Production of Ni3Ti-TiC x intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi-TiC (stoichiometric) or Ni₃Ti-TiC _x (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni₃Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiC _x , for which 0.3 ≤ x ≤ 0.5, has an important controlling effect on the formation of either Ni₃Ti or NiTi; i.e., formation of TiC_0.7 results in a depletion of titanium and formation of Ni₃Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni₃Ti-TiC _x composites are produced. Ignition, combustion, and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiC _x particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented.     

Production of Ni3Ti?TiC x intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi−TiC (stoichiometric) or Ni₃Ti−TiC _x (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni₃Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiC _x , for which 0.3≤x≤0.5, has an important controlling effect on the formation of either Ni₃Ti or NiTi; i.e., formation of TiC_0.7 results in a depletion of titanium and formation of Ni₃Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni₃Ti−TiC _x composites are produced. Ignition, combustion and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiC _x particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented. Guglielmo Gottoli, formerly Graduate Research Assistant, Metallurgical and Materials Engineering Department, Institute for Space Resources, Colorado School of Mines     

Production of Ni3Ti?TiC x intermetallic-ceramic composites employing combustion synthesis reactions

Combustion synthesis (CS) of nickel, titanium, and carbon (graphite) reactant particles can result in NiTi−TiC (stoichiometric) or Ni₃Ti−TiC _x (nonstoichiometric) composites. Since NiTi exhibits both superelasticity and shape memory properties while Ni₃Ti does not, it is important to understand the SHS reaction conditions under which each of these composite systems may be synthesized. The stoichiometry of TiC _x , for which 0.3≤x≤0.5, has an important controlling effect on the formation of either Ni₃Ti or NiTi;i.e., formation of TiC_0.7 results in a depletion of titanium and formation of Ni₃Ti. This deficiency should be considered when developing the SHS reaction. This article examines the SHS conditions under which Ni₃Ti−TiC _x composites are produced. Ignition, combustion and microstructure characteristics of nickel, titanium, and carbon (graphite) particles were investigated as a function of initial relative density and thermophysical properties of the reactant mixture. Combination of the thermophysical properties and burning velocities controlled TiC _x particle size, yielding a dependence of particle size on cooling rate. Theoretical calculations were performed and are in good agreement with the experimental data presented. Guglielmo Gottoli, formerly Graduate Research Assistant, Metallurgical and Materials Engineering Department, Institute for Space Resources, Colorado School of Mines     

The effect of gravity on the combustion synthesis of metal-ceramic composites

The effects of gravity on the combustion characteristics and microstructure of metal-ceramic composites (HfB₂/Al and Ni₃Ti/TiB₂ systems) were studied under both normal and low gravity conditions. Under normal gravity conditions, pellets were ignited in three orientations relative to the gravity vector. Low gravity combustion synthesis (SHS) was carried out on a DC-9 aircraft at the NASA-Lewis Research Center. It was found that under normal gravity conditions, both the combustion temperature and wave velocity were highest when the pellet was ignited from the bottom orientation; i.e., the wave propagation direction was directly opposed to the gravitational force. The SHS of 70 vol pct Al (in the Al-HfB₂ system) was changed from unstable, slow, and incomplete when ignited from the top to unstable, faster, and complete combustion when ignited from the bottom. The hydrostatic force (height × density × gravity) in the liquid aluminum was thought to be the cause of formation of aluminum nodules at the surface of the pellet. The aluminum nodules that were observed on the surface of the pellet when reacted under normal gravity were totally absent for reactions conducted under low gravity. Buoyancy of the TiB₂ particles and sedimentation of the Ni₃Ti phase were observed for the Ni₃Ti/TiB₂ system. The possibility of liquid convective flow at the combustion front was also discussed. Under low gravity conditions, both the combustion temperature and wave velocity were lower than those under normal gravity. The distribution of the ceramic phase, i.e., TiB₂ or HfB₂, in the intermetallic (Ni₃Ti) or reactive (Al) matrix was more uniform.     

Development of TiB<Subscript>2</Subscript> reinforced in-situ Ti aluminide matrix composite through reaction synthesis

TiB₂ reinforced in-situ titanium aluminide matrix composite was made through reaction synthesis process using high purity elemental powders of Ti, Al, Cr, Nb and B. XRD of the synthesized block showed presence of mainly Al₃Ti and TiB₂ phases. To obtain γ Ti aluminide based matrix, the material was homogenized in two phase region (α₂+γ). Presence of γ phase matrix alongwith α₂ was confirmed through XRD, SEM and TEM. Uniform distribution of TiB₂ phase was confirmed through elemental mapping and by analyzing specimens of different locations. Differential scanning calorimetry of powder mixture showed presence of endothermic peak for Al melting and exothermic peak of Ti aluminide and TiB₂ formation.     

X-ray study of phase transformations in martensitic Ni-Al alloys

The formation of the Ni₅Al₃ and Ni₂Al phases in Ni-Al alloys with L1o ↔ B2 thermoelastic martensitic transformation has been studied by X-ray analysis. Ni₅Al₃ can form both the L1o and B2 structures, but the kinetics of L1o → Ni₅Al₃ and B2 → Ni₅Al₃ reactions are significantly different. A homogeneous mechanism for the former reaction and a mechanism of precipitation and growth for the latter are proposed. Ni₂Al forms from the B2 structure by the complex rearrangement of atoms. The initial stage of this reaction proceeds very rapidly and involves segregation of Ni atoms into Ni-rich zones leading to a Ni depletion in the surrounding regions. The nucleation of Ni₂Al retards the Ni₅Al₃ formation, so preaging in the B2 region affects the kinetics of the L1o → Ni₅Al₃ reaction on further aging in the L1o region. The microstructural mechanism for this effect is suggested.     

Reaction synthesis of Ni-Al-based particle composite coatings

Electrodeposited metal matrix/metal particle composite (EMMC) coatings were produced with a nickel matrix and aluminum particles. By optimizing the process parameters, coatings were deposited with 20 vol pct aluminum particles. Coating morphology and composition were characterized using light optical microscopy (LOM), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Differential thermal analysis (DTA) was employed to study reactive phase formation. The effect of heat treatment on coating phase formation was studied in the temperature range 415 °C to 1000 °C. Long-time exposure at low temperature results in the formation of several intermetallic phases at the Ni matrix/Al particle interfaces and concentrically around the original Al particles. Upon heating to the 500 °C to 600 °C range, the aluminum particles react with the nickel matrix to form NiAl islands within the Ni matrix. When exposed to higher temperatures (600 °C to 1000 °C), diffusional reaction between NiAl and nickel produces (γ′)Ni₃Al. The final equilibrium microstructure consists of blocks of (γ′)Ni₃Al in a γ(Ni) solid solution matrix, with small pores also present. Pore formation is explained based on local density changes during intermetallic phase formation, and microstructural development is discussed with reference to reaction synthesis of bulk nickel aluminides.     

Combustion synthesis of Ni3Al and Ni3Al-matrix composites

The self-propagating mode of combustion synthesis (SHS) of Ni₃Al starting from compacts of stoichiometrically mixed Ni and Al powders readily forms fully reacted structures with about 3 to 5 pct porosity, if green density of the compacts is greater than about 75 pct of theoretical. SHS-produced Ni₃Al matrix composites with up to 2 wt pct A1₂O₃ whiskers also have relatively low porosity levels. Porosity increases rapidly with lower green densities, higher Al₂O₃, or SiC whisker contents, and the degree of reaction completeness diminishes. The SiC whiskers undergo reaction with the matrix, while Al₂O₃ whiskers are nonreactive. All of these observations correlate well with temperature measurements made during the course of the reaction. The SHS mode can be achieved with agglomerated particle size ratioD _Al/D _Ni ≥ 1, larger than the limit established from studies of the thermal explosion mode of combustion synthesisD _Al/D _Ni ≃ 0.3. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

Structure of As-cast aluminum matrix composite containing Ni3AI-type intermetallic ribbons

An aluminum matrix composite containing rapidly solidified Ni₇₅Al₂₃B₁Zr₁ (at. pct) ribbons has been fabricated by casting at 700 °C, 715 °C, 730 °C, and 875 °C. Microstructural investigation has shown that the matrix contains particles with a composition between Al₃Ni and eutectic. The interfacial zones composed of several layers with different aluminum and nickel contents are observed around the ribbons. The sequence of layers from the ribbon outward in the specimens fabricated at 700 °C, 715 °C, and 730 °C is as follows: AINi → Al₃Ni₂ → the outer layer between Al₃Ni and eutectic. Composite specimens fabricated at 875 °C contain two types of interfacial zones: a single-layer AINi and a triple-layer zone. The first two layers in the triplelayer zone are exactly the same as their counterparts in the specimens fabricated at lower temperatures. The outer layer has a composition close to the Al₃Ni compound. The thickness of the AINi layer increases continuously with the increasing casting temperature. Within the experimental error, the thickness of the Al₃Ni₂ layer seems to be independent of casting temperature. The thickness of the outer layer in the specimens fabricated at 700 °C to 730 °C (Al₃Ni plus eutectic) increases with the casting temperature. However, the outer layer in the 875 °C specimen (Al₃Ni) is much thinner than the others.     

Phase equilibria in the Ni-Al-Ti system at 1173 k

The phase equilibria at 1173 K have been determined in the Ni-AI-Ti system for Al contents less than 50 at. pct. The extent of theH (Ni₂AlTi) phase field has been established as well as the extent of solubility in the binary compounds γ (Ni₃Al), ν(Ni₃Ti), β₂(NiTi), NiTi₂, and ζ(AlTi₃). Substantial differences were found between the phase equilibria determined in this study and previous studies, in part due to the large solubility of Al in NiTi₂.     

Structure and initial precipitation in a rapidly solidified nickel superalloy

A nickel base superalloy (Nimonic 80A) has been rapidly solidified at cooling rates of between 10⁵ to 10⁶ K.S^-1 by pendant drop melt extraction and by chill block melt spinning in an evacuated chamber backfilled with helium or argon. The internal structure is described in terms of process variables pertaining to rotating chill block quenching techniques. Both transmission electron microscopy and atom-probe field-ion microscopy have been employed to give structural and constitutional data on quenched and aged specimens. The as-quenched structure is homogeneous apart from fluctuations in titanium concentration which upon aging undergoes a spinodal phase decomposition to form disordered Ni₃(Ti,Al,Cr) precipitates in the matrix, which after prolonged aging produces ordered γ (Ni₃(Ti,Al)). inin6 particles form readily on grain boundaries and also appear in conjunction with ordered γ, via a discontinuous reaction, after short aging times.     

Abnormal growth of TiB<Subscript>2</Subscript> crystals during sintering of TiB<Subscript>2</Subscript>-Ni<Subscript>3</Subscript>(Al,Ti) cemented borides

TiB₂-Ni₃(Al,Ti) cermets present both normal and abnormal growth of faceted titanium diboride (TiB₂) grains during liquid-phase sintering. Abnormal grain growth (AGG) is preferentially found at high sintering temperatures in specimens processed from powder mixtures with a wide particle size distribution. The WC additions to the initial powder mixtures have proved efficient in reducing the number and size of these large TiB₂ grains. However, the sinterability of these materials is dramatically reduced, which suggests that TiB₂ AGG control is obtained by decreasing TiB₂ dissolution kinetics in the liquid phase. On the other hand, an alternative method based on intensive powder milling not only reduces TiB₂ AGG but also the porosity levels obtained by previous powder processing routes. TiB₂ cermets produced by aggressive milling present a higher amount of alumina particles in the matrix after sintering, which, in addition, appear more homogeneously dispersed in the microstructure. The distortion produced by these particles on the facets of TiB₂ growing grains suggests a possible dragging effect responsible for the AGG reduction found in these cermets. Moreover, aggressive milling removes large TiB₂ particles from the powder mixtures, which could act as seeds for TiB₂ uncontrolled growth. TiB₂-Ni₃(Al,Ti) cermets obtained by intensive milling combine hardness over 20 GPa with K _IC of about 10 MPa √m, data clearly out of the range covered so far by other TiB₂-based materials.     

Effects of ternary additions on the thermoelastic martensitic transformation of NiAl

Nickel-rich β-NiAl alloys, which are potential materials for high-temperature shape-memory alloys, show a thermoelastic martensitic transformation, which produces their shape memory effect. However, the transformation to Ni₅Al₃ phase during heating of NiAl martensite can interrupt the reversible martensitic transformation; consequently, the shape memory effect in NiAl martensite might not appear after heating. The phase transformation process in binary Ni-(34 to 37)Al martensite was investigated by differential thermal analysis (DTA) method, and we found that the condition of reversible martensitic transformation was not the β → Ni₅Al₃ transformation, but rather the M → Ni₅Al₃ transformation occurring at 250 °C to 300 °C. Therefore, the transformation temperature of M → Ni₅Al₃ determined the highest operating temperature for the shape memory effect. For verifying the critical temperature, the phase transformation process was investigated for eight ternary Ni-33Al-X alloys (X=Cu, Co, Fe, Mn, Cr, Ti, Si, and Nb). Only Ti, Si, and Nb additions were found to be effective in dropping the M _s temperature, and they facilitated the shape memory effect in Ni-33Al-X alloys. In particular, the addition of Si and Nb raised the transformation temperature of M → Ni₅Al₃, a potentially beneficial effect for shape memory at higher temperatures. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

On sintering of Ti–Ni (–TiB2) alloys to near full density

Abstract

Using a combination of mixed elemental powders and TiB₂, a series of Ti–Ni and Ti–Ni–B alloys were optimised for sintering by varying the nickel and boron contents, the particle size of the elemental powders and the compaction pressure. The sintering temperature was maintained at 1200°C to limit the costs of a potential commercial sintering operation. For Ti–Ni alloys, a density of 99% was attained in Ti–7Ni made using fine Ti and Ni powders sintered in the solid state, and from liquid phase sintering of Ti–8Ni made using coarser powders. Porosity was almost eliminated from Ti–7Ni–xB alloys made by adding 1–3%TiB₂ to the coarser Ti and Ni powders. The action of TiB₂ as a sintering aid is possibly owing to a combination of the formation of a small amount of liquid at the sintering temperature and the restriction of grain growth owing to the presence of TiB particles.     

Evolution of boride morphologies in TiAl-B alloys

The solidification of γ-TiAl alloys with relatively low (<2 at. pct) additions of boron is discussed. Binary Ti-Al alloys containing 49 to 52 at. pct Al form primary α-(Ti) dendrites from the melt, which are subsequently surrounded by γ segregate as the system goes through the peritectic reactionL + α →γ. Alloys between 45 and 49 at. pct Al go through a double peritectic cascade, forming primary β-(Ti) surrounded by α-(Ti) and eventually by γ in the interdendritic spaces. Boron additions to these binary alloys do not change the basic solidifi-cation sequence of the matrix but introduce the refractory compound TiB₂ in a variety of mor-phologies. The boride develops as highly convoluted flakes in the leaner alloys, but needles, plates, and equiaxed particles gradually appear as the B content increases above ∼1 at. pct. Increasing the solidification rate initially promotes the formation of flakes over plates/needles and ultimately gives way to very fine equiaxed TiB₂ particles in the interdendritic spaces of the metallic matrix. Furthermore, the primary phase selection in the 49 to 52 at. pct Al range changes from α-(Ti) to β-(Ti) at supercoolings of the order of 200 K. The different boride morphologies are fully characterized, and their evolution is rationalized in terms of differences in their nucleation and growth behavior and their relationship to the solidification of the inter-metallic matrix. Formerly Research Assistant, University of California-Santa Barbara (UCSB) Formerly Professor of Materials and Dean of the College of Engineering at UCSB     

Interface microstructures in the diffusion bonding of a titanium alloy Ti 6242 to an INCONEL 625

A Ti6242 alloy has been diffusion bonded to a superalloy INCONEL 625. The microstructures of the as-processed products have been analyzed using optical metallography, scanning electron microscope (SEM), and scanning transmission electron microscope (STEM) techniques. The interdiffusion of the different elements through the interface has been determined using energy-dispersive spectroscopy (EDS) microanalysis in both a SEM and a STEM. Several regions around the original interface have been observed. Starting from the superalloy INCONEL 625, first a sigma phase (Cr₄Ni₃Mo₂), followed by several phases like NbNi₃, Ŋ/Ni₃Ti, Cr(20 pct Mo), β Cr₂Ti, NiTi, TiO, TiNi, and Ti₂Ni intermetallics, just before the Ti6242 have been identified. Because the diffusion of Ni in Ti is faster than the diffusion of Ti in the superalloy, a Kirkendall effect was produced. The sequence of formation of the different phases were in agreement with the ternary Ti-Cr-Ni diagram.     

Effects of powder processing on densification of titanium diboride based cermets

Abstract

The sinterability of TiB₂-Ni₃(Al,Ti) based cermets has been significantly improved by aggressive milling of the starting TiB₂-Ni-TiAl₃ powder mixtures. This technique improves not only liquid spreading by reducing TiAl₃ particle size but also eliminates alumina agglomerates and the associated porosity found after vacuum sintering. Liquid phase sintering of TiB₂-Ni-TiAl₃ powder mixtures involves the presence of Ni based secondary borides at low temperatures (1200°C), which react afterwards with TiAl₃ particles leading to the formation of the final TiB₂-Ni₃(Al,Ti) eutectic liquid. Apart from improving liquid spreading around TiB₂ grains, aggressive milling is also found to disperse alumina agglomerates, which reduces the porosity associated to these particles. By this refined procedure, the amount of binder phase required for full densification of TiB₂ cermets by sinter hipping has been reduced from a previous limit of 16 vol.-% to 10 vol.-%. The hardness of these TiB₂-10 vol.-%Ni₃(Al,Ti) cermets is in the range of ultrafine WC-Co hardmetals in spite of their much coarser microstructure.