Hydrogen sorption properties of 90 wt% MgH2–10 wt% MeSi2 (Me = Ti, Cr)

The hydriding/dehydriding characteristics of 90 wt% MgH₂–10 wt% MeSi₂ (Me = Ti, Cr) composites, synthesized by ball milling under argon, were studied by Sievert’s type apparatus and by high-pressure differential scanning calorimetry (HPDSC). The composites have demonstrated similar absorption kinetics and capacity at temperature of 300 °C and a pressure of 1 MPa. They showed fast kinetics and achieved absorption capacity higher than 6 wt%. Slightly higher values for the enthalpy of hydriding and dehydriding were obtained at scanning conditions in the HPDSC for the composite 90 wt% MgH₂–10 wt% CrSi₂ compared to those for the composite with addition of TiSi₂.     

Effects of Mo on microstructure of as-cast 28 wt.% Cr–2.6 wt.% C–(0–10) wt.% Mo irons

Microstructures of as-cast 28 wt.% Cr–2.6 wt.% C irons containing (0–10) wt.% Mo with the Cr/C ratio of about 10 were studied and related to hardness. The experimental irons were cast into dry sand molds. Microstructural investigation was performed by light microscopy, X-ray diffractometry, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectrometry. It was found that the iron with about 10 wt.% Mo was eutectic/peritectic, whereas the others with less Mo content were hypoeutectic. The matrix in all irons was austenite, partly transformed to martensite during cooling. Mo addition promoted the formation of M₂₃C₆ and M₆C. At 1 wt.% Mo, multiple eutectic carbides including M₇C₃, M₂₃C₆ and M₆C were observed. M₂₃C₆ existed as a transition zone between eutectic M₇C₃ and M₆C, indicating a carbide transition as M₇C₃(M_2.3C) → M₂₃C₆(M_3.8C) → M₆C. At 6 wt.% Mo, multiple eutectic carbides including M₇C₃ and M₂₃C₆ were observed together with fine cellular/lamellar M₆C aggregates. In the iron with 10 wt.% Mo, only eutectic/peritectic M₂₃C₆ and M₆C were found without M₇C₃. Mo distribution to all carbides has been determined to be increased from ca. 0.4 to 0.7 in mass fraction as the Mo content in the irons was increased. On the other hand, Cr distribution to all carbides is quite constant as ca. 0.6 in mass fraction. Mo addition increased Vickers macro-hardness of the irons from 495 up to 674 HV₃₀. High Mo content as solid-solution in the matrix and the formation of M₆C or M₂₃C₆ aggregates were the main reasons for hardness increase, indicating potentially improved wear performance of the irons with Mo addition.     

Effect of room-temperature aging on shape memory characteristics of Ti–10Nb–10Zr–11Ta (at.%) alloy

In this study, the effect of room-temperature (RT) aging on the shape memory characteristics of the Ti–10Nb–10Zr–11Ta (at.%) alloy was investigated by tensile tests. Ingots were prepared using the arc melting method and then cold-rolled at a reduction of up to 95%. After cold-rolling, the plates were solution-treated at 1173 K for 1.8 ks, followed by aging at RT and temperatures up to ∼573 K for various periods of times. Superior superelasticity was observed at RT in the solution-treated specimen. The critical stress for inducing martensitic transformation (σ_SIM), tensile strength, and critical stress for slip (σ_S) of specimens aged at RT increased with increasing aging time up to 60 days, showing no noticeable changes with further increases in the aging time. On the other hand, in the specimens aged at 373 K, 423 K, and 473 K for 3.6 ks, the values of σ_SIM and the tensile strength increased with increasing aging temperature, while the specimen aged at 573 K exhibited mature fractures. There were little change in σ_SIM and σ_S of the specimen that was solution-treated followed by aging at 373 K for 3.6 ks during RT aging. This result indicated that aging at 373 K resulted in good resistance against the effect of RT aging.     

The effect of annealing on the deformation behaviour and microstructure of crystallized Mg–23.5Ni (wt.%) alloy

Rapidly solidified amorphous Mg–23.5Ni (wt.%) ribbons were crystallized at 300 and 400 °C for 90 min. After annealing at 300 °C the microstructure was heterogeneous, consisting of rounded eutectic–lamellar domains, which contained magnesium grains smaller than 500 nm. In the case of ribbons annealed at 400 °C the microstructure, however, was homogenous, and composed of well-formed magnesium grains and Mg₂Ni particles. At room temperature both crystallized materials were brittle due to the high volume fraction of Mg₂Ni particles, but they exhibited some ductility with increasing test temperature. Above 200 °C, the microstructure of the ribbons annealed at 300 °C was characterised by the formation of particle free zones during the tensile test. This structure was not observed in the material annealed at 400 °C. Deformation behaviour and changes in the microstructure during plastic flow of both crystallized materials were explained according to grain boundary sliding mechanisms.     

Effect of TiB2 content on the characteristics of spark plasma sintered Ti–TiBw composites

Spark plasma sintering (SPS) was employed to fabricate monolithic titanium and in-situ formed TiB whisker (TiB_w) reinforced titanium matrix composites (TMCs) by adding different amounts of TiB₂ as boron source. The sintering process was completed at 1050 °C for 5 min under 50 MPa. The influences of TiB₂ content (0.6–9.6 wt. %) on microstructural evolution and mechanical properties of TMCs were investigated. Thermodynamics, XRD analysis and microstructural investigations confirmed the in-situ formation of TiB_w in the composite samples. However, some semi-reacted TiB₂ phases, surrounded by TiB coronas, were remained in the microstructure due to the unfinished chemical reaction between the components during a short-time sintering process. The results showed that all samples were appropriately densified by SPS process into the almost dense parts with relative density no less than 97.5%. While bending strength decreased and hardness increased with increasing TiB₂ content, the sample with 4.8 wt. % TiB₂ had the maximum tensile strength. Fractographical assessments showed that the addition of TiB₂ hindered the grain growth of titanium matrix. With increasing TiB₂ content, fracture mode changed from a multiple pattern to a predominantly transgranular and brittle state.     

Prediction of the mechanical properties of forged Ti–10V–2Fe–3Al titanium alloy using FNN

In this paper, a fuzzy neural network (FNN) prediction model has been employed to establish the relationship between processing parameters and mechanical properties of Ti–10V–2Fe–3Al titanium alloy. In establishing these relationships, deformation temperature, degree of deformation, solution temperature and aging temperature are entered as input variables while the ultimate tensile strength, yield strength, elongation and area reduction are used as outputs, respectively. After the training process of the network, the accuracy of fuzzy model was tested by the test samples and compared with regression method. The obtained results with fuzzy neural network show that the predicted results are much better agreement with the experimental results than regression method and the maximum relative error is less than 7%. And the optimum matching processing parameters can be quickly selected to achieve the desired mechanical property based on the fuzzy model. It proved that the model has a good precision and excellent ability of predicting.     

Effect of hot extrusion on wear properties of Al–15 wt.% Mg2Si in situ metal matrix composites

Al–15 wt.% Mg₂Si composites were prepared by in situ casting and characterized in wear tests. Previous to the extrusion of specimens at 470 °C – varying extrusion ratio (7.4, 14.1 and 25), the as-cast composites were homogenized at 500 °C for 5 h, followed by slow furnace cooling. The microstructure, hardness and sliding wear behavior were characterized for both, the as-cast and hot extruded composites. Results show that increasing the extrusion ratio causes a significant improvement in hardness and wear resistance. This is ascribed to the observed decrease in average size and better distribution of Mg₂Si particles, in tandem with a remarkable decrease in porosity percentages, which goes from 5.63 in the as-cast condition, to 0.47 at the extrusion ratio of 25. It was found that abrasion is the dominant wear mechanism in all extruded composites, whilst a combination of adhesion and delamination appears to be the governing mechanism for as-cast composites.     

Hyperfine interactions and structural features of Fe–44Co–6Mo (wt.%) nanostructured powders

Nanocrystalline Fe–44Co–6Mo (wt.%) powders have been prepared by high-energy ball milling from elemental Fe, Co and Mo pure powders in a P7 planetary ball mill. The obtained powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Mössbauer spectrometry techniques. The influence of milling process and Mo substitution for Co in equiatomic FeCo have been examined in order to study structural evolution and formation mechanism of nanostructured Fe(CoMo) solid solution. XRD results show the formation of a BCC Fe(CoMo) solid solution (a = 0.2874 nm) where unmixed nanocrystalline Mo with a BCC structure is embedded. Disordered Fe(CoMo) solid solution is characterized by a broad hyperfine magnetic field distribution with two regions centered at B₁ = 35.0 T and B₂ = 30.7 T, respectively, attributed to disordered Fe(Co) solid solution and CoMo enriched environments. Prolonged milling and Mo addition cause the decrease of average hyperfine magnetic field while the average isomer shift remains nearly constant.     

Strain rate dependence of the indentation size effect in Ti–10V–2Fe–3Al alloy

The influence of strain rates on the indentation size effect (ISE) was explored experimentally. A strong ISE phenomenon on the hardness of Ti–10V–2Fe–3Al (Ti-1023) alloy was found when the peak-load was less than 3500?mN, regardless of the variation of loading rates. However, as indenter strain rates increased, the degree of ISE reduced considerably, which was related to the decrease of the internal indentation length scale ranging from 23.08 to 6.80?µm. Furthermore, a positive strain rate sensitivity of the hardness in the whole peak-load range was found in Ti-1023 alloy, which showed a linear function of indenter load. The underlying mechanism was well explained by the variation of geometrically necessary dislocation and statistically stored dislocations in Ti-1023 alloy.     

Characterization of phases in a Mg–6Gd–4Sm–0.4Zr (wt.%) alloy during solution treatment

Phases in as-cast and solution-treated Mg–6Gd–4Sm–0.4Zr (wt.%) alloy have been characterized using transmission electron microscopy in this paper. The intermetallic phase in as-cast microstructure has a face centered cubic crystal structure (a = 2.2879 nm) with a composition of Mg_6.2(Sm_0.56Gd_0.44) and was dissolved after solution treatment. A particulate phase with a face centered cubic crystal structure (a = 0.5502 nm) was found in the solution-treated microstructure and suggested to already exist in as-cast sample as the nucleus for its further growth during solution treatment.     

The effect of alpha phase on flow softening and deformation of Ti–10V–2Fe–3Al

The subtransus deformation behaviour of Ti–10V–2Fe–3Al alloy was investigated. The flow stress curves are featured with significant flow softening, resulting from the breaking up and globularisation of lamellar α phase. The critical major axis of α phase to cause flow softening is 1?µm. Alpha phase acts as a dislocation barrier in the beginning and suppresses the recrystallisation of beta matrix. Dynamic recovery is the main restoration mode for beta matrix and alpha phase just works as a marker of it. Beta phase is dominated by strong {001} and weak {111} orientations. The Burgers relationship and variant selection are observed between α phase and β matrix in a single grain and overall samples, and the subtransus deformation does not destroy it.     

Wear properties of Ti–13Zr–13Nb (wt.%) near β titanium alloy containing 0.5 wt.% boron in dry condition,Hank's solution and bovine serum

The effect of heat treatment on the microstructure, hardness and sliding wear behaviour of Ti–13Zr–13Nb (wt.%) containing 0.5 wt.% B (TZNB) has been studied and compared with that of Ti–13Zr–13Nb (wt.%) (TZN) alloy. The wear properties were tested in dry condition and in simulated body fluid (Hank's solution and bovine serum) to understand the effect of different medium on wear behaviour of the TZNB alloy. Depending on the heat treatment condition the microstructure of the alloy consisted of α/martensite and TiB in β matrix. In general, the hardness of all the heat treated samples varied in a narrow range and in most of the cases addition of boron to the TZN alloy decreased the hardness. Almost all cases, no significant variation of the wear rate in dry condition with heat treatment was observed. Compared with the wear rate in dry condition, the wear rate in Hank's solution of the all the TZNB samples increased substantially. Moreover, the wear was found to be most severe in bovine serum. Addition of boron to TZN alloy did not result in any improvement in the wear resistance in all the media studied.     

Corrosion characteristics of anodized Ti–(10–40wt%)Hf alloys for metallic biomaterials use

The effect of anodizing on corrosion resistance of Ti–xHf alloys has been investigated. Ti–xHf alloys were prepared and anodized at 120, 170 and 220 V in 1 M H₃PO₄ solution, and crystallized at 300 and 500°C. Corrosion experiments were carried out using a potentiostat in 0.15 M NaCl solution at 36.5 ± 1°C. The Ti–xHf alloys exhibited the α′ and anatase phases. The pore size on the anodized surface increases as the applied voltage is increased, whereas the pore size decreases as the Hf content is increased. The anodized Ti–xHf alloys exhibited better corrosion resistance than non-anodized Ti–xHf alloys.     

Hot deformation mechanisms in metastable beta titanium alloy Ti–10V–2Fe–3Al

Abstract

The mechanisms of hot deformation in the β titanium alloy Ti–10V–2Fe–3Al have been characterised in the temperature range 650–850°C and strain rate range 0·001–100 s^-1 using constant true strain rate isothermal compression tests. The β transus for this alloy is ～790°C, below which the alloy has a fine grained duplex +β structure. At temperatures lower than the β transus and lower strain rates, the alloy exhibits steady state flow behaviour while at higher strain rates, either continuous flow softening or oscillations are observed at lower or higher temperatures, respectively. The processing maps reveal three different domains. First, in the temperature range 650–750°C and at strain rates lower than 0·01 s^-1, the material exhibits fine grained superplasticity marked by abnormal elongation, with a peak at ～700°C. Under conditions within this domain, the stress–strain curves are of the steady state type. The apparent activation energy estimated in the domain of fine grained superplasticity is ～225 kJ mol^-1, which suggests that dynamic recovery in the β phase is the mechanism by which the stress concentration at the triple junctions is accommodated. Second, at temperatures higher than 800°C and strain rates lower than ～0.1 s^-1, the alloy exhibits large grained superplasticity, with the highest elongation occurring at 850°C and 0.001 s^-1; the value of this is about one-half of that recorded at 700°C. The microstructure of the specimen deformed under conditions in this domain shows stable subgrain structures within large β grains. Third, at strain rates higher than 10 s^-1 and temperatures lower than 700°C, cracking occurs in the regions of adiabatic shear bands. Also, at strain rates above 3 s^-1 and temperatures above 700°C, the material exhibits flow localisation.     

Microstructure and notched tensile fracture of Ti–6Al–4V to Ti–4.5Al–3V–2Fe–2Mo dissimilar welds

Dissimilar welding of Ti–6Al–4V (Ti-6-4) to Ti–4.5A1–3V–2Fe–2Mo (SP-700) alloys was performed using a CO₂ laser. The microstructure and notched tensile strength (NTS) of the dissimilar welds were investigated in the as-welded and post-weld heat treatment (PWHT) conditions. Moreover, the results were compared with homogeneous laser welds with the same PWHT. The dilution of SP-700 with the Ti-6-4 alloy caused the formation of fine needle-like α + β structures, resulting in the exhibition of a moderately high fusion zone (FZ) hardness of HV 398. The high FZ hardness (HV 438) for the weld with the PWHT at 482 °C was associated with low NTS or high notch brittleness. The fracture appearance of the notched tensile specimen was related to its inherent microstructure. With increasing the PWHT temperature, the thickness of grain boundary α increased, which promoted an intergranular dimple fracture. By contrast, fine shallow dimples were present in the peak-aged weld, which was induced by the refined α + β microstructures in the basket-weave form.     

Low-frequency (10–50 kHz) impedance of polystyrene-onion-like-carbon composites

The impedance of polystyrene-onion-like-carbon (PS-OLC) composites in the low-frequency (10–50 kHz) range has been studied as a function of the OLC weight fraction in the material. The composites were fabricated by rolling of PS filled with OLC powder obtained through the annealing of detonation nanodiamonds at 2140 K. The homogeneity of OLC distribution in the PS matrix has been studied as dependent on the number of rolling stages. It is established that the percolation threshold in PS-OLC composites is achieved at an OLC content of 35–40 wt %.     

Temperature compensating microwave dielectric based on the (Mg0.97Co0.03)2(Ti0.95Sn0.05)O4–CaTiO3 ceramic system

The microstructures and the microwave dielectric properties of the (1 ? x)(Mg_0.97Co_0.03)₂(Ti_0.95Sn_0.05)O₄–xCaTiO₃ ceramic system prepared by the conventional solid-state route were investigated. (Mg_0.97Co_0.03)₂(Ti_0.95Sn_0.05)O₄ possesses high dielectric constant (ε_r = 14.23), high quality factor (Q × f = 188,760 GHz), and negative τ_f value (τ_f = ?55.48 ppm/°C) at 1,390 °C for 4 h. In order to achieve a temperature stable material, CaTiO₃, having a large positive τ_f value of 800 ppm/°C, was added to (Mg_0.97Co_0.03)₂(Ti_0.95Sn_0.05)O₄. Two phase system was confirmed by the X-ray diffraction patterns and the energy-dispersive X-ray analysis. Although the ε_r of the specimen could be boosted by increasing amount of CaTiO₃, it would instead render a decrease in the Q × f. By appropriately adjusting the x value in the (1 ? x)(Mg_0.97Co_0.03)₂(Ti_0.95Sn_0.05)O₄–xCaTiO₃ ceramic system, zero τ_f value can be achieved. A new microwave dielectric material, 0.91(Mg_0.97Co_0.03)₂(Ti_0.95Sn_0.05)O₄–0.09CaTiO₃ ceramic sintered at 1,390 °C had optimal dielectric properties (ε_r = 18.13, Q × f = 87,562 GHz, τ_f = 3.75 ppm/°C) which satisfied microwave applications in resonators, filters and antenna substrates.