhances GFA,as indicated by the increase in the reduced glass transition temperature Trg（=Tg/Tl） and the parameter γ（=Tx/（Tg＋Tl）） with increasing Fe content,and GFA gets deteriorated by further Fe addition（4%）.The addition of Fe also effectively improves the compressive plasticity and increases the compressive fracture strength in these Zr-based BMGs.Compressive tests on BMG sample with 3 mm in diameter and 6 mm in length reveal work-hardening and a certain plastic strain in the alloy containing 2% Fe.The BMG composite containing 4% Fe also exhibits a high fracture strength along with significant plasticity.

Transition of plasticity and fracture mode of Zr-Al-Ni-Cu bulk metallic glasses with network structures

Effect of network structure on plasticity and fracture mode of Zr？Al？Ni？Cu bulk metallic glasses （BMGs） was investigated. The microstructures of transversal and longitudinal sections were exposed by chemical etching and observed by scanning electron microscopy （SEM）. The mechanical properties were examined by room-temperature uniaxial compression test. The results show that both plasticity and fracture mode are significantly affected by the network structure and the alteration occurs when the size of the network structure reaches up to a critical value. When the cell size （dc） of the network structure is ~3μm, Zr-based BMGs characterize in plasticity that decreases with increasingdc. The fracture mode gradually transforms from single 45° shear fracture to double 45° shear fracture and then cleavage fracture with increasingdc. In addition, the mechanisms of the transition of the plasticity and the fracture mode for these Zr-based BMGs are also discussed.

Effects of aspect ratio and loading rate on room-temperature mechanical properties of Cu-based bulk metallic glasses

Room-temperature mechanical properties of Cu50Zr40Ti10-xNix（0≤x≤4,mole fraction,%） bulk metallic glasses （BMG） with aspect ratios in the range of 1：1-2.5：1 and loading rates in the range of1×10^-5-1×10^-2s^-1were systematically investigated by room-temperatureuniaxialcompression test.In the condition of an aspect ratio of 1：1, the superplasticity can be clearly observed for Cu50Zr40Ti10BMG when the loading rate is1×10^-4s^-1, while for Cu50Zr40Ti10-xNix（x=1-3, mole fraction, %） BMGs when the loading rate is1×10^-2s^-1. The plastic strain （εp）, yielding strength （σy） and fracture strength （σf） of the studied Cu-based BMGs significantly depend on the aspect ratio and the loading rate. In addition, theσyof the studied Cu-based BMGs with an aspect ratio of 1：1 is close to the σfof those with the other aspect ratios when the loading rate is1×10^-2s^-1. The mechanism for the mechanical response to the loading rate and the aspect ratiowas also discussed.

Effect of minor-addition of Fe on structural and mechanical properties of CuZrAl bulk metallic glass

The CuZrAl bulk metallic glass with minor-addition of Fe was prepared by rapid quenching method. The structures were examined by X-ray diffraction （XRD）. The effect of Fe on the glass-forming ability was studied by differential scanning calorimetry （DSC）. The minor-addition of Fe obviously extends the supercooled liquid region ΔTx. The plastic strain of the Cu44Zr48Al7Fe bulk metallic glass is about 1.5%. The microstructures were examined by transmission electron microscopy （TEM）. It is found that when 1%-2% Fe （mole fraction） were introduced into the CuZrAl alloy matrix, nanoscale phase separation occurs in the as-prepared Cu44Zr48Al7Fe bulk metallic glass.

Design of high strength Fe-(P,C)-based bulk metallic glasses with Nb addition

Bulk metallic glass（BMG） rods Fe71Mo5-xNbxP12C10B2（x=1,2,3,4 and 5） with diameter of 1 or 2 mm were synthesized by copper mold casting.The effects of Nb substitution for Mo on the structure,thermal and mechanical properties of Fe71Mo5-xNbxP12C10B2 alloys were studied by X-ray diffraction,differential scanning calorimetry and compressive testing.The results show that the substitution of Nb for Mo leads to a decreased glass forming ability,but with plasticity of 1.0%,the fracture strength of Fe71Mo2Nb3P12C10B2 alloy increases up to 4.0 GPa.The improvement of the fracture strength is discussed in terms of the enhancement of atomic bonding nature and the favorite formation of a network-like structure due to the substitution of Nb for Mo.

Combined effect of isothermal annealing and pre-compression on mechanical properties of Cu_(36)Zr_(48)Al_8Ag_8 bulk metallic glass

The combined effects of isothermal annealing and pre-compression on the mechanical properties of Cu36Zr48Al8Ag8 bulk metallic glass （BMG） were investigated. The as-cast specimens were first annealed at 743 K for 10 min, and then pre-compressed under 800 MPa for 1, 3, 5 and 10 h, respectively. The results indicated that annealing resulted in the formation of nanocrystals with a diameter of -10 nm in the amorphous matrix and a drastic decrease of the free volume, leading to complete loss of the plasticity of the BMG. Applying pre-compression under a stress of 800 MPa for a proper duration （5 h） resumed part of the lost free volume in the BMG matrix and therefore partially recovered the plasticity. A very long period of pre-compression （10 h） decreased the free volume again, which was caused by the excessive crystal growth.

Effect of minor Nb addition on mechanical properties of in-situ Cu-based bulk metallic glass composite

In-situ formed （Cu0.6Zr0.3Ti0.1）95Nb5 bulk metallic glass （BMG） composite with Nb-rich dendrite randomly dispersed in hard glassy matrix was prepared by casting into a water-cooled copper mold. The dendrite has much smaller hardness and elastic modulus than glassy matrix, and the stress concentration at interface provides a channel for the initiating and branching of shear bands upon loading, thus leading to a high compressive fracture strain of 6.08% and fracture strength about 2200 MPa. Comparing with other Cu-based BMG composite, the fracture strength of present （Cu0.6Zr0.3Ti0.1）95Nb5 composite is not significantly reduced, indicating that the addition of Nb in the current work is an effective and effortless way to fabricate new practical BMG composites with enhanced strength and good plasticity.

Role of yttrium in glass formation of Ti-based bulk metallic glasses

Starting with Zr of two different purities, Ti-based bulk metallic glasses （BMGs） with a nominal composition of Ti40Zr25Cu12Ni3Be20 were prepared. The effect of the addition of yttrium at levels of 0.2 at.%, 0.4 at.%, 0.5 at.%, 0.6 at.%, and 1 at.% on the glass forming ability （GFA） of the alloy has been investigated by means of metallography, X-ray diffraction, and differential scanning calorimetry. Experimental data in-dicates that high impurity content dramatically reduces the glass forming ability. Microalloying with 0.5 at.% Y is effective in suppressing the crystalline phase formation and alleviating the detrimental effect of oxygen in the low-purity alloy. On the contrary, in the alloy contain-ing high-purity element, the effective yttrium addition is less than 0.4 at.%. The results indicate that the beneficial effect of the optimum dopants is mainly due to scavenging the oxygen impurity via the formation of innocuous phase.

Porous bulk metallic glass fabricated by powder hot pressing

A synthesis method for the production of porous bulk metallic glass (BMG) was introduced. This method utilizes the su- perplastic forming ability of amorphous powder in the supercooled liquid (SCL) state and intenerating salt mixture as a placeholder to produce BMG foam by using a hot die pressing method. Scanning electron microscope (SEM), X-ray diffraction (XRD) and dif- ferential scanning calorimetry (DSC) were employed to characterize the morphologies of foaming structure, the crystallization and the percentage of crystallization of the as-produced porous BMG. The results suggested that the formation of porous structure by su- perplastic forming process is feasible. Good bonding effect was observed between amorphous powder particles. Less than 6.5% of crystalline phases were formed during hot pressing, and less than 5.5% of residual salt was enclosed in the foam. To remove any re- sidual salt particles, salt preforms with three-dimensional network and good connectivity is necessary.