Influence of interface microstructure on the mechanical properties of titanium/17-4 PH stainless steel solid state diffusion bonded joints

In the present study, titanium was diffusion bonded to a type 17-4 precipitation hardening stainless steel in vacuum at different temperatures and times. Bonded samples were characterized using light microscopy, scanning electron microscopy (SEM) and X-ray diffraction technique (XRD). The inter-diffusion of the chemical species across the diffusion interface was evaluated by electron probe microanalysis (EPMA). Up to 850 °C for 60 min, FeTi phase was formed at the diffusion interface; however, α-Fe + λ, χ, Fe₂Ti and FeTi phases and their phase mixtures were formed above 850 °C for 60 min and at 900 °C for all bonding times. The maximum tensile strength of ∼342.4 MPa and shear strength of ∼260.3 MPa along with 12.8% elongation were obtained for the diffusion couple processed at 950 °C. The thicknesses of different reaction products at the bond interface play an important role in determining the mechanical properties of the joints. The residual stress of the bonded joints increases with the increases in bonding temperatures and times.     

Characterization of melded carbon fibre/epoxy laminates

‘Melding’ is a novel in situ method for joining thermosetting composite structures, without the need of adhesives. Laminate joining is achieved using uncrosslinked resin matrix of the pre-preg. This study used Hexply914C pre-preg material to characterize melded CFRP structures produced using the melding method. A designated area of a laminate was maintained at temperatures below 40 °C retaining uncured (B-staged) material, while the remainder of the laminate was cured at 175 °C. After a 2.5 h cure cycle, the cured region showed a high degree of cure (0.88) and glass transition temperature (176 °C). The uncured area of the same laminate was cured in a second stage, simulating an in situ melded joint. By controlling the temperature and duration of the intermediate dwell and affecting minimum viscosity values prior to final cure, low values of porosity (<0.5%) were achieved. The mechanical properties of the resulting joint were consistent throughout the melded laminate. Flexural strength (1600 MPa), flexural modulus (100–105 MPa) and short beam strength (105–115 MPa) values observed where equivalent or greater than those found in the recommended autoclave cured control specimens. After the entire laminate was post cured, glass transition temperatures of 230 °C (peak tan δ) were observed in all areas of the laminate.     

The effect of zirconium addition on the microstructure and properties of chopped carbon fiber/carbon composites

Carbon/carbon composites containing zirconium were prepared using chopped carbon fiber, mesophase pitch and Zr powder by the traditional process including molding, carbonization, densification and graphitization. The influence of Zr on the microstructure and properties of the composites were investigated. Results show that Zr can improve the interface bonding, promote more perfect and larger crystallites and enhance the conductive/mechanical properties of the composites. The high in-plane thermal conductivity of 464 W/(m K) and excellent bending strength of 83.6 MPa was obtained for a Zr content of 13.9 wt% at heat treatment temperature(HTT) of 2500 °C. However the conductive/mechanical properties of the composites decrease dramatically for an higher HTT of 3000 °C. SEM micrograph of the fracture surface for the composites shows that lower disorder crystallite arrangement of fiber and carbon matrix come into being in the composites during HTT of 3000 °C, which should be responsible for the low properties. Correlation between the content of Zr and the microstructure and properties are discussed.     

Evaluation of the microstructure and mechanical properties of diffusion bonded joints of titanium to stainless steel with a pure silver interlayer

Solid-state diffusion bonding of commercially pure titanium to 304 stainless steel using an Ag interlayer was carried out at 825–875 °C under a uniaxial pressure of 8 MPa for 20 min in vacuum. The microstructural observations revealed that the resultant joints were composed of the remnant Ag interlayer, TiAg intermetallic phase and Ti–Ag solid solution. An optimized bonding strength of up to 414 MPa was achieved. Fracture took place through the remnant Ag interlayer during tensile tests and the interfacial TiAg phase exhibited no detrimental effect on the bonding strength. Extensive dimples were observed on the fracture surfaces, indicating that the joints were ductile in nature.     

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.     

Effect of phase transformation and intermetallic compounds on the microstructure and tensile strength properties of diffusion-bonded joints between Ti–6Al–4V and AISI 304L

The occurrence of a phase transformation and the effect of intermetallic compounds on the microstructure and tensile strength properties of diffusion-bonded (DB) joints between Ti–6Al–4V and AISI 304L were studied in the temperature range of 875–950 °C with an interval of 25 °C, a bonding time of 60 min and pressures of 4 MPa and 8 MPa. A maximum tensile strength of 242.6 MPa, was observed for diffusion-bonded joints that were processed at a temperature of 900 °C, bonded for 60 min at a pressure of 4 MPa and annealed for 2 h at 750 °C. Optical microscopy and scanning electron microscopy (SEM) were used to examine the grain growth and the fine details of the interface structure. Energy dispersive X-ray analysis (EDAX) and X-ray diffraction analysis (XRD) revealed the existence of intermetallic compounds and corroborated the phase transformation.     

Influence of carbon-fiber felts on the development of carbon–carbon composites

Oxidized PAN-fiber felt was carbonized to 600, 1000, and 1800 °C, respectively. Different carbon/carbon composites (C/C composites) were prepared from oxidized PAN-fiber felt, the carbonized felts, and resol-type phenol–formaldehyde resin. These composites were then carbonized and graphized at temperatures of between 600 and 2400 °C. The C/C composite made with oxidized PAN-fiber felt showed a strong fiber/matrix bonding, and those developed from the carbonized felt (heat-treatment of 1800 °C) showed a poor fiber/matrix bonding. The graphitized composites reinforced with the oxidized PAN-fiber felt resulted in having a high flexural strength (325 MPa), and the graphitized composites reinforced with the carbonized felt (carbonized at 1800 °C) had a low flexural strength (9 MPa). It was found that the stress-orientation promoted the formation of the anisotropic texture around the fibers as well as between the fibers. This felt may very well be able to provide a low-cost route for producing multidimensional C/C composites.     

Effects of temperature and strain rate on microstructure and mechanical properties of high chromium cast iron/low carbon steel bimetal prepared by hot diffusion-compression bonding

The objective of this study is to develop a hot diffusion-compression bonding process for cladding low carbon steel (LCS) to high chromium cast iron (HCCI) in solid-state. The influence of temperature (950–1150 °C) and strain rate (0.001–1 s⁻¹) on microstructure, hardness and bond strength of the HCCI/LCS bimetal were investigated. The interface microstructure reveals that the unbonded region can only be found for 950 °C due to lack of diffusion, while the intergrowth between the constituent metals occurred at and above 1100 °C. When bonding temperature increases to 1150 °C, a carbide-free zone was observed near the interface on the HCCI layer, and the thickness of the zone decreases with an increase of bonding strain rate. These evolutions indicate that the bond quality was improved by raising temperature and reducing strain rate due to the increase of element diffusion. The hot compression process of the bonding treatment not only changes the carbide orientation of the HCCI, but also increases the volume fraction of Cr–carbide. Based on the microstructural examinations and mechanical tests, the optimum bonding temperature and bonding strain rate are determined to be 1150 °C and 0.001 s⁻¹, respectively.     

The effect of thermal cycling in superplastic diffusion bonding of 2205 duplex stainless steel

In view of the requirement of large cold rolling deformation and bonding pressure in the conventional superplastic diffusion bonding of 2205 duplex stainless steel, a novel method of introducing thermal cycling into the process was proposed. During the thermal cycling process, due to the change of temperature, surface chemical activity of 2205 duplex stainless steel was improved, activity of atoms and grain boundaries were improved, and the recrystallized grains were refined. The shear bond strength of joint prepared in the mode of thermal cycling using specimens with the cold roll reduction of 60% was 15 MPa higher than that of conventional bonding using specimens with the cold roll reduction of 85%. Compared to the shear bond strength of 430 MPa under the specific pressure of 10 MPa after conventional bonding, shear bond strength of 623 MPa was obtained under the condition of T_max = 1000 °C, T_min = 900 °C, cycle number of heating and cooling N = 3, and specific pressure P = 5 MPa.     

Diffusion bonding of titanium alloy to micro-duplex stainless steel using a nickel alloy interlayer: Interface microstructure and strength properties

In the present study, diffusion bonding of titanium alloy and micro-duplex stainless steel with a nickel alloy interlayer was carried out in the temperature range of 800–950 °C for 45 min under the compressive stress of 4 MPa in a vacuum. The bond interfaces were characterised by scanning electron microscopy, electron probe microanalyzer and X-ray diffraction analysis. The layer wise Ni₃Ti, NiTi and NiTi₂ intermetallics were observed at the nickel alloy/titanium alloy interface and irregular shaped particles of Fe₂₂Mo₂₀Ni₄₅Ti₁₃ was observed in the Ni₃Ti intermetallic layer. At 950 °C processing temperature, black island of β-Ti phase has been observed in the NiTi₂ intermetallics. However, the stainless steel/nickel alloy interface indicates the free of intermetallics phase. Fracture surface observed that, failure takes place through the NiTi₂ phase at the NiA–TiA interface when bonding was processed up to 900 °C, however, failure takes place through NiTi₂ and β-Ti phase mixture for the diffusion joints processed at 950 °C. Joint strength was evaluated and maximum tensile strength of ∼560 MPa and shear strength of ∼415 MPa along with ∼8.3% ductility were obtained for the diffusion couple processed at 900 °C for 45 min.     

Cooling process and mechanical properties design of hot-rolled low carbon high strength microalloyed steel for automotive wheel usage

For the purpose of developing Nb–V–Ti microalloyed, hot rolled, high strength automotive steel for usage in heavy-duty truck wheel-discs and wheel-rims, appropriate cooling processes were designed, and microstructures and comprehensive mechanical properties (tension, bending, hole-expansion, and Charpy impact) of the tested steels at two cooling schedules were studied. The results indicate that the steel consists of 90% 5 μm polygonal ferrite and 10% pearlite when subjected to a cooling rate of 13 °C/s and a coiling temperature of 650 °C. The yield strength, tensile strength, and hole-expansion ratio are 570 MPa, 615 MPa, and 95%, respectively, which meet the requirements of the wheel-disc application. The steel consists of 20% 3 μm polygonal ferrite and 80% bainite (granular bainite and a small amount of acicular ferrite) when subjected to a cooling rate of 30 °C/s and a coiling temperature of 430 °C. The yield strength, tensile strength, and hole-expansion ratio are 600 MPa, 655 MPa, and 66%, respectively, which meet the requirements of the wheel-rim application. Both the ferrite–pearlite steel and ferrite–bainite steel possess excellent bendability and Charpy impact property. The precipitation behavior and dislocation pattern are characterized and discussed.     

The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints

Silica carbide modified carbon cloth laminated C–C composites have been successfully joined to lithium–aluminum–silicate (LAS) glass–ceramics using magnesium–aluminum–silicate (MAS) glass–ceramics as interlayer by vacuum hot-press technique. The microstructure, mechanical properties and fracture mechanism of C–C/LAS composite joints were investigated. SiC coating modified the wettability between C–C composites and LAS glass–ceramics. Three continuous and homogenous interfaces (i.e. C–C/SiC, SiC/MAS and MAS/LAS) were formed by element interdiffusions and chemical reactions, which lead to a smooth transition from C–C composites to LAS glass–ceramics. The C–C/LAS joints have superior flexural property with a quasi-ductile behavior. The average flexural strength of C–C/LAS joints can be up to 140.26 MPa and 160.02 MPa at 25 °C and 800 °C, respectively. The average shear strength of C–C/LAS joints achieves 21.01 MPa and the joints are apt to fracture along the SiC/MAS interface. The high retention of mechanical properties at 800 °C makes the joints to be potentially used in a broad temperature range as structural components.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrB2 composites

C/SiC–ZrB₂ composites prepared via precursor infiltration and pyrolysis (PIP) were treated at high temperatures ranging from 1200 °C to 1800 °C. The mass loss rate of the composites increased with increasing annealing temperature and the flexural properties of the composites increased initially and then decreased reversely. Out of the four samples, the flexural strength and the modulus of the specimen treated at 1400 °C are maximal at 216.9 MPa and 35.5 GPa, suggesting the optimal annealing temperature for mechanical properties is 1400 °C. The fiber microstructure evolution during high-temperature annealing would not cause the decrease of fiber strength, and moderate annealing temperature enhanced the thermal stress whereas weakened the interface bonding, thus boosting the mechanical properties. However, once the annealing temperature exceeded 1600 °C, element diffusion and carbothermal reduction between ZrO₂ impurity and carbon fibers led to fiber erosion and a strong interface, jeopardizing the mechanical properties of the composites. The mass loss rate and linear recession rate of composites treated at 1800 °C are merely 0.0141 g/s and 0.0161 mm/s, respectively.     

Tensile properties of hot rolled Mg–3Sn–1Ca alloy sheets at elevated temperatures

Mechanical behavior of hot rolled Mg–3Sn–1Ca (TX31) magnesium alloy sheets were studied in the temperature range 25–350 °C. The microstructure of the alloy consisted of the eutectic structure of α-Mg + Mg₂Sn and a dispersion of needle-like CaMgSn. The highest room-temperature ductility of 18% was obtained by hot rolling of the cast slabs at 440 °C, followed by annealing at 420 °C. The high temperature tensile deformation of the material was characterized by a decrease in work hardening exponent (n) and an increase in strain rate sensitivity index (m). These variations resulted in respective drops of proof stress and tensile strength from 126.5 MPa and 220 MPa at room temperature to 23.5 MPa and 29 MPa at 350 °C. This was in contrast to the ductility of the alloy which increased from 18% at room temperature to 56% at 350 °C. The observed variations in strength and ductility were ascribed to the activity of non-basal slip systems and dynamic recovery at high temperatures. The TX31 alloy showed lower strength than AZ31 magnesium alloy at low temperatures, while it exhibited superior strength at temperatures higher than 200 °C, mainly due to the presence of thermally stable CaMgSn particles.     

Effect of (α + β) heat treatment on microstructure and mechanical properties of (TiB + TiC)/Ti–B20 matrix composite

For the first stage, a metastable β titanium alloy, Ti–3.5Al–5Mo–4V–2Cr–2Sn–2Zr–1Fe reinforced with trace amounts of TiB whiskers and TiC particles was fabricated by vacuum arc melting process and hot forging followed by heat treatment at 780 °C/740 °C, then by aging at 500 °C, 550 °C, 570 °C and 600 °C. For the second stage, the unreinforced titanium alloy was also fabricated by the same process. The microstructural characteristics were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Traces of TiB whiskers and TiC particles (2.2 vol.%) with a volume ratio of 2:3 synthesized in situ exerted a hybrid reinforcing effect on the β titanium alloy. The reinforcements were uniformly distributed in the matrix and the elastic modulus was improved about 25 GPa. Ultimate tensile strength and yield strength achieves about 1625 MPa and 1500MPa respectively, with ductility at 7% when the aging temperature is 500 °C. The ductility of (TiB + TiC)/(Ti–3.5Al–5Mo–4 V–2Cr–2Sn–2Zr–1Fe) matrix composite could be enhanced by increasing the aging temperatures. After 780 °C followed by aging at 570 °C, excellent strength and plasticity properties were obtained (ultimate tensile strength of matrix alloy is 1350 MPa with elongation of 18% and ultimate tensile strength of composite is 1500 MPa with elongation of 13%).     

Bonding strength of heat treated compressed Eastern redcedar wood

The objective of this study was to evaluate effect of heat treatment and compression on some properties of Eastern redcedar (Juniperus virginiana) including bonding strength, hardness and surface quality. Specimens were exposed to three temperature levels of 120 °C, 160 °C and 190 °C for 6 h before they were compressed using 2.5 MPa pressure for 5 min. Polyvinyl acetate (PVAc) bonded specimens showed 23.6% reduction in their shear strength when they were exposed to a temperature of 120 °C. Such strength reduction values were 44.4% and 64.1% for the specimens exposed to temperature levels of 160 °C and 190 °C, respectively. The lowest average Janka hardness value of 214.08 kg was determined for the samples exposed to a temperature of 190 °C while those treated with a temperature of 120 °C had the highest hardness value of 397.73 kg. It appears that combination of heat treatment and compression enchanced overall surface quality of the samples in the form of their roughness determined using stylus type equipment.     

The ambient and high temperature deformation behavior of Al–Si–Cu–Mg alloy with minor Ti,Zr, Ni additions

The principal aim of the present work was to investigate the effects of minor additions of nickel and zirconium on the strength of cast aluminum alloy 354 at ambient and high temperatures. Tensile properties of the as-cast and heat-treated alloys were determined at room temperature and at high temperatures (190 °C, 250 °C, 350 °C). The results show that Zr reacts only with Ti, Si and Al. From the quality index charts constructed for these alloys, the quality index attains minimum and maximum values of 259 MPa and 459 MPa, in the as-cast and solution-treated conditions; also, maximum and minimum values of yield strength are observed at 345 MPa and 80 MPa, respectively, within the series of aging treatments applied. A decrease in tensile properties of ∼10% with the addition of 0.4 wt.% nickel is attributed to a nickel–copper reaction. The reduction in mechanical properties due to addition of different elements is attributed principally to the increase in the percentage of intermetallic phase particles formed during solidification; such particles act as stress concentrators, decreasing the alloy ductility. Tensile test results at ambient temperatures show a slight increase (∼10%) in alloys with Zr and Zr/Ni additions, particularly at aging temperatures above 240 °C. Additions of Zr and Zr + Ni increase the high temperature tensile properties, in particular for the alloy containing 0.2 wt.% Zr + 0.2 wt.% Ni, which exhibits an increase of more than 30% in the tensile properties at 300 °C compared with the base 354 alloy.     

Microstructural development of diffusion-brazed austenitic stainless steel to magnesium alloy using a nickel interlayer

The differences in physical and metallurgical properties of stainless steels and magnesium alloys make them difficult to join using conventional fusion welding processes. Therefore, the diffusion brazing of 316L steel to magnesium alloy (AZ31) was performed using a double stage bonding process. To join these dissimilar alloys, the solid-state diffusion bonding of 316L steel to a Ni interlayer was carried out at 900 °C followed by diffusion brazing to AZ31 at 510 °C. Metallographic and compositional analyses show that a metallurgical bond was achieved with a shear strength of 54 MPa. However, during the diffusion brazing stage B₂ intermetallic compounds form within the joint and these intermetallics are pushed ahead of the solid/liquid interface during isothermal solidification of the joint. These intermetallics had a detrimental effect on joint strengths when the joint was held at the diffusion brazing temperature for longer than 20 min.     

A study on the phase transformation of the nanosized hydroxyapatite synthesized by hydrolysis using in situ high temperature X-ray diffraction

The biodegradable hydroxyapatite (HA) was synthesized by hydrolysis and characterized using high temperature X-ray diffraction (HT-XRD), differential thermal analysis and thermogravimetry (DTA/TG), and scanning electron microscopy (SEM). The in situ phase transformation of the HA synthesized from CaHPO₄·2H₂O (DCPD) and CaCO₃ with a Ca / P = 1.5 in 2.5 M NaOH_(aq) at 75 °C for 1 h was investigated by HT-XRD between 25 and 1500 °C. The HA was crystallized at 600 °C and maintained as the major phase until 1400 °C. The HA steadily transformed to the α-tricalcium phosphate (α-TCP) which became the major phosphate phase at 1500 °C. At 700 °C, the minor CaO phase appeared and vanished at 1300 °C. The Na⁺ impurity from the hydrolysis process was responsible for the formation of the NaCaPO₄ phase, which appeared above 800 °C and disappeared at 1200 °C.     

Alumina–tantalum composite for femoral head applications in total hip arthroplasty

Dense composite laminates of alumina (Al₂O₃) and tantalum (Ta) were fabricated by hot pressing and tested in vitro for potential use as a femoral head material in total hip arthroplasty (THA). Al₂O₃–Ta composite laminates hot pressed at 1450 °C and 1650 °C had flexural strengths of 940 ± 180 MPa and 1090 ± 340 MPa, respectively, which were far larger than the values of 420 ± 140 MPa and 400 ± 130 MPa for Al₂O₃ hot pressed at 1450 °C and 1650 °C, respectively. The interfacial shear strength, determined by a double-notched specimen test, was 310 ± 80 MPa for the composite laminate hot pressed at 1650 °C, indicating strong interfacial bonding between Al₂O₃ and Ta. Scanning electron microscopy (SEM), energy dispersive X-ray (EDS) analysis, and X-ray mapping of polished sections of the hot-pressed laminates showed the presence of an interfacial region formed presumably by diffusion of O (at 1450 °C) or O and Al (1650 °C) from Al₂O₃ into Ta. Composite femoral heads of Al₂O₃ and Ta could combine the low wear of an Al₂O₃ articulating surface with the safety of a ductile metal femoral head.