On the corrosion fatigue behavior of a modified SAE 4135 type steel in a H2S environment

The corrosion fatigue behavior of a modified SAE 4135 steel in a H₂S environment is compared to that of a standard SAE 4135 steel of the same strength level. The modification consists of an increase in molybdenum content from 0.20 to 0.75 pct and the addition of 0.036 pct columbium. It was found that the high cycle fatigue strength of the modified steel was 66 pct higher than that of the standard steel. The improvement is attributed to an increase in resistance to hydrogen embrittlement brought about by a higher tempering temperature which reduces the amount of phosphorous on grain boundaries, as well as the beneficial scavenging effects associated with the increase in molybdenum content and the presence of columbium. Formerly Graduate Student at University of Connecticut, is now with Nippon Kokan, K. K., Fukuyama, Japan.     

Room-Temperature strength and deformation of Tib2-reinforced near-γ titanium aluminides

A series of TiB₂-reinforced near-γ titanium aluminide (Ti-Al) matrix composites have been produced in investment-cast form and characterized with respect to microstructure and tensile deformation. The Ti-Al matrices of the composites examined are based upon the binary composition Ti-47 Al (at. pct), with varying proportions (2 to 6 cumulative percent) of manganese, vanadium, chromium, and niobium. TiB₂ has been introduced into the microstructuresvia XD* processing at levels of 7 and 12 vol pct and compared to unreinforced (0 vol pct TiB₂), base variants. The influences of heat-treatment temperature and time have also been studied for each composition and reinforcement variant. The addition of dispersed TiB₂ leads to a fine, stable, and homogeneous as-cast matrix microstructure. The measured TiB₂ size within the composites examined ranged from 1.4 to 2.6 μm. Increasing the volume fraction of TiB₂ leads to increased elastic moduli, increased ambient temperature tensile strengths, and in general, increased strain-hardening response. In some instances, the overall ductility of the alloy increases with the addition of TiB₂ reinforcement. The flow stresses of both the monolithic and composite variants exhibit conventional power-law plasticity. The results indicate that the strengthening and the flow behavior in these composites are derived from both indirect and direct sources. Strengthening contributions are indirectly derived from the microstructural changes within the matrix of the composite that evolve due to the presence of the reinforcement during its evolution and development, for example, due to grain refinement and reinforcement-derived interstitial solid-solution strengthening. Direct contributions to strength are those that can be specifically attributed to the presence of the reinforcement during deformation,e.g., through the interaction of dislocations with the reinforcing particles. When the estimates of the indirect contributions are isolated and arithmetically removed from the magnitude of the total observed strength of the composite, the increase in flow stress correlates in all instances with the inverse square root of the planar interparticle spacing for all alloy compositions, heat treatments, and levels of strain examined.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.     

Microstructure and properties of In situ Al/TiB2 composite fabricated by in-melt reaction method

A novel in situ reaction process-in-melt reaction method was developed. TiB₂ particles form in situ through the reaction of TiO₂, H₃BO₃, and Na₃AlF₆ in an aluminum alloy melt. The results showed that the in situ TiB₂ particles formed were spherical in shape and had an average diameter of about 0.93 μm. Moreover, the distribution of TiB₂ particles in the matrix was uniform. The interface between the TiB₂ particles and the matrix showed good cohesion. The tensile strength and the yield strength of the composite increase with increasing TiB₂ content. When TiB₂ particle content in the matrix was 10 vol pct, the tensile strength, yield strength, and elongation of Al-4.5Cu/TiB₂ composite were 417 MPa, 317 MPa, and 3.3 pct, respectively.     

Development of Al-Ti-C grain refiners containing TiC

Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first, but was subsequently found to react with the melt forming a sheathing of A1₄C₃ and Ti₃AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable. Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable to that obtainable with the standard TiB₂ commercial grain refiner. TiC particles introducedvia the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals. On leave from Regional Research Laboratory (CSIR), Bhopal, is Research Associate.     

Coating of 6028 Aluminum Alloy Using Aluminum Piston Alloy and Al-Si Alloy-Based Nanocomposites Produced by the Addition of Al-Ti5-B1 to the Matrix Melt

The Al-12 pctSi alloy and aluminum-based composites reinforced with TiB₂ and Al₃Ti intermetallics exhibit good wear resistance, strength-to-weight ratio, and strength-to-cost ratio when compared to equivalent other commercial Al alloys, which make them good candidates as coating materials. In this study, structural AA 6028 alloy is used as the base material. Four different coating materials were used. The first one is Al-Si alloy that has Si content near eutectic composition. The second, third, and fourth ones are Al-6 pctSi-based reinforced with TiB₂ and Al₃Ti nano-particles produced by addition of Al-Ti5-B1 master alloy with different weight percentages (1, 2, and 3 pct). The coating treatment was carried out with the aid of GTAW process. The microstructures of the base and coated materials were investigated using optical microscope and scanning electron microscope equipped with EDX analyzer. Microhardness of the base material and the coated layer were evaluated using a microhardness tester. GTAW process results in almost sound coated layer on 6028 aluminum alloy with the used four coating materials. The coating materials of Al-12 pct Si alloy resulted in very fine dendritic Al-Si eutectic structure. The interface between the coated layer and the base metal was very clean. The coated layer was almost free from porosities or other defects. The coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages (1, 2, and 3 pct), results in coated layer consisted of matrix of fine dendrite eutectic morphology structure inside α-Al grains. Many fine in situ TiAl₃ and TiB₂ intermetallics were precipitated almost at the grain boundary of α-Al grains. The amounts of these precipitates are increased by increasing the addition of Al-Ti5-B1 master alloy. The surface hardness of the 6028 aluminum alloy base metal was improved with the entire four used surface coating materials. The improvement reached to about 85 pct by the first type of coating material (Al-12 pctSi alloy), while it reached to 77, 83, and 89 pct by the coating materials of Al-6 pct Si-based mixed with Al-Ti5-B1 master alloy with different percentages 1, 2, and 3 pct, respectively.     

Chemistry and Mechanism of Interaction Between Molybdenite Concentrate and Sodium Chloride When Heated in the Presence of Oxygen

Roasting of molybdenum concentrates with sodium chloride has high potential and can be an alternative to oxidizing roasting and autoclave leaching; however, the chemistry and mechanism are poorly known. The chemical mechanism of the roasting process between molybdenite concentrate and sodium chloride in the presence of atmospheric oxygen is proposed. It is demonstrated that the process occurs through molybdenite oxidation, up to molybdenum trioxide, with subsequent formation of sodium polymolybdates and molybdenum dioxydichloride from molybdenum trioxide. It is found that the formation of water-soluble sodium polymolybdates from molybdenum trioxide stops over time due to passivation of sodium chloride surface by polymolybdates. It is proved experimentally that preliminary grinding of the mixture in a furnace charge leads to an increase in the polymolybdate fraction of the roasting products, which constitutes approximately 65 pct of molybdenum initially in the roasted mixture against 20 to 22 pct in a nonground mixture (or 75 to 77 pct against 30 to 33 pct of molybdenum in calcine). For the first time, the presence of the Na₂S₂O₇ phase in the calcine was confirmed experimentally. The suggested mechanism gives possible explanations for the sharp increase of MoO₂Cl₂ formation within the temperature range of 673 K to 723 K (400 °C to 450 °C) that is based on the catalytic reaction of molybdenum dioxydichloride from the Na₂S₂O₇ liquid phase as it runs in a melt.     

Boron removal by titanium addition in solidification refining of silicon with Si-Al melt

In order to effectively remove B from Si for its use in solar cells, a process involving B removal by solidification refining of Si using a Si-Al melt with Ti addition was investigated. For clarifying the effect of Ti addition on B removal from the Si-Al melt, TiB₂ solubilities in Si-64.6 at. pct Al melt at 1173 K and Si-60.0 at. pct Al melt at 1273 K were determined by measuring the equilibrium concentrations of B and Ti in the presence of TiB₂ precipitates. The small solubilities of TiB₂ in the Si-Al melt indicate the effective removal of B from the Si-Al melt by Ti addition. Further, solidification experiments of Si-Al alloys containing B by Ti addition were performed, and the effect of Ti addition on the solidification refining of Si with the Si-Al melt was successfully confirmed.     

Microstructure/processing relationships in reaction-synthesized titanium aluminide intermetallic matrix composites

Near-γ TiAl- and Al₃Ti-based intermetallic matrix composites have been produced using in-situ reaction-synthesis techniques. The intermetallic matrices have been reinforced with relatively high loadings (e.g., 20 to 50 vol pct) of dispersed TiB₂ particulates. It is shown that the as-synthesized TiB₂ size is strongly dependent on the specific alloy formulation; specifically, the TiB₂ size tends to increase as the nominal volume percent of TiB₂ in the composite increases. The observed size effect is determined to be associated with the temperature that is attained during the synthesis event, which is established primarily by the net exothermicity of the participating synthesis reaction(s). The exothermicity of the reactions can be assessed through the calculation of a formulations’s adiabatic temperature, which is found to increase with the percentage of TiB₂ over the range of approximately 10 to 60 vol pct. The coupling of a composite’s characteristic adiabatic temperature with the resulting reinforcement size provides direct links among composition, processing, and mechanical performance, since the size of a reinforcing particle is influential in establishing the interparticle spacing, which, in turn, establishes the strengthening potency of the dispersed phase within the composite.     

Microstructure Development,Nanomechanical, and Dynamic Compression Properties of Spark Plasma Sintered TiB2-Ti-Based Homogeneous and Bi-layered Composites

The growing threats due to increased use of small-caliber armor piercing projectiles demand the development of new light-weight body armor materials. In this context, TiB₂ appears to be a promising ceramic material. However, poor sinterability and low fracture toughness remain two major issues for TiB₂. In order to address these issues together, Ti as a sinter-aid is used to develop TiB₂-(x wt pct Ti), (x = 10, 20) homogeneous composites and a bi-layered composite (BLC) with each layer having Ti content of 10 and 20 wt pct. The present study uniquely demonstrates the efficacy of two-stage spark plasma sintering route to develop dense TiB₂-Ti composites with an excellent combination of nanoscale hardness (~36 GPa) and indentation fracture toughness (~12 MPa m^1/2). In case of BLC, these properties are not compromised w.r.t. homogeneous composites, suggesting the retention of baseline material properties even in the bi-layer design due to optimal relief of residual stresses. The better indentation toughness of TiB₂-(10 wt pct Ti) and TiB₂-(20 wt pct Ti) composites can be attributed to the observed crack deflection/arrest, indicating better damage tolerance. Transmission electron microscope investigation reveals the presence of dense dislocation networks and deformation twins in α-Ti at the grain boundaries and triple pockets, surrounded by TiB₂ grains. The dynamic strength of around 4 GPa has been measured using Split Hopkinson Pressure Bar tests in a reproducible manner at strain rates of the order of 600 s^?1. The damage progression under high strain rate has been investigated by acquiring real time images for the entire test duration using ultra-high speed imaging. An attempt has been made to establish microstructure-property correlation and a simple analysis based on Mohr–Coulomb theory is used to rationalize the measured strength properties.     

Grain refinement in aluminum alloyed with titanium and boron

The aluminum corner of the ternary Al-B-Ti diagram was explored. A eutectic: Liq — Al + TiAl₃ + (Al, Ti)B₂ was found at approximately 0.05 wt pct Ti, 0.01 wt pct B; 659.5‡C. TiB₂ and A1B₂ form a continuous series of solid solutions, but no distinct ternary phase was found. The addition of boron to aluminum-titanium alloys expands the field of primary crystallization of TiAl₃ toward lower titanium contents and steepens the liquidus. In equilibrium conditions, pronounced grain refinement is found only in alloys in which TiAl₃ is primary and nucleates the aluminum solid solution before any other impurity can act. The peritectic reaction facilitates this priority but it is not necessary for grain refinement. Because of the low diffusivity of titanium and boron in aluminum, equilibrium is seldom attained and in commercial practice grain refinement by TiAl₃ is found also outside its equilibrium field of primary crystallization.     

High-temperature slow-strain-rate compression studies on CoAI-TiB2 composites

CoAl-base composites containing various volume fractions of TiB₂ particulates in the 1 to 3-μm size range were tested in compression over a range of strain rates at 1100 to 1300 K. Hotpressed and postcompression microstructures were characterized using optical and transmission electron microscopy. For a strain rate of 2 × 10^-6 s^-1, the flow stress at 1300 K of CoAl-20 vol pct TiB₂ was twice that of monolithic CoAl. Correspondingly, the stress exponent of CoAl-20 vol pct TiB₂ at 1300 K was ∼4.5 compared to ∼3.0 for stoichiometric monolithic CoAl. This increased resistance of the composite to deformation at elevated temperatures is attributed to TiB₂ particulate-dislocation interactions. The high-temperature compressive deformation behaviors of NiAl-TiB₂ and CoAl-TiB₂ composites are compared.     

Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys

A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum. Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after the addition of grain refiner. From the results, except for formation of TiB₂, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions of Al-Ti-B master alloys, TiB₂ particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature. Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of the melt.     

The incorporation of self-propagating,high-temperature synthesis-formed Fe-TiB2 into ferrous melts

Steel-matrix particulate composites were processed by direct addition of an Fe-TiB₂ master alloy powder to a BS970:080M30 medium-carbon steel. This powder was produced using a self-propagating, high-temperature synthesis (SHS) reaction and consisted of a dispersion of fine TiB₂ particles (2 to 5 μm), respectively, in an iron binder. The addition of the Fe-TiB₂ powder resulted in the formation a parasitic Fe₂B phase and TiC within the steel microstructure. In response to this, an SHS master alloy composed of Fe-(50 pct TiB₂+50 pct Ti) was manufactured, which, when added to the steel, prevented the formation of Fe₂B and resulted in a composite containing a mixture of TiB₂ and TiC particles. The effect of master alloy composition and addition level on the microstructural phases generated is discussed in detail. The response to heat treatment of composite materials manufactured in this way was also investigated. It was found that an isothermal hold at 840 °C leads to a substantial softening of the material processed using the Fe-TiB₂ additive, while at 1000 °C, a hardness level equivalent to that of the as-cast material was maintained. The same heat treatment of samples in which the formation of Fe₂B was suppressed resulted in no appreciable difference in hardness level or microstructure.     

Electro-extraction of molybdenum from Mo2C-type carbide

A process for the preparation of molybdenum from molybdenum carbide was investigated. It involved fused salt electrolysis of the carbide in an inert atmosphere electrolytic cell using a KCl-K₃MoCl₆ electrolyte. The preferred conditions for electrolysis carried out in a 0.075 m (3 in.) diam cell were: voltage 0.2 to 0.5 V; cathode current density 8000 A/m2 (720 amp/f²);bath temperature 1203 K; and electrolyte composition 7.5 pct molybdenum. Under these conditions, electrolysis in a 0.15 m (6 in.) diam cell charged with 1.5 kg of the carbide yielded a total metal recovery of 71 pct at an average current efficiency of 60 pct. The metal purity was better than 99.9 pct. The electron beam melt hardness for the electro-extracted molybdenum was in the range of 150 to 160 DPH. The paper presents a part of the thesis to be submitted by A. K. SURI to the University of Bombay (India).     

Co-deformation processing and modeling of <Emphasis Type="Italic">In-Situ</Emphasis> multiphase composites

A series of in-situ, deformation-processed metal matrix composites were produced by direct powder extrusion of blended constituents. The resulting composites are comprised of a metallic Ti-6Al-4V matrix containing dispersed and co-deformed discontinuously reinforced-intermetallic matrix composite (DR-IMC) reinforcements. The DR-IMCs are comprised of discontinuous TiB₂ particulate within a titanium trialuminide or near-γ Ti-47Al matrix. Thus, an example of a resulting composite would be Ti-6Al-4V+40 vol pct (Al₃Ti+30 vol pct TiB₂) or Ti-6Al-4V+40 vol pct (Ti-47Al+40 vol pct TiB₂), with the DR-IMCs having an aligned, high aspect ratio morphology as a consequence of deformation processing. The degree to which both constituents deform during extrusion has been examined using systematic variations in the percentage of TiB₂ within the DR-IMC, and by varying the percentage of DR-IMC within the metal matrix. In the former instance, variation of the TiB₂ percentage effects variations in relative flow behavior; while in the latter, varying the percentage of DR-IMC within the metallic matrix effects changes in strain distribution among components. The results indicate that successful co-deformation processing can occur within certain ranges of relative flow stress; however, the extent of commensurate flow will be limited by the constituents’ inherent capacity to plastically deform.     

The constitution of the system Al-Ti-B with reference to aluminum-base alloys

Direct and differential thermal analyses have been used to determine the liquidus temperatures and isothermals in the aluminum-base corner of the Al-Ti-B system, and with microscopical information and electron microprobe analyses provide results on which is based a provisional phase diagram. The TiB₂ liquidus rises very steeply from the aluminum corner of the diagram and is bounded on either side by monovariant reactions which almost coincide with the binary Al-B and Al-Ti liquidus curves. If the Ti:B ratio exceeds 7∶3 wt pct (1∶2 at. pct), TiB₂ precipitates on cooling until there occurs a monovariant reaction involving Al₃Ti, while at lower Ti:B ratios there occurs a monovariant reaction which involves AlB₂. The grain refining action of ternary alloys is discussed with reference to the form of the phase diagram.