Wettability of nickel alloys by Ni-Si-B fillers: influence of substrate oxide reduction and isothermal solidification

The process of wetting of nickel and nickel-chromium substrates by nickel-silicon-boron filler metals is examined. In this system, wetting behaviour is dominated by interactions between the filler metal and the oxide layer on the substrate (which is not completely removed during conventional vacuum brazing). Boron emanating from the filler metal reduces this residual oxide. The extent of wettability is controlled by the onset of isothermal solidification which is a consequence of the removal of boron from the filler metal due to the reaction with the substrate oxide layer.     

Columnar grain development in C-Mn-Ni low-alloy weld metals and the influence of nickel

This paper discusses the development of columnar grains in as-deposited C-Mn-Ni(-Mo) low-carbon low-alloy weld metals and the influence of the alloying elements, particularly nickel. It was found that the austenite columnar grain size prior to the - transformation of the weld metals was mainly controlled by the alloying contents rather than HAZ grains adjacent to the fusion boundaries, although the latter determined the size of the initial columnar grain size. The addition of nickel initially depressed the prior austenite grain size and subsequently dramatically coarsened it. This was related to the nickel equivalent (Ni_eq) of the weld metals and the peritectic reaction during the solidification process. Small columnar grains were associated with a Ni_eq between 3.4 and 6.2% which resulted in a peritectic reaction when the weld melt solidified, whilst a Ni_eq higher than 6.2% produced very large columnar grains because the weld pool would directly solidify into austenite and have a subsequent continuous growth.     

Evaluation of transient liquid phase bonding between nickel-based superalloys

Induction brazing of Inconel 718 to Inconel X-750 using Ni-7Cr-3Fe-3.2B-4.5Si (wt.-%) foil as brazing filler metal was investigated in this paper. Brazing was conducted at the temperature range 1373–1473 K for 0–300 s in a flow argon environment. Both interfacial microstructures and mechanical properties of brazed joints were investigated to evaluate joint quality. The optical and scanning electron microscopic results indicate that good wetting existed between the brazing alloy and both Inconel 718 and Inconel X-750. Microstructures at joint interfaces of all samples show distinct multilayered structures that were mainly formed by isothermal solidification and following solid-state interdiffusion during joining. The diffusion of boron and silicon from brazing filler metal into base metal at the brazing temperature is the main controlling factor pertaining to the microstructural evolution of the joint interface. The element area distribution of Cr, Fe, Si, Ni and Ti was examined by energy dispersive X-ray analysis. It was found that silicon and chromium remain in the center of brazed region and form brittle eutectic phases; boron distribution is uniform across joint area as it readily diffuses from brazing filler metal into base metal. The influence of heating cycle on the microstructures of base material and holding time on the mechanical properties of brazed joint were also investigated.     

Reinitiation of substrate oxide undermining by Ni-P filler metals during wetting of nickel-base alloys

The process of wetting of nickel and nickel-chromium substrates by nickel-phosphorus high-temperature brazing filler metals has been examined. The filler was found to wet the substrate by undermining the oxide layer on the substrate. Evidence was found that the undermining process is halted temporarily at discontinuities in the substrate oxide which typically occur above substrate grain boundaries. It is suggested that it is initially more favourable energetically for the filler to spread along the oxide defects than to continue undermining. Mechanisms for the reinitiation of undermining have been considered. It is proposed that undermining recommences after the onset of isothermal solidification of the filler spreading along oxide defects.     

Microstructure of Ni-10Co-8Cr-4W-13Zr alloy and its bonding behaviour for single-crystal nickel-base superalloy

In the later stages of solidification of zirconium-containing superalloys, the concentration of zirconium in the interdendritic melts is above 10 wt%. The dendrites formed in the early stage of solidification may be considered to be joined by the interdendritic zirconium-rich melt. Based on the composition of the zirconium-rich melt, an Ni-10Co-8Cr-4W-13Zr (wt %) alloy was selected as an interlayer alloy for brazing and transient liquid-phase (TLP) bonding of single crystal superalloys. All the elements in the interlayer alloy are beneficial to the single-crystal superalloys. A bonding microstructure which is very similar to that of the base alloy was obtained by means of the TLP process using this interlayer alloy. In the present work, the microstructural characteristics of the interlayer alloy and the phase relationships in the bond during brazing and TLP processes were investigated.     

Role of the reaction product in the solidification of Ag–Cu–Ti filler for brazing diamond

In order to reveal the mechanism for brazing diamond using Ag–Cu–Ti filler metal, thermoanalysis of elemental metals (silver and copper) either with added diamond micropowder or with added titanium carbide micropowder as nucleant were investigated to detect undercoolings. No undercooling for the solidification of silver with added titanium carbide powder was detected by the thermoanalytical curve, and also no undercooling for copper with added diamond powder was detected. These phenomena suggest that titanium carbide powder acts in the solidification of silver effectively as a nucleant and that diamond powder also acts in the solidification of copper as a nucleant. Fine-grained silver was observed in the micrograph of the silver added with titanium carbide powder. The results of the calculations on the planar disregistry, , and the dispersion energy, Edisp revealed that the Ag (100)–TiC(100) interface and Cu(100)–diamond (100) interface are more stable than the other combinations. The results of undercoolings of various specimens correlated with both planar disregistry and dispersion energy. According to these results, the titanium carbide reaction product is considered to play an important role in the solidification of silver. The brazing strength is considered to arise from the solidification of the brazing filler metal from the titanium carbide reaction product. © 1998 Chapman & Hall     

铝基钎料在SiC及SiC_p/6061复合材料上的润湿性研究

对多种铝基钎料在SiC、6 0 6 1及SiCp 6 0 6 1复合材料上进行了润湿性试验。结果表明 :炉中钎焊时 ,钎料与钎剂的成分、加热温度与保温时间、钎料与钎剂熔化温度的匹配等是影响铝基钎料润湿性的主要因素 ;真空钎焊时 ,镁含量不同的各种含镁Al 2 8Cu 5Si钎料在Al基复合材料连接的温度范围内都不能润湿SiC陶瓷表面 ;配合QJ2 0 1钎剂 ,Al 2 8Cu 5Si 2Mg钎料对 15 %SiCp 6 0 6 1Al复合材料具有良好的润湿性 ,但对 30 %SiCp 6 0 6 1Al复合材料却润湿不良 ;在加钎剂的情况下 ,钎料中的镁反而对在铝合金及铝基复合材料上的润湿性有不利影响 ;在Al 2 8Cu 5Si 2Mg钎料和 15 %SiCp 6 0 6 1Al复合材料的钎焊界面处存在SiC颗粒的偏聚现象     

Effects of starting carbon and solvent-catalyst on the reaction sintering of diamond

Polycrystalline diamond sintered compact was prepared under high pressure and temperature conditions (7 GPa, 1700° C, 10 to 15 min) from purified natural graphite (NG) or graphitized pitch coke (GPC) using iron, cobalt or nickel as the catalytic active metal and titanium or zirconium as the solvent metal. The effects of the combinations of starting carbon and solvent-catalyst on the transformation behaviour and morphology of the converted diamond were investigated in relation to the starting powder compositions. Diamond crystals converted from NG tended to have euhedral habits, when twin crystals were occasionally found due to a relatively rapid conversion and growth rate of diamond. On the other hand, a skeletal structure of diamond was easily formed by a mild conversion from GPC in the 15 to 30 vol% ( 40 to 60 wt%) nickel solvent-catalyst. The degree of catalytic action in the 8A group of 3d transition metals for the GPC system was in the order: Ni Co > Fe. The grain growth of the converted diamond was depressed by the addition of 4A transition metals (titanium or zirconium) which results in the fine-grained and homogeneous sintered microstructure.     

Void formation in wide gap brazing using prepacks of nickel base braze mixes

Abstract

The various types of void formed in wide gap brazed joints of C1023 nickel base superalloy produced using prepacks of nickel base braze mixes have been investigated and systematically categorised. Of particular interest are interfacial and interstitial voids and unwetted pockets, which are features frequently found in joints brazed using such a technique. Examination of brazed joints produced under a wide range of conditions revealed that during heating to the brazing temperature, the braze mix partially sinters together, causing the prepack to shrink towards the centreline, leaving two channels next to the joint faying surfaces. At the same time, relatively large pockets of free space are created within the partially sintered mass of prepack. At the brazing temperature, the filler metal deposited at the gap mouth becomes molten and this molten filler is drawn into the gap preferentially through the fine capillary paths in the partially sintered mass of prepack. The relatively large pockets of free space in the prepacks as well as the channels adjacent to the joint faying surfaces are bypassed by the molten filler metal owing to their lack of capillarity. As a result, the various brazing defects described above will be formed should solidification occur before these empty spaces are filled. The effects of materials and process variables on the formation of various brazing defects are briefly discussed.

MST/1925     

Thermoelectric properties of fine grained (75% Sb2Te3-25% Bi2Te3)p-type and (90% Bi2Te3-5% Sb2Te3-5% Sb2Se3)n-type alloys

The thermoelectric properties of fine-grained alloys prepared by either cold pressing and sintering or hot pressing in the range 5–50 m are compared with single-crystal best-direction values. It is shown that for thep-type alloy, almost the entire thermoelectric properties are recovered, i.e. the figure of merit for the finest grain size is almost the same as the best single-crystal value. The same trend is observed in then-type alloy except that 90% of the single-crystal figure of merit is recovered. These results are discussed in terms of a model which suggests that degradation of favourable thermoelectric properties by powdering the alloys is compensated by (1) decrease of thermal conductivity due to scattering of phonons at grain boundaries for grain sizes that are comparable to the mean free path of phonons; and (2) retention of some of the anisotropic properties of the single crystal in the fine-grained compacts.     

Effect of holding time on vacuum brazing for a stainless steel plate–fin structure

This paper presents a vacuum brazing of 304 stainless steel plate–fin structures with nickel-based BNi-2 filler metal. The effect of brazing holding time on tensile strength and microstructure has been investigated, aiming to obtain the optimal brazing holding time. The microstructure in brazing joint consists of diffusion-affected zone (DAZ), interface reaction zone (IRZ), isothermally solidified zone (ISZ) and athermally solidified zone (ASZ). The structure in the fillet is composed of solid solution, nickel silicon, nickel boron compound and a mixture with nickel silicon and nickel boron. The tensile strength increases along with the increase of holding time, but decreases when the holding time is over 25 min. A maximum tensile strength of 65.1 MPa is obtained with 25 min holding time. Too short holding time will make boron diffuse insufficiently and generate a great deal of brittle boride components, and too long holding time will make the base metal dissolve into the filler metal excessively and creates more corrosion voids.     

The influence of grain size,stress and temperature on the steady state creep of a 25 Cr-20 Ni austenitic stainless steel without precipitates

This work was performed in order to study the steady state creep behaviour of a modified 25 Cr-20 Ni stainless steel which has no precipitates. The test temperature range was 1171 to 1211 K, the stress range 4.9 to 19.6 MPa, and the grain size was 40 to 600m. The steady state creep rate, , decreases with increase in grain size, especially at the lowest stress; is proportional to 1/d ² at 4.9 MPa, whered is a mean grain diameter. The variation of with grain size is smaller in the middle and coarse-grained specimens than in the fine-grained specimens, the stress exponent,n, gradually decreases from ~ 4 to ~ 1.5 with reducing stress, but in the middle- and coarse-grained specimens, a discontinuous point appears on a log versus log plot. The activation energy for the steady state creep of the coarse-grained specimens tends to be larger than that of the fine-grained specimens, and the tendency is remarkable in the higher stress level. It is indicated that the creep mechanisms in the fine-grained specimens are essentially different from those in the coarse-grained specimens, and that the creep at the lowest stresses and smallest grain size is similar to that predicted by a vacancy creep model involving grain-boundary diffusion.     

Modifizierte Nickelbasisstandardlote für korrosiv belastete Bauteile

Modified nickel based standard brazing filler metals for units under corrosive loads Nickel‐based alloys are presently used as brazing filler metals for components which undergo mechanical stress in corrosive conditions, f. e. heat exchangers. When soldering chrome containing steel parts with nickel based brazing filler metals additionally containing boron and silicon a reaction of chrome and boron can occur. This evolution of chromium borides, depositing on grain boundaries, causes a lack of chrome in the steel part. A drop of the chrome content in the parts below 13 % leads to a loss of corrosion resistance. It is possible to change the microstructure of brazing joints by modification with chromium and molybdenum. Continuous brittle phases could be successfully avoided with this modification. Furthermore it could be shown that the choice of additives, the heating respectively cooling rate and the brazing temperature have important influence on the microstructure evolution and therefore on the mechanical and corrosive properties of joints.     

Influence of bonding variables on transient liquid phase bonding behavior of nickel based superalloy IN-738LC

The effect of process variables on the microstructure and properties of transient liquid phase (TLP) bonded IN-738LC superalloy was investigated using AMS 4776 filler metal. Continuous centerline eutectic phases, characterized as nickel-rich and chromium-rich borides, were observed at the joints with incomplete isothermal solidification. The eutectic width decreased with the increase of holding time and the increase in initial gap size resulted in thicker eutectic width in the samples bonded at the same temperature and for equivalent holding times. In contrast to the conventional expectation of the increase in the rate of isothermal solidification with the increase of temperature, rate decrease was observed with the increase of temperature to 1150 °C. The investigations demonstrated that low isothermal solidification rate was not only due to the enrichment of liquid phase with the base alloying elements such as Ti but also because of the reduction of solid solubility limit of B in the base metal contributed to the reduction of isothermal solidification rate. Microhardness and shear strength tests were carried out in order to investigate mechanical properties of the bonded samples. In the bonding condition in which isothermal solidification did not completely occur, eutectic constituent with the highest hardness in the bond region was the preferential failure source. The results showed that homogenized joints had the highest shear strength.     

Counteraction of particulate segregation during transient liquid-phase bonding of aluminium-based MMC material

Particle segregation during transient liquid-phase bonding of aluminium-based metal matrix composite material using copper filler metal was investigated. Segregation was promoted by the slow movement of the solid-liquid interface during isothermal solidification and alumina particles with diameters less than 30 Μm were segregated when the copper foil thickness exceeded 5 and 15 Μm for the base metals examined. When bonding at 853 K, the liquid widths produced using these copper foil thiciknesses were almost identical to the median inter-particle spacing in the base metals investigated. When the amount of liquid formed at the bonding temperature decreased below a critical level, the test specimens broke apart immediately following the joining operation. The minimum film thickness of copper for satisfactory joint strength increased from 0.6 Μm to 2.4 Μm, when the heating rate to the bonding temperature decreased from 1 Ks^?1 to 0.01 Ks^?1.     

High resistivity BaTiO3 ceramics sintered in CO-CO2 atmospheres

A barium titanate ceramic containing 13.5 mol % calcium zirconate was doped with up to 3 mol % of oxides of various metals that were considered likely on the basis of ionic size and valence to enter the small cation lattice with charge less than 4+. It was hoped in this way to compensate electrically for loss of oxygen during sintering in CO-CO₂ mixtures, so as to obtain high resistivity dielectrics and allow the use of base metal electrodes in a monolithic capacitor. Doping with approximately 0.5 mol % Mn, Co, or Mg produced the highest resistivities, and dielectrics with nickel electrodes and relative permittivity up to 10 000 were obtained with resistivity in excess of 10¹² cm at room temperature. When the doped ceramic sintered in contact with nickel, the grain structure and permittivity-temperature characteristics depended on the oxygen partial pressure of the sintering atmosphere, apparently influenced by dissolution of Ni into the ceramic.     

Microstructure evolution of Mg–Al–Zn alloys during compression test at low strain and temperature

The microstructure evolutions and texture changes during the compression test were investigated using an extruded magnesium alloy with average grain sizes of 11.4 and 49.6 μm. The deformation twins were formed in all the samples; however, a comparison of the fraction of deformation twins on the effect of grain size and initial texture, i.e., the cutting position (normal or parallel to the extrusion), showed that the fine-grained alloy and/or the sample with the normal-cut to the extrusion had a lower fraction of deformation twins. On the other hand, the texture change showed different tendencies depending on the grain size and/or the initial texture. In the coarse-grained alloy, since the dominant deformation mechanism was the deformation twins, the lattice was rotated without relation to the initial texture. However, in the fine-grained alloy, even the applied strain of 0.20, the intensity peaks existed at 10-10 and the basal texture remained in the sample with the parallel- and normal-cut to the extrusion, respectively. This resulted from the difference in the fraction of deformation twins and the occurrence of partial grain boundary sliding.     

Influence of oxygen-rich inclusions on the γ→α phase transformation in high-strength low-alloy (HSLA) steel weld metals

Reducing the oxygen content of two submerged-arc, high-strength, low-alloy (HSLA) steel weld metals has been shown to depress transformation temperatures and produce a marked change in the resultant microstructures. In weld metal of composition 0.12wt% C, 1.35wt% Mn, 0.29 wt% Si, 0.03 wt% Nb reducing the oxygen content from about 300 ppm to about 60 ppm decreased the transformation initiation temperature by about 30° C and changed the microstructure from acicular ferrite to parallel lath ferrite. In weld metal of composition 0.1 wt% C, 0.8 wt% Mn, 0.1 wt% Si, 0.01 wt% Nb reducing the oxygen content from about 600 ppm to about 300 ppm decreased the transformation initiation temperature by approximately 20° C and favoured the development of ferrite side-plates and acicular ferrite at the expense of the polygonal ferrite microstructure. In both weld metals the depressed transformation temperature is thought to be due to the larger -phase grain size developed when the volume fraction of small de-oxidation products is reduced. The marked microstructural change from fine-grained acicular ferrite to parallel lath ferrite which occurred when virtually all the de-oxidation products were removed suggests that these small de-oxidation products may also be of fundamental importance to the nucleation of acicular ferrite.     

Transient liquid phase diffusion bonding of oxide dispersion strengthened nickel alloy MA758

Abstract

Transient liquid phase diffusion bonding has been used to join an oxide dispersion strengthened (ODS) nickel alloy (MA758) using an amorphous metal interlayer with a Ni–Cr–B–Si composition. A microstructural study was undertaken to investigate the effect of parent metal grain size on the joint microstructure after isothermal solidification. The ODS alloy was bonded both in fine grain and recrystallised conditions at 1100°C for various hold times. The work shows that the final joint grain size is independent of the parent alloy grain structure and the bonding time. However, when the alloy is bonded in the recrystallised condition and given a post-bond heat treatment at 1360°C, the joint grain size increases and a continuous parent alloy microstructure across the joint region is achieved. If MA758 is bonded in the fine grain condition and then subjected to a recrystallising heat treatment at 1360°C, the grains at the joint appear to increase in size with increasing bonding time. The joint grains are generally larger than those produced when the alloy is bonded in the recrystallised condition. The differences in microstructural developments across the joint are discussed in terms of stored strain energy of the parent metal grains.     

Interfacial reactions between titanium film and single crystal α-Al2O3

Titanium is commonly used to join metals and ceramics by active metal brazing methods. In this work, titanium was sputter deposited on to single-crystal -Al₂O₃ substrates and the interfacial reactions between the titanium film and the Al₂O₃ substrate were studied. Al₂O₃ was reduced by titanium when samples were annealed at 973 and 1173 K for 300 s in an argon gas flow. Metallic aluminium was produced at the interface, and this diffused from the interface into the titanium film. At 1173 K, the intermetallic compound Ti₃Al and the intermediate titanium oxides, such as Ti₂O and TiO, were formed. The Al⁰ diffusion is important in stimulating interfacial reactions.