Vacuum Hot Pressed Research Articles

Microstructural Heterogeneity and Property Variations in Cast and Vacuum Hot-Pressed CoCrPtB Alloy.

Limited research has been undertaken regarding the homogeneity of CoCrPtB alloy billets. A CoCrPtB alloy was processed through casting and vacuum hot pressing. This investigation delved into the interconnection between the secondary dendrite arm spacing (SDAS) in the as-hot-pressed samples and their corresponding attributes, specifically Vickers hardness and magnetic properties. Systematic sampling was conducted on the cross-sectional layer and longitudinal surface. Upon examination of the cross-sectional layer proximate to the uppermost region of the hot casting, a discernible parabolic trend was observed for the SDAS that exhibited a gradual increment from the peripheral regions toward the central area along the width. Simultaneously, the fraction of the dendrite phase displayed a consistent linear decline, attaining its peak value at the central portion of the billet. Conversely, on the longitudinal surface, SDAS and the fraction of the dendrite phase remained fairly uniform within the same column sampling regions. However, a notable divergence was identified in the central section, characterized by an augmented SDAS and diminished dendrite phase content. This inherent microstructural inhomogeneity within the CoCrPtB alloy engendered discernible disparities in material properties.

Role of multi-walled carbon nanotube particles on the wear behaviour of vacuum hot pressed Mg-AZ91 composite

The demerit of Mg-AZ91 alloy is poor wear resistance. In order to increase the wear resistance and decrease the CoF, MWCNT is reinforced in various wt% (0.5, 1, & 1.5) into Mg-AZ91 by a vacuum hot press technique. The OM image of the specimens shows the strong metallurgical bonding between matrix material, uniform dispersion of MWCNT for 0.5 and 1 wt% composites and agglomeration of MWCNT for 1.5 wt% composite. XRD analysis confirms MWCNT presence and peak shift towards a higher angle in the composites. MWCNT reinforced 1 wt% composite has exhibited the highest hardness values of 97.90 HV0.3, good wear resistance (wear rate = 3.28 × 10-5 g/m) due to the strengthening by grain size reduction and a low CoF value of 0.32 due to the self-lubrication effect of MWCNT. SEM investigation confirms that surface fatigue is the dominant wear mechanism in all specimens.

Tribological behaviors of vacuum hot-pressed ceramic composites with enhanced cyclic oxidation and corrosion resistance

Wear failure is a bottleneck restricting applications and developments of Ti3SiC2 ceramic. Particles reinforced composites provide an effective strategy to resist wear. In this work, Ti(C,N) particles are used as reinforcements, and Ti3SiC2/Ti(C,N) composite is fabricated by vacuum hot-pressing. Scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffract meter (XRD) are used to investigate composite morphologies, compositions and phases before and after hot-pressing. Meanwhile, high-temperature cyclic oxidations and tribological behaviors of composites under various loads, speeds and Ti(C,N) contents are characterized. Results show that as-prepared composite is relatively dense, and Ti(C,N) addition plays an important role in particle reinforcement of Ti3SiC2. Meanwhile, its hardness, wear resistance, cyclic oxidation resistance and corrosion resistance are significantly improved. In addition, wear characteristics and mechanisms of composites under different loads and speeds are analyzed in details. This work shows great potentials in developing engineering applications of ceramics, especially in high-temperature, oxidizing, frictional and corrosive environments.

Synthesis and Thermoelectric Property Elucidation of Mechanically Alloyed and Vacuum Hot Pressed Single-Phase AlSb

AlSb is a line compound and the synthesis of single phase bulk AlSb has always proven to be a significant challenge. Synthesis of bulk single phase AlSb by powder metallurgy has not yet been reported. This work shows a novel synthetic route for successful production of single phase bulk AlSb using mechanical alloying and vacuum hot pressing. Thermoelectric properties of the semiconductor were measured and reported for the first time using powder metallurgy process. The intrinsic semiconductor showed a moderate Seebeck coefficient and a high thermal conductivity, consequently a low thermoelectric figure of merit (ZT). The thermoelectric figure of merit was also calculated and is shown in this study.

Synthesis of Tin-Doped FeVSb Half-Heusler System by Mechanical Alloying and Evaluation of Thermoelectric Performance

In the current work, we tried to investigate and compare the thermoelectric performance of the FeVSb1−xSnx (x = 0.0–0.05) half-Heusler system. The elemental mixture was formulated and subjected to mechanical alloying, and subsequently consolidated using vacuum hot pressing. X-ray diffractometry analysis was used to observe phase transition during the process, and scanning electron microscopy was utilized for microstructure analysis. After vacuum hot pressing, a near single phase of FeVSb1−xSnx (x = 0.0–0.05) was observed with small portions of second phases. The vacuum hot-pressed samples were analyzed to study the thermoelectric properties as a function of temperature up to 980 K. The n-type conduction was confirmed from Seebeck and Hall coefficients in the test range. In addition, the electrical conductivity showed combined conduction behavior in the test range, while the thermal conductivities showed abnormal behavior. Mass fluctuation scattering and changing in carrier concentration might have taken place during doping. The Seebeck coefficient and electrical conductivity of the system also decreased after doping. Dimensionless figure of merit values were calculated and compared with the results of analogue studies. The maximum ZT value was obtained for x = 0.01 at 553 K.

Vacuum hot pressed AZ31/UCF/AZ31 composite sheets: Microstructure and mechanical properties

AZ31 Mg alloy/unidirectional carbon fiber (UCF)/AZ31 composite sheets were fabricated with AZ91 Mg alloy powder as an adhesive using vacuum hot pressing (VHP) at various temperatures. Both the wettability and bonding strength of the UCF/AZ91 interfaces increased with increasing VHP temperature, resulting in an improvement of the mechanical properties of the composite sheets. The fracture mechanism of the composite sheets at low VHP temperature (500 °C) was the pullout of UCFs and delamination. In contrast, at 560 °C, partial debonding between the UCFs and AZ91 was the dominant failure mechanism, and an S-type fracture path surrounding the UCFs was observed. The bonding strength of the UCF/AZ91 interface was weaker than that of the AZ91/AZ31 interface for this composite. The sheet prepared at 585 °C exhibited the largest ultimate tensile strength of 252 MPa and an elongation of 13.2%. This behavior was attributed to the enhancement of the interfacial bonding strength due to the formation of Al4C3 and MgAl2C2 phases at the UCF/AZ91 interface. The hardness and modulus reached maximum values of 9.9 and 61 GPa, respectively, near the UCF/AZ91 interface.

Effect of Interface Evolution on Thermal Conductivity of Vacuum Hot Pressed SiC/Al Composites

The SiC/Al composites have been fabricated by a vacuum hot pressing (VHP) process in order to study the effect of interface evolution on the global thermal conductivity (TC). By optimizing the VHP parameters of sintering temperature and time, the three different kinds of SiC/Al interface configurations, that is, non‐bonded, diffusion‐bonded, and reaction‐bonded interfaces, are formed and identified by measurement of relative density, X‐ray diffraction, scanning and (high‐resolution) transmission electron microscopy. The VHPed composite sintered at 655 °C for 60 min is fully dense and presents a tightly‐adhered and clean SiC/Al interface at the nanoscale, the ideal diffusion‐bonded interface being the most favorable for minimizing interfacial thermal resistance, which in turn results in the highest TC of around 270 W/mK.

Elastic precursor decay in S-200F beryllium

We have performed a series of plate impact experiments on vacuum hot-pressed (VHP) S-200F Be at peak shock stresses between 2.1 and 23.0 GPa to gain insight into the dynamic strength (Hugoniot elastic limit (HEL)), equation-of-state, and damage behavior of this technologically important material. In this paper we focus on our VISAR observations of the evolution of elastic precursor amplitude with Be target thickness in a series of plate impact experiments conducted in both transmission and reverse geometry. We observe monotonic decay in precursor amplitude with run distance for sample thicknesses between 4 and 8 mm and present the HEL values obtained from these experiments. We will discuss the observed precursor decay with respect to the relative roles of twinning and dislocation-mediated slip in the overall dynamic material mechanical response.

Vacuum hot-pressed beryllium and TiC dispersion strengthened tungsten alloy developments for ITER and future fusion reactors

Beryllium and tungsten have been selected as the plasma facing materials of the ITER first wall (FW) and divertor chamber, respectively. China, as a participant in ITER, will share the manufacturing tasks of ITER first-wall mockups with the European Union and Russia. Therefore ITER-grade beryllium has been developed in China and a kind of vacuum hot-pressed (VHP) beryllium, CN-G01, was characterized for both physical, and thermo-mechanical properties and high heat flux performance, which indicated an equivalent performance to U.S. grade S-65C beryllium, a reference grade beryllium of ITER. Consequently CN-G01 beryllium has been accepted as the armor material of ITER-FW blankets. In addition, a modification of tungsten by TiC dispersion strengthening was investigated and a W–TiC alloy with TiC content of 0.1wt.% has been developed. Both surface hardness and recrystallization measurements indicate its re-crystallization temperature approximately at 1773K. Deuterium retention and thermal desorption behaviors of pure tungsten and the TiC alloy were also measured by deuterium ion irradiation of 1.7keV energy to the fluence of 0.5–5×1018D/cm2; a main desorption peak at around 573K was found and no significant difference was observed between pure tungsten and the tungsten alloy. Further characterization of the tungsten alloy is in progress.

Effect of powder oxidation on densification and properties of vacuum hot pressed Cu–Cr–Nb alloy

The densification behavior of oxidized Cu-8 at% Cr-4 at%Nb alloy powders by vacuum hot pressing and subsequent mechanical properties of alloy compares well with that of fresh powders with no oxides and hydrogen reduced powders sintered using same conditions. The reason for such observation is the presence of high vacuum and carbon atmosphere due to graphite die at the furnace temperature helping in the decomposition of oxide layer. Reduction of oxides of copper controls the overall densification process. A new mechanism for the densification of air-oxidized powders during vacuum hot pressing is also put forward. Present work indicates that reduction of powders is not needed if consolidated by vacuum hot pressing.

Summary of beryllium qualification activity for ITER first-wall applications

Beryllium is considered as an armor material for the ITER first wall. The ITER Final Design Report 2001 identified the reference grades S-65C vacuum hot pressed (VHP) from Brush Wellman and DShG-200 from the Russian Federation. These grades have been selected based on excellent thermal fatigue/shock behavior and the available comprehensive database. Later, Chinese and Russian ITER Parties proposed their new grades: CN-G01 (from China) and TGP-56FW (from Russia). To assess the performance of these new grades, the ITER Organization, Chinese and Russian Parties established a program for the characterization of these materials. A summary of the published data and new results are presented in the paper. It was concluded that the proposed Chinese (CN-G01) and Russian (TGP-56FW) beryllium grades can be accepted. Three grades of beryllium are now available for the armor application for the ITER first wall: S-65, CN-G01 and TGP-56FW.

Thermal Stability of Vacuum Hot Pressed Bulk Nanostructured Al-Cu Alloys

Bulk nanostructured Al-4Cu alloy was synthesized by high energy ball milling followed by vacuum hot pressing. Its thermal stability was investigated up to 450°C using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The results showed that grain growth in this Al-4Cu alloy was very limited and grain sizes in the range of 100 nm were still present in the alloys after exposure to 450 °C corresponding to a temperature as high as 0.77 T/Tm. The TEM investigations reveal that such a high thermal stability against grain growth observed in this alloy is attributed to the presence of ultrafine dispersoids.

Thermoelectric Properties of Bi2Te3 Material Doped with Lanthanum by Mechanical Alloying

The Bi2Te3, (Bi1‐xLax)2Te3 (x=0.002, 0.003) and Bi2(Te1‐yLay)3 (y=0.0015, 0.002) alloys were fabricated by mechanical alloying and vacuum hot pressing, and their thermoelectric properties were characterized. The Seebeck coefficient of vacuum hot pressed specimens doped with lanthanum was higher than that of Bi2Te3. The electrical conductivity was improved at specimens doped with relatively small amount of lanthanum improved than that of Bi2Te3. The maximal dimensionless figure of merit in Bi2(Te0.9985La0.0015)3 doped with relatively small amount of lanthanum was 0.48 at 473 K.

Thermoelectric properties of mechanically alloyed iron disilicides consolidated by various processes

Mn doped p-type iron disilicide powders have been produced by a mechanical alloying process. As-milled powders were of metastable state and mostly transformed to β-FeSi2 phase by subsequent isothermal annealing. As-milled powders were consolidated by various processes such as sintering of the cold compact in vacuum, vacuum hot pressing (VHP), and spray drying/atmospheric plasma thermal spraying. Phase transitions during the processes were investigated using XRD, EDS, and SEM. As-consolidated specimens consisted of a mixture of α-Fe2Si5 and e-FeSi phases, which were gradually transformed into a thermoelectric semiconducting α-FeSi2 phase by subsequent isothermal annealing in the vicinity of 845°C in vacuum. However, some residual α and e phases remained even after prolonged annealing. Thermoelectric properties were measured as a function of temperature and correlated with phase transformation. They showed optimum values in the vacuum hot pressed specimen due to a higher fraction of β phase and/or higher density.

EFFECT OF EXTRUSION PARAMETERS ON STRUCTURE AND PROPERTIES OF 2124 ALUMINUM ALLOY MATRIX COMPOSITES

Discontinuously reinforced aluminum matrix composites (DRA) have been attracting attention because of their amenability to undergo deformation processing by conventional metalworking techniques. Extrusion is used in processing of DRA composites for consolidation, redistribution of reinforcements, and shape forming. The important parameters that control the extrusion process are temperature and strain rate, which is a function of several equipment/extrusion parameters. Vacuum hot-pressed (VHP) 2124 Al/30 SiCp composite billets were extruded at different ram speeds (1, 10, 100 mm sec−1) and using different extrusion ratios (4:1, 10:1, and 20:1). The extruded samples were studied for their integrity, microstructure, and mechanical properties. The integrity of the extruded composite rod was very good at minimum extrusion speed of 1 mm sec−1, whereas 100 mm sec−1 extrusion speed resulted in extensive fir tree cracking. Extrusion of VHP billets, with necklace structure, resulted in elongated alternate stringers of matrix and SiCp in the extrusion direction. Matrix stringer width and aspect ratio were found to vary with extrusion ratio. Because of the microstructural refinement, both the strength and ductility of the metal matrix composites (MMCs) were improved. Microhardness of the matrix stringers was found to be a function (power relation) of their width, irrespective of the location and extrusion ratio.

Formation of a TiN/Ti 5Si 3 nano-grain composite by a non-equilibrium PM process

In the present paper, the authors have proposed a non-equilibrium powder metallurgy (PM) process which enables improved formability and the production of a (TiN+Ti 5Si 3) composite. The microstructure and high-temperature deformation behavior of Ti–Si 3N 4 mechanically alloyed (MA) powder compacts were investigated. Powders of the elements Ti and Si 3N 4 of composition Ti–20 mass%Si 3N 4 were blended for MA. The MA process for 720 ks resulted in the formation of amorphous and α-Ti phases. These phases changed to TiN, Ti 2N and Ti 5Si 3 phases after heat treatment at elevated temperatures. A (TiN+Ti 5Si 3) ultra-fine microduplex structure was obtained after heat treatment at 1473 K for 3.6 ks. The amorphous and α-Ti phases were stable even after vacuum hot pressed (VHP) at 803 K, while they disappeared and α-Ti, Ti 2N and Ti 5Si 3 phases appeared in 813 and 823 K VHP specimens. Compression tests revealed that the 803 K VHP specimen with the non-equilibrium phases had the lowest flow stress at 993 K and an initial strain rate of 4.2×10 −4 s −1 in these three VHP specimens. The slower strain rate test produced a larger amount of harder phases such as TiN, Ti 2N and Ti 5Si 3. Therefore, there exists an appropriate condition for a low temperature and high strain rate forming process. A (TiN+Ti 5Si 3) microduplex structure was also obtained in the specimen compressed to 25% after annealing at 1473 K for 3.6 ks.

金属間化合物の粉末冶金と諸物性 Ｔｉ‐４１ｍｏｌ％Ａｌ合金のメカニカルグラインディング粉末およびその燃結体の微細組織

The microstructure of mechanically ground (MG) Ti-41 mol%Al alloy powder made by a vibratory ball mill, in the as-milled state as well as after heat treatment or vacuum hot pressing, was investigated. A new method for preparing TEM samples was employed which resolved the microstructure of the MG powder, whose particle sizes were more than 10μm in diameter. The alloy powder milled for 720ks was composed of fine grained α2 (Ti3Al), nanocrystals of Al supersaturated α-Ti and an amorphous phase. The a nanocrystals and the amorphpos phase presumably caused by milling of α2 and γ phases, respectively. The vacuum hot pressed compact showed ultra-fine equiaxed grain structure, with an average grain size of approximately 400nm. The similarity of the microstructure to that of mechanically alloyed (MA) compacts strongly suggests that the formation mechanism of the ultra-fine grain structure of the compacts is the same in both MG and MA processes.

Properties of monolithic and composite NiAl processed by hydrostatic extrusion and vacuum hot-pressing

Nickel aluminide intermetallics and composites have been processed by hydrostatic extrusion or vacuum hot-pressing. Hydrostatic extrusion at both 298 K and 573 K was used to process nominally stoichiometric cast and previously extruded NiAl, while vacuum hot-pressing of prealloyed NiAl (45 at.% Al) powders was additionally used to process NiAl and NiAl+TiB 2 particulate composites. The microstructures, hardnesses, and compressive yield strengths of the processed materials were evaluated. It is shown that NiAl can be hydrostatically extruded at both temperatures, although substantial cracking was observed in the material extruded at 298 K. Hydrostatic extrusion at 573 K leads to significant hardness and strength increases without producing fracturing around the hardness indentations. Reinforcement with TiB 2 in the hot-pressed materials similarly provided an increase in both the compressive strength and hardness, although cracking around the hardness indents was observed. The fracture toughnesses of the hydrostatically extruded cast/extruded NiAl, the vacuum hot-pressed powder processed monolithic NiAl, and the composite NiAl intermetallics were determined in accordance with standard testing techniques. The effects of TiB 2 reinforcement particle size and distribution at 10 vol.% loading on the composite fracture toughness were also characterized. Monolithic NiAl demonstrated toughness values of about 5 MPa√m. Reinforcement additions to NiAl resulted in a toughness increase while the range of reinforcement distributions and sizes tested did not significantly change the measured composite toughness values. Significant increases in toughness and R-curve behavior were obtained in NiAl hydrostatically extruded at 573 K. The fracture behavior of the monolithics and composites is discussed in light of the deformation and fracture mechanisms operating at the crack tips, aided with in situ fracture studies and post-failure analyses.

Hot-hardness comparisons among isostructural Be12X intermetallic compounds

Hot hardness has been measured in seven isostructural Be12X compounds: Be12Cr, Be12Mn, Be12V, Be12Mo, Be12Ti, Be12Ta, and Be12Nb. All vacuum-hot-pressed (VHP) materials exhibited similar hardness levels (800 to 1000 kg/mm2) at low temperatures (<600 °C), but sharply diverged at higher temperatures. Most compounds showed a sharp decrease in hardness above about one-half their melting temperatures except for Be12Nb and Be12Ti which maintained strength until higher temperatures. Ductile-brittle transition temperatures (DBTT's) of 600, 625, 690, 700, 700, and 850 °C were determined for Be12Cr, Be12Mn, Be12V, Be12Mo, Be12Ti, and Be12Nb, respectively. Hot-isostatically-pressed (HIP) and VHP Be12Nb heats both showed comparable behavior, indicating that the temperature required for macroscopic plasticity is not dependent on processing conditions. Differences in slip characteristics were observed between beryllides with similar melting temperatures, but differing DBTT's. Dislocations were activated on multiple slip systems in Bei2V during compressive deformation at 800 °C. Similar behavior was not observed in Be12Nb until test temperatures above 1000 °C. It is proposed that dislocation mobility may be related to the stability between Be12X and Be17X2 phases. Larger refractory metal atoms such as Nb may form faulted layers related to Be17X2 structures and thereby impede deformation at intermediate temperatures. Smaller atoms such as Cr, Mn, V, and Mo do not form a stable Be17X2 phase and thereby allow enhanced dislocation mobility at these temperatures.

Effect of homogenization treatment on fatigue behaviour of 2124Al/20 vol % SiCp composite

The fatigue behaviour of 2124Al/20 vol % SiCp composite was studied in the as vacuum hotpressed condition, as well as after a homogenization treatment subsequent to vacuum hot pressing. It was found that there was a significant improvement in the tensile strengths, fatigue threshold stress intensity range, ΔKth, and cyclic fracture toughness, Kfc, as a result of the homogenization treatment. The improvement in the properties of the composite after homogenization is attributed to the dissolution of the coarse intermetallic precipitates present in the composite in the as-vacuum hot-pressed condition.