Microstructure and Interfacial Reactions During Vacuum Brazing of Stainless Steel to Titanium Using Ag-28 pct Cu Alloy

Microstructural evolution and interfacial reactions during vacuum brazing of grade-2 Ti and 304L-type stainless steel (SS) using eutectic alloy Ag-28 wt pct Cu were investigated. A thin Ni-depleted zone of α -Fe(Cr, Ni) solid solution formed on the SS-side of the braze zone (BZ). Cu from the braze alloy, in combination with the dissolved Fe and Ti from the base materials, formed a layer of ternary compound τ2, adjacent to Ti in the BZ. In addition, four binary intermetallic compounds, Cu3Ti2, Cu4Ti3, CuTi and CuTi2 formed as parallel contiguous layers in the BZ. The unreacted Ag solidified as islands within the layers of Cu3Ti2 and Cu4Ti3. Formation of an amorphous phase at certain locations in the BZ could be revealed. The β -Ti(Cu) layer, formed due to diffusion of Cu into Ti-based material, transformed to an α -Ti + CuTi2eutectoid with lamellar morphology. Tensile test showed that the brazed joints had strength of 112 MPa and failed at the BZ. The possible sequence of events that led to the final microstructure and the mode of failure of these joints were delineated

Diffusion bonding of commercially pure titanium to 304 stainless steel using copper interlayer

Diffusion bonding was carried out between commercially pure titanium (cpTi) and 304 stainless steel (304ss) using copper as interlayer in the temperature range of 850–950 °C for 1.5 h under 3 MPa load in vacuum. The microstructures of the transition joints were revealed in optical and scanning electron microscopy (SEM). The study exhibits the presence of different reaction layers in the diffusion zone and their chemical compositions were determined by energy dispersive spectroscopy. The occurrence of different intermetallic compounds such as CuTi2, CuTi, Cu3Ti2, Cu4Ti3, FeTi, Fe2Ti, Cr2Ti, T2 (Ti40Cu60−xFex; 5 < x < 17), T3 (Ti43Cu57−xFex; 21 < x < 24) and T5 (Ti45Cu55−xFex; 4 < x < 5) has been predicted from the ternary phase diagrams of Fe–Cu–Ti and Fe–Cr–Ti. These reaction products were confirmed by X-ray diffraction technique. The maximum bond strength of 318 MPa (99.7% of Ti) was obtained for the couple bonded at 900 °C due to better coalescence of mating surface. With the rise in joining temperature to 950 °C, decrease in bond strength occurs due to formation of brittle Fe–Ti bases intermetallics. At a lower joining temperature of 850 °C, bond strength is also lower due to incomplete coalescence of the mating surfaces

Microstructure and interfacial reactions during active metal brazing of stainless steel to titanium

Microstructural evolution and interfacial reactions during active metal vacuum brazing of Ti (grade-2) and stainless steel (SS 304L) using a Ag-based alloy containing Cu, Ti, and Al was investigated. A Ni-depleted solid solution layer and a discontinuous layer of (Ni,Fe)2TiAl intermetallic compound formed on the SS surface and adjacent to the SS-braze alloy interface, respectively. Three parallel contiguous layers of intermetallic compounds, CuTi, AgTi, and (Ag,Cu)Ti2, formed at the Ti-braze alloy interface. The diffusion path for the reaction at this interface was established. Transmission electron microscopy revealed formation of nanocrystals of Ag-Cu alloy of size ranging between 20 and 30 nm in the unreacted braze alloy layer. The interdiffusion zone of β-Ti(Ag,Cu) solid solution, formed on the Ti side of the joint, showed eutectoid decomposition to lamellar colonies of α-Ti and internally twinned (Cu,Ag)Ti2 inter- metallic phase, with an orientation relationship between the two. Bend tests indicated that the failure in the joints occurred by formation and propagation of the crack mostly along the Ti- braze alloy interface, through the (Ag,Cu)Ti2 phase layer

Microstructural evolution during reactive brazing of alumina to Inconel 600 using Ag-based alloy

A metal ceramic bonding process was developed to produce vacuum tight alumina Inconel 600 joints using an Ag-based active metal brazing alloy that can withstand continuous operating temperature up to 560 degrees C. The microstructure and microchemistry of the braze zone was examined using extensive microanalysis of the constituent phases and a mechanism for the interfacial reactions responsible for the bonding is proposed. Prolonged heat treatment at 400 and 560 degrees C under simulated in-service conditions revealed that the microstructure of braze zone of the joints was stable and maintained leak-tightness and strength. The bond strength of the interface was high enough to cause failure in the alumina side of the joints. Failure of the joints was caused by initiation of crack on the surface of alumina as a result of high tensile residual stress adjacent to the metal ceramic interface. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Diffusion characteristics in the Cu-Ti system

The formation and growth of intermetallic compounds by diffusion reaction of Cu and Ti were investigated in the temperature range 720-860 degrees C using bulk diffusion couples. Only four, out of the seven stable intermediate compounds of the Cu-Ti system, were formed in the diffusion reaction zone in the sequence CuTi, Cu4Ti, Cu4Ti3 and CuTi2. The activation energies required for the growth of these compounds were determined. The diffusion characteristics of Cu4Ti, CuTi and Cu4Ti3 and Cu(Ti) solid solution were evaluated. The activation energies for diffusion in these compounds were 192.2, 187.7 and 209.2 kJ mol(-1) respectively, while in Cu(Ti), the activation energy increased linearly from 201.0 kJ mol(-1) to 247.5 kJ mol(-1) with increasing concentration of Ti, in the range 0.5-4.0 at.%. The impurity diffusion coefficient of Ti in Cu and its temperature dependence were also estimated. A correlation between the impurity diffusion parameters for several elements in Cu matrix has been established

Mid-crustal ramping of the Main Himalayan Thrust in Nepal to Bhutan Himalaya: New insights from analogue and numerical experiments

The mid-crustal ramp on the Main Himalayan Thrust (MHT) in the Lesser Himalaya Sequence (LHS) is the most critical geometrical asperity to trigger major seismic events in the Himalaya, including the 2015 Mw 7.8 Gorkha, Nepal earthquake. However, it is still not well understood what caused the MHT to ramp up at laterally varying locations in central to eastern Himalaya during the southward propagation of the Himalayan tectonic wedge. Based on laboratory and numerical model experiments, approximated to the central (Nepal) and eastern (Bhutan) Himalayan geological settings, we show that the coal-bearing Gondwana horizon in the stratigraphic sequence has played a crucial role in producing the mid-crustal ramp on the MHT. During the southward propagation of the Himalayan wedge, the mechanically weak Gondwana sequence resulted in strain localization at its northern edge, and it eventually caused the MHT to ramp from a depth of >20 km to ~6–7 km. Combining our experimental and field observations with the geophysical sections, we interpret that a strike-wise non-uniform occurrence of the Gondwana horizon is responsible for the MHT to ramp at different locations, separated by N-S trending natural barriers, like tear faults. The weak-zone model also explains the rock uplift versus erosion pattern recorded in central Nepal. The ramp-induced surface uplift rates, calculated from our model, are in good agreement with those reported from central Nepal (U* ~ 0.28, normalized to the convergent rate)

Methane Hydrate Formation and Dissociation in the Presence of Silica Sand and Bentonite Clay

The formation and dissociation of methane hydrates in a porous media containing silica sand of different sizes and bentonite clay were studied in the presence of synthetic seawater with 3.55 wt% salinity. The phase equilibrium of methane hydrate under different experimental conditions was investigated. The effects of the particle size of silica sand as well as a mixture of bentonite clay and silica sand on methane hydrate formation and its dissociation were studied. The kinetics of hydrate formation was studied under different subcooling conditions to observe its effects on the induction time of hydrate formation. The amount of methane gas encapsulated in hydrate was computed using a real gas equation. The Clausius-Clapeyron equation is used to estimate the enthalpy of hydrate dissociation with measured phase equilibrium data

Methane Hydrate Formation and Dissociation in the Presence of Silica Sand and Bentonite Clay

International audienceThe formation and dissociation of methane hydrates in a porous media containing silica sand of different sizes and bentonite clay were studied in the presence of synthetic seawater with 3.55 wt% salinity. The phase equilibrium of methane hydrate under different experimental conditions was investigated. The effects of the particle size of silica sand as well as a mixture of bentonite clay and silica sand on methane hydrate formation and its dissociation were studied. The kinetics of hydrate formation was studied under different subcooling conditions to observe its effects on the induction time of hydrate formation. The amount of methane gas encapsulated in hydrate was computed using a real gas equation. The Clausius-Clapeyron equation is used to estimate the enthalpy of hydrate dissociation with measured phase equilibrium data

Stability of microstructure and its evolution during solid-state annealing of Al(2)O(3)-Inconel 600 brazed couples

The stability of the microstructure of alumina-Inconel 600 brazed joints was investigated under simulated in-service conditions by subjecting them to prolonged heat treatment at 400 and 560 degrees C. The evolution of the microstructure and microchemistry of the brazing zone was examined using extensive microanalysis of the constituent phases. The layered structure of the brazing zone transformed to homogeneous, near-equilibrium, two-phase microstructure after heat treatment at 560 C. Solid-state interdiffusion was identified as a primary factor responsible for such copious modification of the microstructure. Intermediate temperature heat treatment at 400 degrees C revealed that the mechanism of transformation of the microstructure was globulization of the Ni overlayer and dissolution of Mo from the metallization layer into the Ni-rich phase. The migration behavior of each of the elements, in response to heat treatment, was analyzed. Cr was found to diffuse out of Inconel and form a layer of Cr(2)O(3) at the alumina-brazing alloy interface. The bond strength of the interface was high enough to cause cohesive failure in the alumina side of the joints. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved