Ni- and Cu-free Ti-based metallic glasses with potential biomedical application

In the present work Ti–Fe–Si and Ti–Fe–Si–X (X = Zr, Pd, Ge) glassy alloys are discussed as potential biomedical materials. Depending on composition and experimental conditions these alloys possess glassy, quasicrystalline or crystalline structure. The glassy state and crystallization behavior of the melt spun ribbons were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and the Hank's solution was used as simulated body fluid for corrosion tests. Ternary Ti–Fe–Si alloys near the Ti₆₅Fe₃₀Si₅ eutectic point were prone to form quasicrystals if the cooling rate was not high enough to retain amorphous structure. The compositions on the steeper side of the eutectic point could be vitrified. The results indicate that small additions of Zr can have a positive effect on glass formation, while additions of Ge, Pd may have a detrimental effect by promoting crystallization. Ti–Fe–Si and Ti–Fe–Si–Zr alloys exhibited high corrosion properties, superior to that of pure Ti and most of Ti-based glassy alloys reported in the literature. Being free of Ni and Cu this group of alloys may be considered for possible biomedical application.     

Evolution of the amorphous structure in melt-spun Ti50Ni25Cu25 alloy subjected to high pressure torsion deformation

Peculiarities of structure and mechanical behaviour of amorphous Ti₅₀Ni₂₅Cu₂₅ alloy were the focus of this research. The melt-spun ribbons of amorphous Ti₅₀Ni₂₅Cu₂₅ were subjected to high pressure torsion (HPT) at temperatures of 20–150 °C in order to modify their structure and mechanical behaviour. Some features of obtained HPT-processed samples were compared with initial state with the help of x-ray diffraction (XRD), transmission electron microscopy (TEM) and nanohardness testing. Analysis of structural data and mechanical behaviour allowed us to assume that severe plastic deformation (SPD) processing of melt-spun Ti₅₀Ni₂₅Cu₂₅ alloy might lead to the formation of the structure similar to the new kind of noncrystalline state – “nanoglass” state.     

Microstructure and mechanical properties of a spray-formed Ti-based metallic glass former alloy

A Ti_45.8Zr_6.2Cu_39.9Ni_5.1Sn₂Si₁ composite plate was spray deposited on a copper substrate. From the bottom substrate-contact surface to the upper free surface of the deposit, a layered microstructure evolution in the order of fully amorphous (for the region 1-2 mm perpendicularly away from the substrate), amorphous/nanocrystalline (3-4 mm from the substrate), ultrafine-grained crystalline (5-6 mm from the substrate) and micron-sized crystalline phases (7-8 mm from the substrate) was observed. The oversprayed powders bellow 50 μm exhibit fully amorphous structure, while the ones above 50 μm show certain crystallization behavior. The fracture strength of 1.58-1.85 GPa with obvious plastic strain can be achieved under compressive tests for the spray-formed deposit. Spray forming can therefore produce bulk-sized high strength Ti-based alloy which evolves gradually from certain non-equilibrium towards equilibrium during deposition, which were considered to be attributable to the chill effect at the bottom substrate-contact surface and the following heat entrapment from the successively deposited droplets or powders.     

Conversion of isothermal and isochronal crystallization in a supercooled liquid through additivity rule

In the present work, the compatibility of additivity rule with the crystallization behavior of an Au₅₀Cu_25.5Ag_7.5Si₁₇ glass forming alloy on the conditions of both isothermal and isochronal transformations is investigated. Taking advantages of the ultrafast heating and cooling rates of the Flash DSC technique, we verified the additivity rule in a solidification process for the first time. It is proved to be effective in predicting the beginning and the end of isochronal crystallizations during continuous heating and cooling from the kinetic analysis of corresponding experimental isothermal transformation. The predicted isochronal curves are fairly in accordance with the experimental results, especially for the incubation and crystallization processes during rapid solidification of the glass forming liquid. The compatibility of additivity rule enables a conversion between isothermal and isochronal transformations for the crystallization of supercooled liquid even during solidification from the melt.     

Influence of powder size on the crystallization behavior during laser solid forming Zr55Cu30Al10Ni5 bulk amorphous alloy

The Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glasses (BMGs) were prepared using laser solid forming (LSF) process from the plasma rotating electrode process (PREP) powder. The effect of the powder size on the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) was investigated. It was found that the as-prepared powders were composed of the amorphous phase and Al₅Ni₃Zr₂-type phase. The RZ mainly kept the amorphous state after LSF. The residual Al₅Ni₃Zr₂-type phase could be observed in RZ only if the powder size was larger than 106 μm. Meanwhile, the NiZr₂-type nanocrystals at the boundary of RZ primarily formed from the solidification of remelted liquid. With the increase of the powder size, the lower overheating temperature and shorter existing time of the molten pool enhanced the heredity of Al₅Ni₃Zr₂ clusters and other intermetallic clusters in remelted alloy melt, which decreased the thermal stability of the already-deposited layer. The volume fraction of crystallization in the deposit increased with the increase in powder size. There was no crystallization occurred in the HAZ between the adjacent tracks and layers for the deposit prepared by the powder with the size range of 53–75 μm. However, the wide crystalline band with Al₅Ni₃Zr₂-type faceted phase, CuZr-type dendrite, CuZr₂-type spherulite and NiZr₂-type nanocrystal were observed in the entire HAZ for the deposit prepared by the powder with the size range of 106–150 μm. The finer powder was benefit to prepare the BMGs by LSF.     

FeSiBPNbCu alloys with high glass-forming ability and good soft magnetic properties

Effects of Cu addition on the glass-forming ability (GFA), thermal stability, magnetic properties and crystallization process of (Fe_0.76Si_0.09B_0.1P_0.05)_99−xNb₁Cu_x (x = 0, 0.25, 0.5, 0.75, 1) alloys were investigated. The introduction of Cu effectively stimulates the precipitation of the α-Fe(Si) without obvious deterioration of the GFA, and successfully modifies the simultaneous precipitation of α-Fe(Si), Fe₂B and Fe₃(B,P) phases in (Fe_0.76Si_0.09B_0.1P_0.05)₉₉Nb₁ alloy into separable precipitation of each phase at different temperatures during annealing, leading to the enhancement of soft magnetic properties. The saturation magnetic flux density of the representative (Fe_0.76Si_0.09B_0.1P_0.05)_98.25Nb₁Cu_0.75 alloy could be enhanced from 1.43 to 1.51 T after annealing at 530 °C for 10 min due to the precipitation of α-Fe(Si) nanoparticles with a diameter of about 22 nm dispersing randomly in the amorphous matrix. The integration of high GFA and excellent soft magnetic properties makes the FeSiBPNbCu alloys promising soft magnetic materials for industrial applications.     

Microstructural analysis of Zr55Cu30Al10Ni5 bulk metallic glasses by laser surface remelting and laser solid forming

The crystallization behavior of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy during laser solid forming (LSF) was analyzed. Since laser surface remelting (LSM) is a key process for the LSF, the crystallization behavior of as-cast Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glasses (BMGs) during LSM was also investigated. It was found that the amorphous state of the as-cast BMGs was maintained when they were repeatedly remelted four times in a single-trace LSM, and as for the LSF of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy, the crystallization primarily occurred in the HAZ between the adjacent traces and layers after the two layers were deposited. The as-deposited microstructure exhibited a series of phase evolutions from the molten pool to the HAZ as follows: the amorphous → NiZr₂–type nanocrystal + amorphous → NiZr₂–type equiaxed dendrite + amorphous → Cu₁₀Zr₇–type dendrite + NiZr₂–type nanocrystal. Among these microstructural patterns, the NiZr₂–type nanocrystals and equiaxed dendrites primarily formed from the rapid solidification of the remelted liquid in the laser processing process, and the Cu₁₀Zr₇–type dendrites in the HAZ primarily formed by the crystallization of pre-existed nuclei in the already-deposited amorphous substrate.     

Bulk metallic glasses based on precious metals: Thermal treatments and mechanical properties

The thermomechanical behavior of precious based metallic glasses has been investigated. Their compositions are free of nickel for biocompatibility. The gold-based BMG has a gold content resulting in 18 Karats alloy, a supercooled liquid region of 43 °C and a casting diameter up to 5 mm in rod. The compositions of platinum and palladium based BMGs are interesting as they can be formed into bulk glassy rods with diameter up to 15 and 30 mm respectively. The platinum-based BMG has a platinum content resulting in 850 Pt grade with a supercooling region reaching 58 °C. The palladium-based BMG is principally composed of 40 wt.% palladium and 32 wt.% platinum, with a large supercooling region reaching 73 °C.The thermoplastic deformation of these BMG has been examined using thermomechanical analyser (TMA) and the results show that the alloys can be easily processed in the supercooled liquid region. During thermal processing, crystallization must be controlled since it improves hardness and elastic modulus, but embrittles the alloys and stops the deformation. The high hardness of Au-, Pt- and Pd- base BMGs (respectively 340, 420, 460 HV) twice the value of conventional precious metals, coupled with good properties for superplastic forming in the supercooled liquid region made them promising materials for watch making and jewelry applications.     

Effects of electrochemical hydrogenation of Zr-based alloys with high glass-forming ability

Zr–(Ti)–Cu–Al–Ni metallic glasses exhibit a high thermal stability corresponding to a wide undercooled liquid region. Depending on their composition, the formation of metastable intermediate phases, e.g. a quasicrystalline phase is possible. The combination of early and late transition metals makes these alloys very interesting regarding their interaction with hydrogen. Amorphous Zr₅₅Cu₃₀Al₁₀Ni₅, Zr₆₅Cu_17.5Al_7.5Ni₁₀ and Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ ribbons were prepared by melt spinning and their microstructure and thermal behaviour was checked by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in 1 vol.-% HF was investigated in 0.1 M NaOH by applying potentiodynamic polarisation techniques. Galvanostatically hydrogenated samples were characterised by XRD, DSC, TEM and thermal desorption analysis (TDA). For amorphous Zr₅₉Ti₃Cu₂₀Al₁₀Ni₈ samples an increase in electrochemical surface capacity by two orders of magnitude is observed after pre-etching. Compared to the quasicrystalline and crystalline alloy, the hydrogen reduction takes place at significantly lower overpotentials. Zr-based alloys cathodically absorb hydrogen up to H/M=1.65 while keeping the amorphous structure. Already small amounts of hydrogen cause a significant decrease of the thermal stability and changes in the crystallisation sequence. The hydrogen desorption is a two-stage process: (T<623 K) hydrogen desorption from high interstitial-site energy levels and (T>623 K) zirconium hydride formation and subsequent transformation under hydrogen effusion. Hydrogen suppresses the oxygen-triggered formation of metastable phases upon heating and supports primary copper segregation. At very high H/M ratios, severe zirconium hydride formation causes the crystallisation of new compounds.     

Magnetic properties and magnetocaloric effect of FeCrNbYB metallic glasses with high glass-forming ability

The influences of Cr addition on the Curie temperature (T_C), glass-forming ability (GFA), and magnetocaloric effect were investigated in FeCrNbYB metallic glasses. It was found that the addition of Cr element slightly decreases the GFA and saturation magnetization, whereas effectively modulates T_C. By the method of copper mold casting, bulk metallic glasses (BMGs) with critical diameters up to 5 mm can be obtained in Fe_68−xCr_xNb₄Y₆B₂₂ (x = 2–6) alloys. The resulting metallic glasses exhibit T_C of 271–367 K and excellent magnetocaloric properties, including magnetic entropy change of 0.76–1.05 J/kg K, and refrigerant capacity of 83–93 J/kg under a low field change of 1.5 T. In addition, they exhibit a wide supercooled liquid region of 116–135 K. The successful synthesis of the FeCrNbYB BMGs with near room-temperature magnetocaloric properties is encouraging for the future development of Fe-based BMGs as a new magnetic refrigerant in magnetic cooling system.     

Production methods and properties of engineering glassy alloys and composites

Since the first synthesis of glassy alloys in 1960, a great large number of studies have been carried out in basic science and technological aspects. At present, glassy alloys and composites have been used as functional and structural materials. The production method for their practical materials was limited to a quenching type technique for many years before around 1990, but the finding of bulk glassy alloys caused a drastic change in production method of glassy alloys to a copper mold casting type technique which has enabled the production of glassy alloys in a three dimensional bulk form. Furthermore, glass alloy composites produced by semi-solid progressive solidification have been successfully developed recently, even using impure charge materials. The drastic changes in the production method and material form have also resulted in significant extension of application fields of glassy alloys. This paper aims to review the production methods and properties of glassy alloys and composites with useful critical sizes above 100 μm or below 4 μm in thickness for glassy powder in conjunction with the present situation of applications for their glassy alloys and composites which do not belong to materials with an ordinary thickness size range of 10–50 μm.     

Fe-based soft magnetic amorphous alloys with high saturation magnetization above 1.5 T and high corrosion resistance

Fe-rich amorphous alloys with minor-addition of Cr and/or Nb were examined with the aim of developing Fe-based amorphous alloys exhibiting simultaneously high saturation magnetization above 1.5 T and good corrosion properties. Fe₈₂Cr₂B₈P₄Si₃C and Fe₈₂NbB₉P₄Si₃C amorphous alloys were found to exhibit high saturation magnetizations of 1.49 T and 1.57 T, respectively, and rather good corrosion resistance in 3.5 mass% NaCl solution at 298 K. The minor-addition of Cr or Nb enables the formation of amorphous alloy particles without harmful oxide layer by water atomization process which makes these alloys suitable for applications as soft magnetic core materials. The addition of 1 at% Nb improved the corrosion resistance through the increase in E_corr value, which makes easy to reach passive state, and the suppression of pitting corrosion. Besides, it has been proved that the simultaneous addition of Nb and Cr has an effect on forming protective passive film.     

Effect of cracking in diffusion aluminide coatings on their cyclic oxidation performance on Ti-based IMI-834 alloy

Diffusion aluminide coatings improve the high temperature oxidation resistance of Ti-alloys. This study evaluates the oxidation resistance of a Al₃Ti type aluminide coating and a Pt-aluminide coating on Ti-alloy IMI-834, at 650, 750 and 850 °C under cyclic oxidation conditions in air. Both coatings provide good oxidation resistance, however, the extent of through-thickness cracking in coating and localized oxidation degradation of substrate increases with thermal cycling. At high temperature of 850 °C, TiO₂ outgrowths emanate from these cracks, resulting in a prominent mud-crack pattern on the surface. The possible effect of such cracking on long-term oxidation properties of coatings has been discussed.     

Low temperature heat capacity and electrical resistivity of the Ti40Zr25Cu12Ni3Be20 glass forming alloy

Gaining knowledge of electronic structure provides useful information for understanding unique properties of metallic glasses. In this study, low temperature heat capacity and electrical resistivity of the glass forming Ti₄₀Zr₂₅Cu₁₂Ni₃Be₂₀ alloy with glassy, quasicrystalline, or crystalline states below 300 K were investigated. The precipitation of the I-phase was revealed in the initial crystallization process of the Ti₄₀Zr₂₅Cu₁₂Ni₃Be₂₀ BMG. The glassy state has higher state density at Fermi level than its quasicrystalline or crystalline counterparts, which could be interpreted by the electron localization in glassy state as well as a pseudo-Brillouin zone formed nearby Fermi surface in the quasicrystalline state. None of the three states showed superconductivity phenomenon down to 1.9 K. Temperature dependence of resistivity for both the glassy state and the quasicrystalline state exhibited negative temperature coefficient and was less sensitive to temperature than the crystalline state. The electrical resistivity showed a smaller value for the I-phase than that for the glass due to lower structural integrity of I-phase. Electrical resistivity as well as heat capacity measurements indicated that the electronic structure of the quasicrystalline state is quite similar to glassy state but far from crystalline state.     

On the applicability of a mesoscopic interface sliding controlled model for understanding superplastic flow in bulk metallic glasses

It is demonstrated that a mesoscopic interface sliding controlled flow model, which has already been shown to account for superplastic deformation in different types of crystalline materials, is also capable of describing superplastic flow in bulk metallic glasses. The only difference is that the random high-angle grain boundaries in crystalline materials along which deformation is concentrated, have to be replaced by the transient interfaces which are formed by interconnecting shear transformation zones in the region of homogeneous flow in bulk metallic glasses. Comparison with experimental results concerning superplastic flow in eight bulk metallic glasses shows that the numerical solutions obtained in the paper for the transcendental stress–strain rate equation of superplastic deformation lead to accurate predictions.     

A quinary Ti–Zr–Hf–Be–Cu high entropy bulk metallic glass with a critical size of 12 mm

A quinary Ti₂₀Zr₂₀Hf₂₀Be₂₀Cu₂₀ high entropy bulk metallic glass (HE-BMG), with a critical diameter (D_c) of 12 mm—the largest size in the reported quinary high entropy alloys (HEAs), has been successfully prepared by copper mold casing. This novel HE-BMG possesses a supercooled liquid region ΔT (=T_x − T_g) of 78 K, indicating a better thermal stability than those of other HE-BMGs. In addition, the HE-BMG exhibits a relatively good compressive plasticity (2.2 ± 0.1%) among the HE-BMGs. The newly developed HE-BMG may offer insights for the indepth understanding of the fundamental issues associated with the glass formation and the unique structure-property relationship when combining the features of HEAs and BMGs together.     

Diffusion bonding of a Zr-based metallic glass in its supercooled liquid region

Based on the analyses of the influences of temperature and pressure on diffusion behavior with time, a Zr-based bulk metallic glass is successfully bonded in its supercooled liquid region. The temperature is found to be a key factor for the success of the bonding process if sufficient pressure and time are adopted. Three-point bending tests show that the flexural performance of the samples bonded with various parameters is significantly different. The parameters in favor of diffusion bonding do not automatically benefit the performance of the bonded materials. Diffusion makes the nano-voids shrink to form metallurgical bonding at the interface and it causes structural relaxation leading to the embrittlement inside bulk. Balance between the two aspects is the key to the bonding success and the utility of the bonded materials.     

Novel multilayer structure design of metallic glass film deposited Mg alloy with superior mechanical properties and corrosion resistance

A novel multilayer structure of metallic glass film deposited Mg alloy was designed and fabricated by combining surface mechanical attrition treatment and magnetron sputtering. The multilayer structure consists of amorphous layer and gradient structure sequentially from surface, following with the coarse grains inside. Gradient structure significantly decreases hardness mismatch between the amorphous layer and the matrix of Mg alloy. Wear resistance is increased by 2 orders for this novel multilayer structure with the simultaneous improvement of corrosion resistance.     

Amorphization and nanostructure synthesis in Al alloys

The recent innovations in metallic glasses have led to new alloy classes that may be vitrified and a re-examination of the commonly used criteria for glass formation and stability. In one case large, bulk volumes may be slowly cooled to the glassy state that signifies a nucleation-controlled synthesis. The other important case is represented by Al and Fe based marginal glass formers that have been synthesized under growth controlled kinetic conditions mainly by rapid solidification processes. With marginal glass forming alloys the termination of the amorphous state upon heating is often characterized by a primary crystallization reaction that yields a high number density in the range of 10²¹–10²² m⁻³ of Al nanocrystals (15–20 nm in diameter) dispersed within a residual amorphous matrix. At the same time the results from alternate synthesis strategies involving intense cold rolling reveal that the primary crystallization reaction can be bypassed during deformation alloying of elemental multilayers or enhanced during deformation bonding of amorphous ribbons. The kinetics analysis of the crystallization behavior provides insight into the origin of the dispersed nanocrystal and amorphous matrix microstructures, the important influence of heterogeneous catalysts and an effective assessment of the overall stability that are essential for microstructure control.