Softening of α-Ti by electrochemically introduced hydrogen

The substantial and permanent modification of the elastic and inelastic properties of α-Ti have been achieved by hydrogen charging with cathodic polarization in an alkaline solution at RT, with the effects being dependent on the hydrogen state of the metal. The decrease in the elastic modulus to values close to those characteristic for bones have been caused by the distortion of the Ti lattice due to the supersaturation of the solid solution with hydrogen and (or) due to the formation of hydride precursors and precipitates. The low, but still detectable decrease in the elastic modulus has been observed by the formation of a compact hydride layer. On the basis of a thorough examination of the effect of cathodic polarization on the formation and morphology of the hydride phase and on the state of the α-Ti lattice, the electrochemical parameters provided: (1) the presence of hydrogen in supersaturated solid solution, (2) the formation of hydride precipitates of various stoichiometry and (3) the formation of the compact hydride surface layer have been determined.Taking into account the hydrogen induced metal softening, the easiness of electrochemical alloying the metal with hydrogen and no health hazard produced by this element when dissolved in body liquids, the electrochemical hydrogen treatment of Ti implants can be considered as very promising.     

Influence of N and Fe on α-Ti precipitation in the in situ TiC–titanium alloy composites

High strength with high ductility can be achieved in the titanium alloys by using metal precipitated ceramic particle as reinforcement. In this work, α + β or β-Ti alloy composites were prepared with α-Ti precipitated TiC particles. A series of Ti–Fe–C–N alloys were prepared and a constitutional diagram was constructed as a function of N and Fe contents. Two criteria were identified for the formation of α-Ti precipitation. One is the existence of Ti₂C phase and the other is the presence of α-Ti phase in the matrix. The mechanism of α-Ti formation from the Ti₂C phase is discussed.     

In situ nitridation of titanium–molybdenum alloys during laser deposition

In situ nitridation during laser deposition of titanium–molybdenum alloys from elemental powder blends has been achieved by introducing the reactive nitrogen gas during the deposition process. Thus, Ti–Mo–N alloys have been deposited using the laser engineered net shaping (LENS^TM) process and resulted in the formation of a hard α(Ti,N) phase, exhibiting a dendritic morphology, distributed within a β(Ti–Mo) matrix with fine scale transformed α precipitates. Varying the composition of the Ar + N₂ gas employed during laser deposition permits a systematic increase in the nitrogen content of the as-deposited Ti–Mo–N alloy. Interestingly, the addition of nitrogen, which stabilizes the α phase in Ti, changes the solidification pathway and the consequent sequence of phase evolution in these alloys. The nitrogen-enriched hcp α(Ti,N) phase has higher c/a ratio, exhibits an equiaxed morphology, and tends to form in clusters separated by ribs of the Mo-rich β phase. The Ti–Mo–N alloys also exhibit a substantial enhancement in microhardness due to the formation of this α(Ti,N) phase, combining it with the desirable properties of the β-Ti matrix, such as excellent ductility, toughness, and formability.     

Effect of hydrogen on phase and structural transformations in titanium alloys of different classes

We study the laws of phase and structural transformations in titanium alloys under the action of dissolved hydrogen and in the course of subsequent degassing. We show the possibility to control structure formation in them with the help of alloying with hydrogen and to obtain structures that cannot be obtained by conventional technological methods. It has been established that the alloying system determines the character of interaction of these alloys with hydrogen in the course of hydrogenation. After additional alloying with hydrogen, intermediate hydrides are formed in titanium alloys with β-isomorphic stabilizers (V, Nb), Ti Cr₂ intermetallic compound appears in alloys with β-eutectoid stabilizer (Cr), and Ti₃Al compound is formed in alloys with high aluminum content. After vacuum annealing of hydrogen-containing specimens at 600°C, a composite heterophase (α + β + α₂) or (α + β + TiCr₂) structure is formed, and intermetallic particles in it have an incoherent boundary. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 3, pp. 33–39, May–June, 2006.     

Structural, electrical and thermodynamical aspects of hydrogenated La-Ni-Si alloy

The structural, electrical and thermodynamic properties of a La-Ni-Si [La = 28.9%, Ni = 67.5%, Si = 3.6%] alloy have been investigated. Powder XRD results show that the lattice constants and unit cell volume of the alloy increase after hydrogen storage. It was also found that the resistance of the alloy increased with dissolved hydrogen concentration. Hydrogen absorption pressure composition isotherms have also been investigated which show the presence of two single a and β regions and one mixed (α + β) phase. The thermo-dynamic parameters viz. the relative partial molar enthalpy (ΔH) and relative partial molar entropy (ΔS) of dissolved hydrogen, are found to be in the range 8–18 kJ (mol H)^-1 and 25–63 JK^-1 (mol H)^-1. From the dependence of ΔH on the hydrogen concentration,X, the different phases [α, α + β, β] and phase boundaries of the alloy-H system are identified. Thermal conductivity and diffusivity of La-Ni-Si and its hydride have been measured at room temperature by using TPS technique. Thermal conductivity was found to decrease due to absorbed hydrogen in the alloy.     

Formation mechanism of hydride precipitation in commercially pure titanium

Since titanium has high affinity for hydrogen and reacts reversibly with hydrogen,the precipitation of titanium hydrides in titanium and its alloys cannot be ignored.Two most common hydride precipitates in α-Ti matrix are γ-hydride and δ-hydride,however their mechanisms for precipitation are still unclear.In the present study,we find that both γ-hydride and δ-hydride phases with different specific orienta-tions were randomly precipitated in the as-received hot forged commercially pure Ti.In addition,a large amount of the titanium hydrides can be introduced into Ti matrix with selective precipitation by using electrochemical treatment.Cs-corrected scanning transmission electron microscopy is used to study the precipitation mechanisms of the two hydrides.It is revealed that the γ-hydride and δ-hydride precipita-tions are both formed through slip + shuffle mechanisms involving a unit of two layers of titanium atoms,but the difference is that the γ-hydride is formed by prismatic slip corresponding to hydrogen occupy-ing the octahedral sites of α-Ti,while the δ-hydride is formed by basal slip corresponding to hydrogen occupying the tetrahedral sites of cα-Ti.     

Hydrogen-induced phase transformations in alloys based on Sm Co<Subscript>5</Subscript> under pressures of up to 650 kPA

The interaction of alloys based on SmCo₅ with hydrogen is studied by the methods of differential thermal and X-ray phase diffraction analyses under initial pressures of hydrogen of 200, 300, 400, 500, and 650 kPa at temperatures of up to 1223°K. The hydride of the alloy is formed up to a temperature of 343°K. Within the temperature ranges 388–408°K and 488–523°K, hydrogen is released from the hydrides of phases of the alloy. Within the temperature range 823–863°K, the alloy partially disproportionates into Sm H_x and Co. At 1008–1053°K, SmH_x undergoes partial decomposition and the SmCo₅ and Sm₂Co₁₇ phases are detected. The Co, SmCo₅, and Sm₂Co₁₇ phases exist at temperatures above 1168–1188°K. The compositions of the phases depend on the duration of interaction of the alloy with hydrogen. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 94–98, January–February, 2007.     

Control of the structure of titanium alloys by the method of thermohydrogen treatment

We show the possibility of controlling the structure of titanium alloys of different classes by means of their thermohydrogen treatment and consider the process of structure formation in shaped castings of VT20L alloy under additional alloying with hydrogen. Depending on the hydrogen content in the course of thermohydrogen treatment, the transformation of a course-lamellar structure into a fine-lamellar one with α-particle sizes from several micrometers to nanometers is possible. We describe the mechanism of phase and structural transformations in titanium alloys with a heightened content of β-eutectoid stabilizers (Ti-12Cr) or aluminum (VT6, VT23) under the action of hydrogen. We also show the possibility of producing a composite structure of the β-phase and TiCr₂ intermetallic compound or noncoherent particles of the α₂-and α-phases, depleted with aluminum. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 28–34, May–June, 2008.     

LaNi5- and RT3-based (R = Ce, Nd, Gd, Er; T = Co, Ni, Fe) hydrides prepared at low temperatures and H2 pressures

Hydride phases based on the intermetallic compounds LaNi₅, CeCo₃, NdNi₃, GdFe₃, DyCo₃, and ErNi₃ have been synthesized at a hydrogen pressure of 10 MPa and a temperature of 273 K. The phase composition of the synthesized materials and the lattice parameters of the hydride phases have been determined by X-ray diffraction. During storage in air at room temperature, the hydrides decompose more slowly than do their analogs synthesized at low pressure. The hydrogen content of the hydrides is higher than or similar to that of hydride phases synthesized at high pressure. X-ray diffraction results for the low-temperature RT₃-based intermetallic hydrides demonstrate that their lattice is expanded to a lesser extent than that of their high-pressure analogs.     

Kinetics of hydrogen liberation from stoichiometric and nonstoichiometric magnesium hydride

We have carried out a systematic study of the kinetics of decomposition of stoichiometric and nonstoichiometric magnesium hydride. Using the thermal-desorption methods in barometric modification and mathematical modeling, we have shown that hydrogen desorption from stoichiometric MgH₂ proceeds in two stages: the formation of nuclei of the metal phase and hydrogen liberation through surface islands of the metal phase under the limiting influence of desorption rate. In the course of hydrogen liberation from partially hydrogenated magnesium, the first stage is usually absent. We have established that the influence of other reactions on the overall degasification rate is at least much smaller than the effect of desorption from the surface of α-Mg. We have obtained estimates of the parameters affecting the desorption kinetics. Finally, we have shown that, for analysis of the kinetics of hydrogen liberation from MgH₂, the choice of physically substantiated models has serious advantages as compared with the Avrami-Erofeev approach. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 25–35, September–October, 2007.     

Prediction of enthalpy of formation and Gibbs energy change in pseudo-binary (Ti–Zr)(Fe–Cr)<Subscript>2</Subscript> and pseudo-ternary (Ti–Zr)(Fe–Cr)<Subscript>2</Subscript>-H system using extended Miedema model

The thermodynamic model proposed by Miedema is capable of predicting the enthalpy of formation (ΔH) and relative stability of phases in binary but not in ternary or multi-component systems. While developing nanocrystalline binary/ternary metal hydrides for compressor-driven reversible heating–cooling applications, it is necessary to identify appropriate alloy compositions with suitable hydrogen storage capacity and reversible hydrogen absorption–desorption capability. Accordingly, a suitable modification of the Miedema model is proposed in the present study for calculating ΔH of AB₂ type of pseudo-binary (Ti–Zr)(Fe–Cr)₂ and pseudo-ternary (Ti–Zr)(Fe–Cr)₂-H alloys. Subsequently, Gibbs energy (ΔG) of the possible phases is estimated to predict relative phase stability/equilibrium in a given system. It is shown that grain size or interfacial energy contribution exerts a significant influence on ΔG and relative stability of the phases beyond a critical value/limit. Finally, the predicted phase equilibrium from this model-based calculation is validated by suitable comparison with relevant experimental data reported in the literature.     

Hydrogen-enhanced stress-corrosion cracking of aluminum alloys

We present the results of the investigation of Al-Zn-Mg and Al-Zn-Cu-Mg alloys in NaCl solutions at low strain rates. The contribution of hydrogen to the process of stress-corrosion cracking is analyzed by taking into account the influence of the admixtures of arsenic trioxide and residual hydrogen (remaining after the processes of release and cathodic polarization) on the susceptibility of metals to this kind of cracking. A mechanism of hydrogen-assisted stress-corrosion cracking taking into account the time dependence of the microstructure of grain boundaries, concentration of hydrogen, and its distribution is suggested on the basis of the concept of critical concentration of hydrogen. Technical University of Gdansk, Poland. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 20–26, July–August, 1998.     

Interaction of ZrFe<Subscript>2</Subscript> doped with Ti and Al with hydrogen

The influence of doping with Ti and Al on the structure and hydrogen sorption properties of ZrFe₂ was studied by XRD, XRSMA, and measurement of hydrogen absorption and desorption isotherms at pressure up to 300 MPa. The hydrogen capacity and equilibrium desorption pressures of hydrides decrease with increasing Al content at a constant ratio of Ti and Zr. The increase in the Ti content at a constant content of Al in alloys also leads to a decrease in hydrogen capacity; however, the equilibrium desorption pressures of hydrides increase considerably. Zr_1−x Ti _x (Fe_1−y Al _y )₂ (x= 0.2–0.8; y = 0.05–0.4) alloys were investigated.     

V系储氢合金及其合金化

概述了V系储氢合金的研究现状,涉及V系储氢合金的氢化物相结构、合金化元素及第二相对合金吸、放氢性能的影响:V系储氢合金随吸氢量的增加,氢化物结构发生bcc→bct→fcc转变,同时其稳定性呈降低趋势;合金元素通过改变V与H的亲和力以及氢化物的稳定性来影响合金的储氢性能;第二相的出现对合金电化学性能、吸放氢动力学有明显的影响作用.在上述分析的基础上,对V系bcc固溶体储氢合金今后的研究进行了展望.     

An electron metallographic study of pressure die-cast commercial zinc–aluminium-based alloy ZA27

The microstructure of ZA27 pressure die-castings was examined by scanning and transmission electron microscopy after ageing for 5 years at ambient temperatures. Solidification began with the formation of compact aluminium-rich α′ dendrites and tiny rounded α′ particles, followed by the peritectic reaction whereby a zinc-rich β phase formed around the edges of the primary phases. The extremely high cooling rate during solidification reduced the extent of the peritectic reaction so that the liquid became highly enriched with zinc and solidification was completed by eutectic formation of β and η phases, the β joining the peritectic β and the η remaining in the interdendritic regions. On rapid cooling after casting through the eutectoid transformation temperature, the β phase decomposed eutectoidally into well-formed lamellae or semi-particulate irregular particles of α and η, and some lamellar colonies spread into the low-aluminium α′-phase cores of the dendrites to form coarse lamellar products. The bulk of the α′, however, decomposed into a very fine mixture of zinc-rich phases in an aluminium matrix. These structures are consistent with solidification under conditions of high undercooling. Enclosed within the α constituent of the decomposed peritectic and eutectic β phases were small particles of a phase which was identified as the transitional α′m phase containing 30.2%Al or 14.8%Al, with an fcc crystal structure and lattice parameter (at 14.8%Al) of about 0.395 nm. It had a symmetrical cube/cube orientation relationship with the surrounding α phase. This metastable phase was probably stabilized by copper. Copper became concentrated in the eutectic liquid during the first stages of solidification, and was rejected from the liquid in the form of discrete irregular particles, 1–2 μm in diameter, during eutectic solidification. After solidification, copper was also rejected from solid solution in the zinc-rich η phase in the form of a dense precipitation of small particles of 70–120 nm diameter and 2–3 nm thick. Both of these particles were identified as the metastable cph ε-phase (CuZn4) with lattice parameters a = 0.274 nm, c = 0.429 nm, and c/a = 1.566. This revised version was published online in November 2006 with corrections to the Cover Date.     

Structure and physicomechanical properties of eutectic Ti-Si-X alloys

We study the structure and physicomechanical properties of various eutectic alloys of Ti-Si-Zr, Ti-Si-B, and Ti-Si-Ga systems. It is shown that Ti-Si-Zr alloys with elevated concentrations of Zr reveal, due to the presence of (Ti, Zr)₂ Si dispersed silicides with sizes of about several hundred nanometers, improved mechanical properties as compared with the properties of alloys based on Ti₅Si₃ silicides. The cast eutectic alloy of the Ti-Si-Zr-Sn system with a plasticity of ∼ 1.7% is obtained for the first time. The formation of superfine eutectics based on the Ti₆Si₂B ternary compound in alloys of the Ti-Si-B system enables one to obtain titanium composites with improved refractory properties and elevated moduli of elasticity (of about 150 GPa or, after additional alloying, as high as 165 GPa). This can be promising for the development of new refractory titanium composite materials with elevated stiffness. The analysis of the combined effect of gallium and silicon in Ti-Si-Ga alloys reveals the possibility of getting titanium materials with high heat resistance, i.e., materials based on the (α-Ti(α₂-Ti₃Ga) + Ti₅ (Si, Ga)₃ binary eutectics. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 35–42, May–June, 2008.     

Processing of a porous titanium alloy from elemental powders using a solid state isothermal foaming technique

The authors have conducted a preliminary investigation with regard to the potential to manufacture porous titanium alloys for biomedical applications using toxic-free elemental powders, i.e., Ti, Nb, Ta, Zr, in combination with the pressurised gas bubble entrapment method and in contrast to standard processing routes that generally utilise prealloyed powder containing potentially toxic elements. Elemental powder compacts were either hot isostatic pressed (HIP-ed) at 1000°C and then foamed at 1150°C or else HIP-ed at 1100°C and foamed at 1350°C. Porous α + β alloys containing up to 45 vol% of porosity in the size range 20–200 μm were successfully produced, thus highlighting the potential of this manufacturing route. It was expected that further optimisation of the processing route would allow full development of the preferred β-Ti phase (from the point of view of elastic modulus compatibility between implant and bone) with this being the subject of future work by the authors.     

Developing aluminium–zinc-based a new alloy for tribological applications

In order to develop aluminium–zinc-based a new alloy for tribological applications, six binary Al–Zn and seven ternary Al–25Zn–(1–5)Cu were prepared by permanent mould casting. Their microstructure and mechanical properties were investigated. Dry sliding friction and wear properties of the ternary alloys were investigated using a pin-on-disc machine. Surface and subsurface regions of the wear samples were studied with scanning electron microscopy (SEM). The highest hardness and tensile strength were obtained with the Al–25Zn alloy among the binary ones. The microstructure of this alloy consisted of aluminium-rich α and eutectoid α + η phases. Addition of copper to this alloy resulted in the formation of θ (CuAl₂) phase. The hardness of the ternary alloys increased with increasing copper content. The highest tensile and compressive strengths and wear resistance and the lowest friction coefficient were obtained from the ternary Al–25Zn–3Cu alloy. The dimensional change measured on ageing (stabilization) of this alloy was found to be much lower than that obtained from the copper containing zinc-based alloys. Microstructural changes were observed below the surface of the wear samples of the Al–25Zn–3Cu alloy. These changes were related to the heavy deformation of the surface material due to normal and frictional forces, and smearing and oxidation of wear material. Adhesion was found to be the main wear mechanism for the alloys tested.     

The solidification process of Al–Mg–Si alloys

The microstructure and solidification process of three Al–Mg–Si alloys with different magnesium contents have been studied using optical microscopy and the electron probe X-ray microanalysis. The results showed that Al–Mg–Si alloys possessed fairly complicated solidification path: L→α-Al+L1→α-Al+Al15Si2(FeMn)3+L2→α-Al+Al15Si2(FeMn)3+ (α-Al+Mg2Si)+L3→α-Al+Al15Si2(FeMn)3+(α-Al+Mg2Si)+(α-Al+Mg2Si+Al15Si2(FeMn)3), and wide solidification temperature of 75 °C. The magnesium content in the alloys greatly influenced the as-cast microstructure. The higher the magnesium content, the more Mg2Si structure was present. Iron and manganese segregated to the finally solidified zone, which resulted in the formation of ternary eutectic structure. Although their content in the alloys was very low, their effect on solidification behaviour cannot be ignored. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of hydrogen on the low-temperature mechanical properties of V-5 at % Ti alloys

The effects of grain size and hydrogen in solid solution or as hydrides on the strength and ductility of V-5 at % Ti was studied over the temperature range 15–448 K. Comparison of the strength and ductility characteristics of hydrogenated alloys where hydrides were not observed down to 78 K (1.8 and 1.9 at % H alloys) or where hydrides were observed to form near 230 K (3.8 and 3.9 at % H alloys) indicated that the presence of hydride precipitates had no apparent influence on the strength or ductility characteristics. It appears that the main consequence of hydride precipitation is that hydrogen is removed from solid solution making strengthening less effective than expected based on the total hydrogen content. Decreasing grain size from 31 m to 8 m had no apparent effect on ductility in the nonhydrogenated alloys (< 0.05 at % H) but it did increase the strength over most of the temperature range and especially at 15 K. In the hydrogenated alloys this decrease in grain size lowered the transition temperature about 10 K and it appreciably increased the degree of ductility return at 78 K and below. The ductility return below 78 K peaked near 50 K before decreasing below 30 K with the improvement in ductility being greatest in the alloys with the lower hydrogen contents.