Dynamic compaction of titanium Aluminides by Explosively Generated Shock Waves: Microstructure and Mechanical Properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[131] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.     

Dynamic compaction of titanium aluminides by explosively generated shock waves: Microstructure and mechanical properties

A detailed microstructural analysis and evaluation of the mechanical properties of titanium aluminides consolidated by novel shock processes^[13] are presented. Successful consolidation was obtained and was evidenced by strong bonding between individual particles. Additions of Nb and Ti and Al elemental powders resulted in enhanced interparticle bonding through intense plastic deformation of Nb and shock-induced reactions between Ti and Al. Rapid cooling of interparticle molten layers yielded amorphous Ti-Al alloys; this interparticle melting and rapid cooling are a unique feature of shock processing. Embrittlement of individual particles of Ti₃Al-based alloy after exposure to 550 °C and 750 °C was observed. There is evidence of phase transformation after preheating the powder, and this fact can explain the high density of cracks obtained with this alloy after high-temperature shock consolidation. Mechanical properties of the Ti₃Al-based alloy were determined at room temperature and the fracture modes were studied. The microstructural observations are correlated with the mechanical properties.     

Interface characterization of ceramic fiber-reinforced ti alloy composites manufactured by infrared processing

Interfacial reactions between several ceramic fibers (SCS-0, SCS-6, and carbon fibers) and a liquid titanium-nickel-copper alloy were investigated using electron microscopic analysis. Composite spec-imens were produced using a rapid infrared manufacturing (RIM) process. In SCS-O/Ti alloy com-posites, SiC dissolved in the alloy. The main reaction product was discontinuous agglomerates of titanium carbide which formed from the reaction between dissolved carbon and titanium. Polygonal precipitates of Ti₅Si₃, which are believed to have formed during cooling, were also noticed. Two distinct interface morphologies were observed in these composites: uniform fronts caused by iso-thermal dissolution and scalloped fronts formed as a result of an accelerated dissolution mechanism caused by localized heating. The presence of the accelerated dissolution mechanism suggests that SiC fibers cannot be infiltrated with liquid titanium alloys without applying a coating. In the C/Ti system, carbon fibers reacted with the liquid alloy to form a continuous layer of Ti_xC_1-x. Further growth of this layer occurred by the diffusion of carbon atoms across the reaction product. In SCS-6/Ti alloy composites, free carbon present in the coating formed a discontinuous layer of Ti^C,^, whereas SiC particles dissolved in the alloy. Due to channeled dissolution in the coating, the accel-erated dissolution mechanism was not observed in these composites. As a result, the presence of the carbon-rich coating prevented degradation of the fibers. Although the coating present on SCS-6 fibers moderately retarded reactions in the SiC/Ti alloy composite system during infrared liquid infiltration, it is recommended that the fibers be coated with pure carbon to effectively limit the attack of the fiber by molten titanium. Formaly Postdoctoral Fellow, Department of Materials Science and Engineering, University of Cincinnati     

Microstructure Refinement After the Addition of Titanium Particles in AZ31 Magnesium Alloy Resistance Spot Welds

Microstructural evolution of AZ31 magnesium alloy welds without and with the addition of titanium powders during resistance spot welding was studied using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The fusion zone of AZ31 magnesium alloy welds could be divided into columnar dendritic zone (CDZ) and equiaxed dendritic zone (EDZ). The well-developed CDZ in the vicinity of the fusion boundary was clearly restricted and the coarse EDZ in the central region was efficiently refined by adding titanium powders into the molten pool, compared with the as-received alloy welds. A microstructural analysis showed that these titanium particles of approximately 8 μm diameter acted as inoculants and promoted the nucleation of α-Mg grains and the formation of equiaxed dendritic grains during resistance spot welding. Tensile-shear testing was applied to evaluate the effect of titanium addition on the mechanical properties of welds. It was found that both strength and ductility of magnesium alloy welds were increased after the titanium addition. A TEM examination showed the existence of an orientation matching relationship between the added Ti particles and Mg matrix, i.e., [ 0 1[`1]0 ]<sub>\textMg</sub> //  [ 1[`2] 1[`3] ]<sub>\textTi</sub>  \textand ( 000 2 )<sub>\textMg</sub> //  ( 10[`1]0)_\textTi \left[ {0 1\bar{1}0} \right]_{\text{Mg}} // \, \left[ { 1\bar{2} 1\bar{3}} \right]_{\text{Ti}} \,{\text{and}}\,\left( {000 2} \right)_{\text{Mg}} // \, ( 10\bar{1}0)_{\text{Ti}} in some grains of Ti polycrystal particles. This local crystallographic matching could promote heterogeneous nucleation of the Mg matrix during welding. The diameter of the added Ti inoculant should be larger than 1.8 μm to make it a potent inoculant.     

Characterization of Ti carbosulfide precipitation in Ti microalloyed steels

The morphology and composition of the Ti carbosulfides observed in a family of steels containing 0.05 to 0.25 wt pct Ti were determined using optical and electron microscopy, electron microprobe analysis, and energy-dispersive X-ray (EDX) and secondary ion mass spectrometer (SIMS) techniques. It is demonstrated that the Ti carbosulfide phase has a Ti: S mole fraction ratio of 2∶1 and contains an appreciable level of carbon, its identity being Ti₄C₂S₂. The solubility product of Ti₄C₂S₂ in austenite is derived to be log [Ti] [C]^0.5[S]^0.5=−15,600/T+6.50 and that of TiS to be log [Ti] [S]=−17.640/T+8.20. The former lies between the values for TiN and TiC, whereas the latter is more soluble than TiC. Stringer inclusions consisting of globular Ti₄C₂S₂ surrounded by elongated MnS were observed in the steels with 0.05 to 0.18 wt pct Ti. The volume fraction of the stringers is shown to be related to the sulfur partition coefficient through an empirical power law function. W.J.Lju, formerly with the Department of Metallurgical Engineering, McGill University     

Relation between microstructure, composition, and hot cracking in Ti-stabilized austenitic stainless steel weldments

A stabilized, fully austenitic alloy D9, a 15Cr-15Ni-2Mo stainless steel with a titanium addition corresponding to UNS 38660, is a candidate material for the fuel-clad and wrapper applications of the Prototype Fast Breeder Reactor (PFBR). The fully austenitic microstructure and the presence of titanium in this alloy lead to high susceptibility to hot cracking during welding. The fusion-zone and the heat-affected zone (HAZ) cracking susceptibility of alloy D9 was studied at three titanium levels, 0.22, 0.32, and 0.42 pct, all other elements remaining constant. The longitudinal and transverse Varestraint (Transvarestraint) hot-cracking tests were used to evaluate fusion-zone and HAZ cracking. The results showed that titanium increases cracking in the fusion zone by 15 to 20 pct in the range of Ti levels studied. The microanalysis of fusion-zone hot cracks using electron probe microanalysis (EPMA) showed an enrichment of Ti, C, N, and S along cracks and in the interdendritic regions. The corresponding phases were identified as TiC, TiC_0.3N_0.7, and the carbosulfides Ti₂CS and Ti₄C₂S₂, which are believed to form eutectics with austenite to produce cracking. The amounts of these phases increased with increasing Ti content. In the HAZ, a similar relation between titanium level and cracking was found. The comparison of the weldability of the D9 with an FA mode type 321 revealed that Ti-bearing eutectics were responsible for a high degree of cracking irrespective of the solidification mode. The results show that in the D9, the ratio of Ti to C and N must be controlled to minimize cracking.     

Synthesis of ultrafine TiC1–X and Ti(CN) powders via planetary milling

Abstract

Ultrafine TiC_1-X and Ti(C,N) powders 100-200 nm in size were synthesised through mechanical alloying for cutting tool applications. Elemental titanium and carbon were processed by a high energy planetary mill in air, argon and nitrogen atmosphere. With variations in the Ti/C ratios, the characteristics of synthesised powders were investigated. Of three atmospheres, argon was effective in synthesising pure TiC_1-X powders. However, the presence of oxygen seemed to interfere with carbon and nitrogen diffusion into TiC lattice. Nitrogen exhibited a stronger affinity with titanium than carbon in forming the Ti(C,N) phase. This study shows the parameters used to determine the final compositions of synthesised TiC_1-X and Ti(C_1-XN_X).     

Interaction coefficients in the iron-carbon-titanium and titanium-silver systems

A liquid Fe-C-Ti system was studied by establishing an iso-titanium-activity state for ternary samples at 1600 °C through the medium of a bath of liquid silver which permits diffusion of titanium only. From the two iso-titanium-activity lines obtained, the self-interaction coefficients of titanium and interaction coefficients of carbon on titanium in liquid iron were estimated:ε _Ti ^Ti = 4.67, ρ_Ti ^Ti = 0.32, ε_Ti ^C = −11.94, ρ_Ti ^C = −4.52, ρ_Ti ^Ti,C = −9.96 An experimental study has been made of the distribution of titanium between liquid silver and liquid iron at 1600 °C. By the use of the interaction coefficients of titanium and r_Ti ^o in liquid iron, the thermodynamic parameters of titanium in liquid silver were determined asr _Ti ^o _Ag = 2.44 X 10⁻³, (ε_Ti ^Ti _Ag = −6.17, (ρ_Ti ^Ti)_Ag = −16.3     

The microstructure of selected annealed vanadium-base alloys

The microstructures of three vanadium-base alloys in the referenced annealed state were investigated by analytical transmission electron microscopy (TEM) and X-ray diffraction. The most common precipitates in all three alloys were particles of fcc TiN_1−x−yC_xO_y. In the alloy V-15Cr-5Ti, particles of type M₂₃C₆ were also found. Additional phases in the alloys V-3Ti-1Si and V-20Ti include Ti_1.7P- and Ti₈S₃-type precipitates. Precise lattice parameters of the matrices and the titanium carbonitrides were also measured. Weight percentages of the combined precipitates were determinedvia electrolytic extraction procedures.     

Evolution of microstructures in the nickel modified titanium trialuminides near the L12 phase field

This study focuses upon the evolution of microstructures during solidification processing of several intermetallic alloys around the Ll₂ phase in the Al-rich corner of the Al-Ti-Ni ternary system. The alloys were produced by double induction melting and subsequent homogenization followed by furnace cooling. The microstructure was characterized by means of optical and scanning electron microscopy with energy-dispersive spectroscopy (EDS) analysis and X-ray diffraction. The microstructural evolution in homogenized alloys was dependent on both nickel and titanium content. Very fine precipitates of Al₂Ti were observed within the Ll₂ phase in alloys containing 62 to 65 at. pct Al and at least 25 at. pct Ti. The Al₂Ti precipitates are stable at least up to 1000 °C and undergo complete dissolution at 1200 °C. In alloys containing around 66 at. pct Al and 25 to 31 at. pct Ti, phases such as Al₃Ti, Al₅Ti₂, and Al₁₁Ti₅ were observed. A modified room temperature isotherm in the Al-Ti-Ni ternary system is proposed, taking into account the existence of Al₂Ti, Al₁₁Ti₅, Al₅Ti₂, and Al₃Ti in equilibrium with the Ll₂ phase. It seems that at room temperature, the Ll₂ phase field for homogenized alloys is extremely small. It will be practically impossible to obtain a single-phase microstructure at room temperature in the Al-Ti-Ni ternary alloys after homogenization at 1000 °C followed by furnace cooling. S. BISWAS, formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo     

The effects of thermomechanical processing on the precipitation in an industrial dual-phase steel microalloyed with titanium

Analytical transmission electron microscopy was employed to characterize the precipitation at each step of the fabrication process and thermomechanical treatment of an industrial dual-phase steel microalloyed with titanium. Theoretical thermodynamic calculations as well as experimental analysis showed that more than half of the titanium carbosulfide (Ti₄C₂S₂) precipitates would dissolve during reheating at 1240 °C. Despite this dissolution at 1240 °C, the remaining titanium carbonitrides and carbosulfides were effective in pinning austenitic grain boundaries, keeping the austenitic grain size at around 40 μm (at 1240 °C). It is also shown that, during hot rolling, there exist three regions of titanium carbide precipitation. The first is defined by an increase of titanium carbide precipitation due to deformation. The second region is marked by the insignificant change in precipitation. The third region is indicated by another increase in precipitation due to the austenite-to-ferrite transformation. The experimental and theoretical results on the contribution of TiC precipitation to hardening of ferrite (Orowan mechanism) were in excellent agreement, showing that TiC precipitates have the most important effect on increasing the yield strength, overshadowing the austenitic grain-boundary pinning contributions by Ti(C,N) and Ti₄C₂S₂ precipitates.     

Influence of Cr removal on the microstructure and mechanical behaviour of a high-entropy Al0.8Ti0.2CoNiFeCr alloy fabricated by powder metallurgy

We report a systematic study on the influence of Cr removal on the microstructure and mechanical behaviour of an ultra-fine grained (UFG) high-entropy alloy (HEA), Al_0.8Ti_0.2CoNiFeCr, fabricated via spark plasma sintering (SPS) of mechanically alloyed (MA’ed) powders from constituent elemental powders. The MA’ed Al_0.8Ti_0.2CoNiFeCr powders consist principally of a BCC phase (～85 vol.-%) with a small amount of FCC phase (～15 vol.-%), whereas the MA’ed Al_0.8Ti_0.2CoNiFe powders present similar phases to those in the MA’ed Al_0.8Ti_0.2CoNiFeCr powders. Interestingly, the SPS processed UFG Al_0.8Ti_0.2CoNiFeCr alloy contains mostly an FCC phase (～78 vol.-%) and some BCC phase (～22 vol.-%); in contrast, the SPS processed UFG Al_0.8Ti_0.2CoNiFe alloy consists of a slightly enriched BCC phase (～53 vol.-%) and an FCC phase (～47 vol.-%). In addition, the SPS processed Al_0.8Ti_0.2CoNiFe alloy exhibits slightly higher yield strength, compressive strength and hardness but lower plasticity than those of the SPS processed Al_0.8Ti_0.2CoNiFeCr alloy.

Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.     

Synthesis of ductile titanium-titanium boride (Ti-TiB) composites with a beta-titanium matrix: The nature of TiB formation and composite properties

The study focused on the in-situ synthesis of titanium (Ti)-titanium boride (TiB) composites with β phase in the matrix by reaction sintering of TiB₂ with Ti and alloying element powders. The goal was to examine the nature of TiB whisker formation in three different kinds of powder mixtures: (1) β-Ti alloy powders and TiB₂; (2) α-Ti powder, a master alloy (Fe-Mo) powder containing the β-stabilizing elements, and TiB₂; and (3) α-Ti powder, a β-stabilizing elemental powder (Mo or Nb), and TiB₂. The effects of powder packing and the relative locations of powder particles on the morphological changes in TiB whisker formation and their growth were studied at processing temperatures ranging from 1100°C to 1300°C. The morphology, size, and distribution of whiskers were found to be influenced by the powder-packing conditions. A large particle-size ratio in bimodally packed mixtures led to the formation of a TiB monolithic layer around β grains. With a relatively finer starting powder, smaller size ratio, and trimodal packing arrangement, the TiB whiskers were found to be distributed more homogeneously in the matrix. The study also used the X-ray direct comparison method and the structure factor for the β phase to determine the volume fraction of TiB phase from X-ray data. Tensile tests and fractographic investigations were carried out on selected composites. The evolution of the composite microstructure, the influence of powder-packing variables, and the morphology and growth of TiB whiskers and their effect on mechanical properties are discussed.     

Auger electron analysis of the initial oxidation of titanium aluminides based on Ti-48Al

The surface of a Ti-48 at. Pct Al alloy was examined by Auger electron microscopy to study oxidation at room temperature. On exposure to air at room temperature, both Al and Ti oxides were observed together with an abundance of C. The amount of C was always larger in the two-phase α₂ + γ region compared to the single-phase γ region. The Ti oxides formed on the surface of they grains were primarily Ti₂O₃ rather than TiO₂. On depth profiling with Ar⁺ ion sputtering, lower oxide states of Ti were found. This was attributed to either the Ar⁺ ion sputtering or the fact that the inner layers of oxide represented oxides of Ti in their lower valence states. The A1₂O₃ was stable and did not exhibit any transient oxidation states. The dominant oxidation product on the surface of sputtered single-phase γ grains after an 84-hour exposure in the ultrahigh vacuum Auger chamber at room temperature is A1₂O₃. A depletion of C and O occurred beneath the oxide surface in some γ grains. The chemical shift between the Al L_2,3MM and A1₂O₃ L_2,3(A1)M_(O)M_(O) peaks in the Auger spectrum of A1₂O₃ formed on the γ phase in TiAl was found to be 11 eV. Y.T. Peng, Graduate Student, Formerly with the Materials Science and Engineering Program, University of Texas at Arlington, Arlington, TX 76019,     

Syntheses of full-density nanocrystalline titanium nitride compacts by plasma-activated sintering of mechanically reacted powder

Nearly equiatomic nanocrystalline titanium nitride (Ti₅₆N₄₄) powder with an average grain size of 5 nm has been synthesized by ball milling elemental Ti powder under nitrogen gas flow at room temperature. During the first stage of reactive ball milling (RBM) (time <3.6 ks), the metallic Ti powder tends to agglomerate to form powder particles with a larger diameter. At the second stage (3.6 to 22.0 ks), the agglomerated particles of Ti fragment to form smaller particles. These smaller particles that have new or fresh surfaces begin to react with the milling atmosphere (nitrogen) during the third stage of milling (22 to 86 ks) to form TiN powder coexisting with unreacted Ti powder. Toward the end of milling (86 to 173 ks), a single phase of nanocrystalline TiN (NaCl structure) is obtained. The powder of this end-product has a spherical-like morphology with an average particle size of about 0.4 μm diameter. A sintering procedure using plasma activation has been employed to consolidate the powder particles at several stages of the RBM. The as-milled and as-consolidated powders have been characterized as a function of the RBM time by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, and chemical analyses. Density measurements of the consolidated samples show that after 86 to 173 ks of the RBM time, the compacted samples are essentially fully dense (above 96 pct of the theoretical density for TiN). The results also show that the consolidated TiN compacts still maintain their unique nanocrystalline properties with an average grain size of about 65 nm. The hardness and some mechanical properties of the consolidated TiN compacts have been determined as a function of the RBM time.     

Shock densification/hot isostatic pressing of titanium aluminide

Consolidation of rapidly solidified titanium aluminide (Ti₃Al) powders employing explosive shock pressure followed by hot isostatic pressing (“hipping”) was carried out successfully. Shock densification was achieved by using a double tube design in which the flyer tube was explosively accelerated, impacting the powder container. Elemental mixtures of Ti (15 wt pet) and Al (15 wt pct) powders were added to intermetallic compound powders (Ti₃Al). Hipping was used to chemically induce bonding between Ti₃Al particles. The highly exothermic reactions were activated by hipping at 1000 ‡C and enhanced the bonding between the inert intermetallic powders. Compression tests indicated strong bonding between Ti₃Al particles. Well-bonded Ti₃Al compacts having an average ultimate compressive strength of 2 GPa and compressive fracture strain of 20 pct were produced by this technique. The ultimate tensile strengths, due to the presence of flaws in the microstructure (microcracks and voids) and intergranular fracture observed in the reacted regions, were much lower (~250 MPa).     

Preparation and thermal stability of Zr-Ti-Al-Ni-Cu amorphous powders by mechanical alloying technique

This study examined the amorphization feasibility of Zr_70−x−y Ti _x Al _y Ni₁₀Cu₂₀ alloy powders by the mechanical alloying (MA) technique. According to the results, after 5 to 7 hours of milling, the mechanically alloyed powders were amorphous basically in the ranges of 0 to 12.5 at. pct Ti and 2.5 to 17.5 at. pct Al. These ranges are larger than those of bulk amorphous alloys prepared by a squeeze mold casting technique. Most of the amorphous mechanically alloyed powders exhibited a wide supercooled liquid region of more than 60 K before crystallization. The glass-transition and crystallization temperatures of mechanically alloyed samples were different from those prepared by squeeze casting. It is suspected that different thermal properties arise from the introduction of impurities during the MA process. The amorphization behavior of Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ was examined in detail. The X-ray diffraction and extended X-ray absorption fine structure (EXAFS) results show the fully amorphous powders formed after 5 hours of milling. A kinetically modified thermodynamic phase transformation process was observed for the glass-transition behavior in the Zr₅₀Ti_7.5Al_12.5Ni₁₀Cu₂₀ amorphous powder.     

Calculation of the Ti(C y N1−y )−Ti4C2S2−MnS-austenite equilibrium in Ti-bearing steels

A thermodynamic model is presented for the equilibria among various precipitates (Ti(C _y N_1−y ), Ti₄C₂S₂, and MnS) and austenite containing six alloying elements (C, Mn, N, S, Si, and Ti). This model is applied to four microalloyed steels with Ti levels of 0.05, 0.11, 0.18, and 0.25 pct. The calculations show that the Ti in these steels cannot be completely dissolved over the austenite temperature range. However, the compositions of the undissolved Ti carbonitrides differ significantly from pure TiN, as 10 to 40 pct of the nitrogen is replaced by carbon. An expression for the Gibbs energy for the formation of Ti₄C₂S₂ in austenite is estimated. The present predictions are compared with those of the Hudd, Jones, and Kale (HJK) model; considerable differences are observed at temperatures below 1250°C.     

Phase transitions in reactive formation of Ti5Si3/TiAl in situ composites

A new method is developed for preparing Ti₅Si₃/TiAl in situ composites by incorporating metastable phases (called metastable precursors) into TiAl (a mixture of elemental Ti and Al) matrix powders. Metastable precursors with a starting composition of Ti-14Al-21Si are prepared by mechanical alloying (MA). They have been proven through X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) observations to be mainly consisting of mixtures of nanostructured solid solutions and milling-formed TiAl compound. Particularly, phase reactions and transitions in the precursors and the composites during heating have been investigated in detail by using diffraction thermal analysis (DTA) in conjunction with XRD. It has been found that Ti₅Si₃ is in situ formed through a phase transition chain, TiSi₂ → Ti₅Si₄ → Ti₅Si₃. When the composite powder (precursor, Ti and Al) is heated, a combustion reaction first occurs in the matrix, which results in the formation of TiAl₃ and/or TiAl followed by the completion of the previously mentioned silicide transitions in a very short time. Scanning electron microscope (SEM) observations indicated the locations of reinforcements in the reaction-formed composite, and TEM observation provided some details of the structures for the reinforcements and their neighborhood. This method is intriguing because a designed phase hierarchy is possible.