A study on preparation of Mo–0.6Ti–0.2Zr–0.02C alloy by mechanical alloying and hot isostatic pressing,and its characterization

Mo–0.6Ti–0.2Zr–0.1C alloy was prepared by mechanical alloying (MA) and subsequently consolidated by powder processing techniques. The pellets prepared from the fine size MA powder showed a high rate of densification during sintering in the temperature range of 1300–1500 °C. Close to theoretical density was attained by hot isostatic pressing (HIP) at 1250 °C and TEM studies revealed the uniform distribution of complex carbide precipitates (<100 nm) in the fine grain microstructure of the consolidated alloy. The alloy consolidated by HIP showed a high hardness of the order of 500 HK due to the presence of the carbides in the fine grain microstructure.     

Fabrication and evaluation of oxidation resistance performance of open-celled Fe(Al) foam by space-holder technique

First, fabrication of Fe(Al) open-cell foams by using space-holder technique, in which NaCl powder and polyester resin were utilized as space holder and binder, was investigated. Fe(Al) powder were prepared via mechanical alloying method after 5 h milling process. Then, the influences of fabrication parameters, alloying element Al content, time of milling process, sintering temperature and soaked time on the oxidation resistance performance of foam specimens at 800 °C were mainly studied. The experimental results exhibit that alloying with 2 wt.% Al resulted in acceleration of the oxidation rate of foam samples, additionally, alloying with 4 wt.% Al had a little positive effect on oxidation rate at 800 °C. However, the oxidation resistance of the 10 wt.% Al-containing sample greatly improves compared to pure Fe. In addition, the oxidation resistance performance of prepared foam was improved by increasing the milling time up to 15 h, and followed by a decrease gradually. The increase of sintering temperature and the prolongation of soaked time to 1100 °C and 90 min, respectively had significant positive effect on the oxidation resistance.     

Microstructural and mechanical characterisation of 7075 aluminium alloy consolidated from a premixed powder by cold compaction and hot extrusion

The present work concerns the processing of 7075 Al alloy by cold compaction and hot extrusion of a premixed powder. To this end, a premixed Al–Zn–Mg–Cu powder, Alumix 431D, was uniaxially cold pressed at 600 MPa into cylindrical compacts 25 mm in diameter and 15 mm thick. Subsequently, selected green compacts were subjected to either a delubrication or presintering heat treatment. Extrusion of the powder compacts was performed at 425 °C using an extrusion ratio of 25:1. No porosity was present in the microstructures of the extruded alloys. Heat treatment prior to extrusion had a great effect on the degree of alloy development in powder compacts and, as a direct consequence, remarkably affected the extrusion process and the as-extruded microstructures and mechanical properties of the processed materials. Hot extrusion caused banded structures for the alloys consolidated from the green and delubricated powder compacts. The alloy extruded from the presintered powder compact showed a fine, recrystallized microstructure which resulted in a superior combination of mechanical properties for the consolidated material.     

Ultra-high-temperature tensile properties and fracture behavior of ZrB2-based ceramics in air above 1500 °C

Nearly fully dense ZrB₂–SiC–graphite composites were fabricated from commercially available powder at 1900 °C by hot pressing. The tensile strength of ZrB₂-based ceramics was measured in air up to 1750 °C, which is the first reported tensile strength measurement in air above 1500 °C. A mechanical testing apparatus capable of testing material in ultra-high temperature under air atmosphere was built, evaluated, and used. Tensile strength was measured as a function of temperature up to 1750 °C in air. The respective average values of the tensile strength measured at 1550 °C, 1650 °C, and 1750 °C are 58.4, 44.8, and 21.8 MPa, which are 49.4%, 37.9%, and 18.4% of their room-temperature strength (118.2 MPa), respectively. Moreover, the tensile fracture behaviors and mechanism of ZrB₂-based ceramics at different testing temperatures were discussed based on microstructure characterization.     

Development of NiFe-CNT and Ni3Fe-CNT nanocomposites by mechanical alloying

NiFe-CNT and Ni₃Fe-CNT nanocomposites were fabricated by high energy mechanical alloying method. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and optical microscopy were employed for evolution of phase composition, morphology and microstructure of the powder particles. Ball milled powders were heat treated at 500 °C for 1 h to release the milling induced stresses. Bulk samples were prepared by sintering of cold pressed (300 MPa) samples at 1040 °C for 1 h. XRD patterns of powders, as-milled and after annealing at 500 °C did not show any peak related to CNTs or excess phases due to the interaction between CNTs and matrix. SEM micrographs showed that the addition of CNTs caused a reduction of powder particles size. The hardness value of as-milled NiFe and Ni₃Fe powders reach to 660 and 720 HV, respectively. According to optical microscopy evaluations, the amount and size of the porosities of the composites bulk samples decreased in comparison with matrix ones.     

Nanocrystalline Al/Al12(Fe,V)3Si alloy prepared by mechanical alloying: Synthesis and thermodynamic analysis

Al–Al₁₂(Fe,V)₃Si nanocrystalline alloy was fabricated by mechanical alloying (MA) of Al–11.6Fe–1.3V–2.3Si (wt.%) powder mixture followed by a suitable subsequent annealing process. Structural changes of powder particles during the MA were investigated by X-ray diffraction (XRD). Microstructure of powder particles were characterized using scanning electron microscopy (SEM). Differential scanning calorimeter (DSC) was used to study thermal behavior of the as-milled product. A thermodynamic analysis of the process was performed using the extended Miedema model. This analysis showed that in the Al–11.6Fe–1.3V–2.3Si powder mixture, the thermodynamic driving force for solid solution formation is greater than that for amorphous phase formation. XRD results showed that no intermetallic phase is formed by MA alone. Microstructure of the powder after 60 h of MA consisted of a nanostructured Al-based solid solution, with a crystallite size of 19 nm. After annealing of the as-milled powder at 550 °C for 30 min, the Al₁₂(Fe,V)₃Si intermetallic phase precipitated in the Al matrix. The final alloy obtained by MA and subsequent annealing had a crystallite size of 49 nm and showed a high microhardness value of 249 HV which is higher than that reported for similar alloy obtained by melt spinning and subsequent milling.     

High temperature tensile deformation behavior and failure mechanisms of an Al–Si–Cu–Mg cast alloy — The microstructural scale effect

In this study the high temperature tensile deformation behavior of a commercial Al–Si–Cu–Mg cast alloy was investigated. The alloy was cast with two different cooling rates which resulted in average secondary dendrite arm spacing of 10 and 25 μm, which is typical of the microstructure scale obtained from high pressure die casting and gravity die casting. Tensile tests were performed at different strain rates (10^− 4 s^− 1 to 10^− 1 s^− 1) and over a wide temperature range from ambient temperature to 500 °C. The fine microstructure had superior tensile strength and ductility compared to the coarse microstructure at any given temperature. The coarse microstructure showed brittle fracture up to 300 °C; the fracture mode in the fine microstructure was fully ductile above 200 °C. The fraction of damaged particles was increased by raising the temperature and/or by microstructure coarsening. Cracks arising from damaged particles in the coarse microstructure were linked in a transgranular-dominated fashion even at 500 °C. However, in the fine microstructure alloy the inter-dendritic fracture path was more prevalent. When the temperature was raised to 300 °C, the concentration of alloying elements in the dendrites changed. The dissolution rates of Cu- and Mg-bearing phases were higher in the fine microstructure.     

Structure and mechanical properties of the annealed TZAV-30 alloy

The Ti–30Zr–5Al–3V (wt.%, TZAV-30) alloy having good mechanical properties is a potential structural material to apply in the aerospace industry. The microstructure and mechanical properties of ZTAV-30 alloy underwent various annealing heat treatments were investigated. The specimens annealed from 500 to 800 °C are composed of α and β two phases. No compound is detected in specimens annealed in that temperature range. The microstructure of annealed specimens is characterized as a typical basketweave microstructure. Three microstructural parameters, thickness of plate α phase, relative fraction of β phase and aspect ratio of α grains, were measured in those annealed specimens. As the alloy annealed in the range from 500 to 800 °C, the average thickness of plate α grains increases with the increasing annealing temperature from 500 to 700 °C but decreases while annealed at 800 °C. The fraction of retained β phase increases with annealing temperature. And the aspect ratio of plate α grains decreases firstly but increases while the annealing temperature is higher than 700 °C. As the variation of those three microstructural parameters, the strength of examined alloy varies from 1269 to 1355 MPa for tensile strength and from 1101 to 1190 MPa for yield strength, inversely, the elongation changes in the range from 12.7% to 8.4%. The strengthening and toughening mechanism of the TZAV-30 alloy with basketweave microstructure is also discussed in this paper.     

Microstructure,corrosion resistance and cytocompatibility of Mg–5Y–4Rare Earth–0.5Zr (WE54) alloy

Conventionally cast Mg–5Y–4Rare Earth–0.5Zr alloy (WE54) was solution treated (525 °C/8 h — T4) and one part subsequently aged (200 °C/16 h — T6). Powder from the cast WE54 alloy prepared by gas atomizing was consolidated by extrusion at 250 °C or 400 °C. Dense triangular arrangement of prismatic plates of transient D0₁₉ and C-base centered orthorhombic phases precipitated in the α-Mg matrix during the T6 treatment. Both alloys prepared by powder metallurgy exhibit similar microstructure consisting of ~ 4–6 μm α-Mg matrix fibers surrounded by particles of the equilibrium Mg₅(Y, Nd) phase and of oxides. Open circuit potential and polarization resistance in the isotonic saline (9 g/l NaCl/H₂O) were monitored for 24 h. The corrosion rate of the T4 and T6 treated alloys was about 80 times lower than that of commercial Mg. Both alloys prepared by powder metallurgy exhibited approximately 8 times higher corrosion resistance than commercial Mg. The human MG-63 osteoblast-like cells spreading and division in the extracts (0.28 g in 28 ml of EMEM) of all 4 alloys were monitored by cinemicrography for 24 h. The MG-63 cells proliferate without cytotoxicity in all extracts.     

Densification behavior,mechanical properties and thermal shock resistance of tungsten alloys fabricated at low temperature

The low-temperature shrinkage of tungsten was greatly accelerated by the addition of trace Nb and Ni, and the addition of trace Nb and Ni also significantly promoted the final sintering density. The 99.1% of theory density for W–0.1 wt.%Nb–0.1 wt.%Ni material sintered at 1600 °C was obviously greater than 93.7% of theory density for W material sintered at 2000 °C. Ball milling treatment played an important role in promoting the sintering densification of W–0.1 wt.%Nb–0.1 wt.%Ni powder, and the powder milled for 10 h (W10) could be sintered to near full density (99.4% of theory density) at 1600 °C. The ball milling for 15 h has no effect in improving the sintering density, but it induced rapid growth of tungsten grains. The microhardness and tensile strength of the sintered tungsten alloys were highly dependent on its sintering density and grain size. Improving the sintering density while controlling the grain growth could effectively promote the microhardness and tensile strength. Furthermore, the improvement of thermal shock resistance of the W10 alloy was due to good microstructure and the increase in the tensile strength.     

Nanocrystalline matrix TiC–Al3Ti and TiC–Al3Ti–Al composites produced by reactive hot-pressing of milled powders

Mechanically alloyed nanocrystalline TiC powder was short-term milled with 40 vol.% of Al powder. The powders mixture was consolidated at 1200 °C under the pressure of 4.8 GPa for 15 s and at 1000 °C under the pressure of 7.7 GPa for 180 s. The bulk materials were characterised by X-ray diffraction, light and scanning electron microscopy, energy dispersive spectroscopy, hardness, density and open porosity measurements. During the consolidation a reaction between TiC and Al occurred, yielding an Al₃Ti intermetallic. The microstructure of the produced composites consists of TiC areas surrounded by lamellae-like regions of Al₃Ti intermetallic (after consolidation at 1200 °C) or Al₃Ti and Al (after consolidation at 1000 °C). The mean crystallite size of TiC is 38 nm. The hardness of the TiC–Al₃Ti and TiC–Al₃Ti–Al composites is 13.28 GPa (1354 HV1) and 10.22 GPa (1041 HV1) respectively. The produced composites possess relatively high hardness and low density. The results obtained confirmed satisfactory quality of the consolidation with keeping a nanocrystalline structure of TiC.     

Mechanical properties and toughening mechanism of WC-doped ZrB2–ZrSi2 ceramic composites by hot pressing

WC-doped ZrB₂–ZrSi₂ ceramic composites were fabricated by hot pressing at temperatures ranging from 1450 °C to 1550 °C. The influence of ZrSi₂ content on the mechanical properties of the composites was investigated by means of three point bending test and single edge notched-beam test, respectively. The microstructure and phase composition were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis. The results revealed that: (i) the highest relative density was 99.5% for the composite fabricated at 1550 °C; (ii) the doping of WC refined the grain size and led to an anisotropic grain growth which was evidenced by the occurrence of elongated grains; (iii) the highest strength and fracture toughness were 585 MPa and 6.87 MPa m^1/2, respectively; (iv) the main toughening mechanism was considered as the pull out of elongated grains and the deflection of cracks.     

The synthesis and characterization of ultrafine grain NiAl intermetallic

The nanocrystalline NiAl powders were synthesized by mechanical alloying (MA), and the ultrafine grain NiAl bulk materials were subsequently consolidated by vacuum hot-pressed sintering. The microstructure and mechanical properties of milled powders and bulk materials were characterized. The results reveal that the NiAl powders were synthesized after 1.67 h of milling and the grains of NiAl were refined to 18 nm after 22 h of milling. During milling, the temperature rise caused by MA led to the annealing effect and consequently resulted in the abnormal decrease in microstrain and microhardness. NiAl bulk material with a relative density of 99.4% was prepared after sintering at 1300 °C and its grain size was about 400 nm. Due to fine-grain strengthening, the compressive stress and compressive strain of NiAl bulk material were significantly improved at room temperature.     

Effect of post weld heat treatment on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel weldments

The effect of postweld heat treatment (PWHT) on the microstructure and mechanical properties of ITER-grade 316LN austenitic stainless steel joints with ER316LMn filler material was investigated. PWHT aging was performed for 1 h at four different temperatures of 600 °C, 760 °C, 870 °C and 920 °C, respectively. The microstructure revealed the sigma phase precipitation occurred in the weld metals heat-treated at the temperature of 870 °C and 920 °C. The PWHT temperatures have the less effect on the tensile strength, and the maximum tensile strength of the joints is about 630 MPa, reaching the 95% of the base metal, whereas the elongation is enhanced with the rise of PWHT temperatures. Meanwhile, the sigma phase precipitation in the weld metals reduces the impact toughness.     

The influence of chemical composition and thermo-mechanical treatment on Ti–Nb–Ta–Zr alloys

Ti–Nb–Ta–Zr quaternary alloying system is very promising for biomedical alloys. It is due to good mechanical properties and corrosion resistance of titanium alloys. Moreover no potentially harmful elements are contained in this system.Mechanical properties were influenced by changing the chemical composition and by various heat-treatment operations. The alloys were prepared by arc melting and then they were hot forged (900–1000 °C). After solution treatment 850 °C/0.5 h/water quenched, cold swaging was carried out with section reduction about 85%. As final heat treatment aging at 450 °C/8 h/furnace cooling was used.Mechanical properties were measured from tensile tests results at cold swaged and aged specimens. The microstructure was observed by using light microscopy and transmission electron microscopy (TEM)-thin foils method. X-ray diffraction analysis reveals the phase composition. By using these techniques the changes in microstructure caused by precipitation during aging treatment were clarified. After aging, the presence of ω or α phases may occur. Influence of changing Zr and Ta contents on mechanical properties and also on precipitation of secondary phases during aging treatment was observed.     

Development of high performance powder metallurgy steels by high-energy milling

High-energy milling is considered to be one of the most efficient techniques for producing materials that have a well-controlled chemical composition and microstructural features that are difficult to obtain using other synthesis routes. In this study, the mechanical alloying technique (MA) was used to develop special powder metallurgy (PM) steels with two different types of properties. The use of this technique is essential for obtaining the target microstructure, which ensures the desired performance of the resulting material. Oxide dispersion strengthened (ODS) ferritic steels were produced using MA based on the prealloyed grade Fe–20Cr–5Al and Fe–14Cr–5Al–3 W steels with the addition of Ti and Y₂O₃ as reinforcements. The incorporation of Y₂O₃ enables a homogeneous dispersion of nano-oxides and nano-clusters in a submicron-grained structure that should enhance the mechanical properties up to 600 °C. In addition, the base alloying system, Fe–Cr–Al (Ti), should enable the development of protective oxide layers through high-temperature treatments, which improves the compatibility with the environment, avoids liquid–metal embrittlement and contributes to the increase of the mechanical response up to 600 °C. Furthermore, microalloyed powders can be obtained using MA and consolidated with a pressure-assisted sintering process in an attempt to control the final grain size, to achieve microalloyed steels, and to achieve an extraordinary balance of properties.     

Study on preparation and mechanical property of nanocrystalline NiAl intermetallic

Nanocrystalline NiAl materials were fabricated using mechanical alloying and hot-pressing sintering technique. The crystal structural and microstructure of milled powders during mechanical alloying, and the microstructure and mechanical properties of bulk NiAl intermetallic were characterized. The results show that B2 ordered nanocrystalline NiAl powders were successfully synthesized by solid-state diffusion via the gradual exothermic reaction mechanism during mechanical alloying. Scanning electron microscope image confirmed that the powder particles were flat and flake shape in the early stage of milling, but changed to a spherical shape with the crystallite size about 30 nm after the milling. After sintering, the crystal structure of nanocrystalline NiAl intermetallic was assigned to B2 order NiAl phase with the average crystallite size about 100 nm. The nanocrystalline NiAl intermetallic exhibited prominent room temperature compressive properties, such as the true ultimate compressive strength and the fracture strain were 2143 MPa and 32.2%, respectively. The appearances of vein-like patterns on the fracture surface of NiAl intermetallic materials indicated that the fracture mechanism could be characterized as ductile fracture. It can be concluded that higher sintering density and nanocrystalline of NiAl intermetallic were benefited for the improvement of mechanical properties.     

Ultrasound-assisted brazing of Cu/Al dissimilar metals using a Zn–3Al filler metal

Ultrasound-assisted brazing of Cu/Al dissimilar metals was performed using a Zn–3Al filler metal. The effects of brazing temperature on the microstructure and mechanical properties of Cu/Al joints were investigated. Results showed that excellent metallurgic bonding could be obtained in the fluxless brazed Cu/Al joints with the assistance of ultrasonic vibration. In the joint brazed at 400 °C, the filler metal layer showed a non-uniform microstructure and a thick CuZn₅ IMC layer was found on the Cu interface. Increasing the brazing temperature to 440 °C, however, leaded to a refined and dispersed microstructure of the filler metal layer and to a thin Al_4.2Cu_3.2Zn_0.7 serrate structure in the Cu interfacial IMC layer. Further increasing the brazing temperature to 480 °C resulted in the coarsening of the filler metal and the significantly growth of the Al_4.2Cu_3.2Zn_0.7 IMC layer into a dendrite structure. Nanoindentation tests showed that the hardness of the Al_4.2Cu_3.2Zn_0.7 and CuZn₅ phase was 11.4 and 4.65 GPa, respectively. Tensile strength tests showed that all the Cu/Al joints were failed in the Cu interfacial regions. The joint brazed at 440 °C exhibited the highest tensile strength of 78.93 MPa.     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Influence of solution treatment on microstructure and mechanical properties of a near β titanium alloy Ti-7333

The effect of solution treatment on the microstructure and mechanical properties of Ti-7333, a newly developed near β titanium alloy, was investigated. Compared to Ti-5553 and Ti-1023, Ti-7333 possesses the slowest α to β dissolution rate, allowing a wider temperature window for processing. The rate of β grain growth decreases with the increase of soaking time and increases with the increase of solution temperature. The β grain growth exponents (n) are 0.30, 0.31, 0.32 and 0.33 for solution treatment temperature of 860 °C, 910 °C, 960 °C and 1010 °C, respectively. The activation energy (Q_g) for β grain growth is 395.6 kJ/mol. Water cooling or air cooling after solution treatment have no significant influence on microstructure, which offers large heat treatment cooling window. However, under furnace cooling, the fraction of α phase increases sharply. α phase maintains strictly the Burgers orientation relation with β phase ({0 0 0 1}_α//{1 1 0}_β and 〈1 1 −2 0〉_α//〈1 1 1〉_β), except the α_p particles formed during forging. The tensile strength decreases with the increase of the solution temperature when only solution treatment is applied, whereas the ductility increases gradually. When aging is applied subsequently, the tensile strength increases with the increase of the solution temperature and the ductility decreases gradually.