The Fe-rich corner of the metastable C-Cr-Fe liquidus surface

The liquidus surface of the C-Cr-Fe system has been experimentally determined in the Fe-rich region —C ≤6 wt pct, Cr ≤40 wt pct —using a sensitive differential thermal analysis technique, along with optical and scanning electron microscopy and X-ray diffraction. Previous liquidus surfaces for this system have differed on the extent of the (Cr,Fe)₂₃C₆ liquidus field, with one version reporting its existence at ∼20 wt pet Cr, and others finding that it did not occur at Cr levels of less than ∼60 wt pct. The present investigation provides evidence in favor of the second contention, with the (Cr,Fe)₂₃C₆ field not being detected at Cr ≤40 wt pct. Changes are proposed to the accepted liquidus surface in respect of the compositions of the invariant reactions—L + αδFe ⇌γFe + (Cr,Fe)₇C₃ andL + (Cr,Fe)₇C₃ ⇌γFe + (Fe,Cr)₃C —and of the monovariant eutectic valley—L⇌ γFe + (Cr,Fe)₇C₃.     

On the improvement of creep strength and ductility of Ni-20 pct Cr by small zirconium additions

The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically 20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only in heavily necked parts of the samples. In addition to Ni₅Zr and ZrO₂ inclusions, a Zr₄C₂S₂ carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20 wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor alloying additions in constructing and using fracture mechanism maps.     

High Temperature Phase Stability and Microstructural Characterization of D9 Stainless Steel-Zirconium Metal Waste Form Alloys

Zirconium present in stainless steel-zirconium metal waste form (MWF) alloys form Ni?CZr and Fe?CZr intermetallic phases which act as a sink for radionuclide and improve resistance to localized corrosion as well as selective radionuclide leaching. The present study looks into the behavior of Zr intermetallics in MWF alloys with the variation of Zr content after heat treatments. Two MWF alloys of D9 SS (Ti modified 15Cr?C15Ni?C2.5Mo stainless steel) with 8.5 and 17?wt% Zr were heat treated at 1,323?K for 2 and 5?h and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The stability of the Zr intermetallic compounds was examined by high temperature XRD. The results from XRD study showed the presence of NiZr, Ni₅Zr, Ni₇Zr₂, FeZr₂, and Fe₃Zr peaks along with fcc Fe based solid solution. The MWF alloy with 17?wt% Zr exhibited ??-ferrite peak in as-cast condition which was not observed after heat treatment. From the SEM micrograph the agglomeration of intermetallic phases was observed after heat treatment and the grain size of the intermetallic phases increased with duration of heat treatment. The high temperature XRD study revealed that all the intermetallic phases were stable up to 1,173?K and above this temperature Ni?CZr intermetallics started disappearing. However Fe?CZr intermetallics were stable till 1,373?K. The paper presents the high temperature phase stability of D9 SS-Zr MWF alloys.     

The forming of Zr3Al-base alloys

Tubes have been formed from the intermediate phase Zr₃Al (L1₂ type) which is of potential use as a structural element in thermal nuclear power reactors. The procedure is to extrude Zr/Zr₂Al two-phase ingots at temperatures above 1270 K in theβ Zr + Zr₂Al two-phase field and then transform the extruded product at lower temperatures to Zr₃Al via the peritectoid transformation Zr + Zr₂Al → Zr₃Al. For small tubes (⪝ 3.2 cm OD) the extrusion constants at 1375 and 1425 K, respectively, are ≃ 400 and ≃ 300 MN/m² for the conditions chosen. The experimental extrusions indicate no fundamental barriers to forming pressure tubes from Zr-Al alloys containing 7.6 to 9.0 wt pct Al.     

Role of Mo and W during sensitization of superaustenitic stainless steel—crystallography and composition of precipitates

This study concerns the crystallographic identification and compositions of precipitates formed in superaustenitic stainless steel. Three experimental alloys, all containing 24 wt pct Cr, 22 wt pct Ni, and 0.5 wt pct N but with varying amounts of Mo and W, were investigated after sensitization heat treatment (aging) at 900 °C. The contents of Mo and W in the three alloys were 7 wt pct Mo, (6 wt pct Mo + 2 wt pct W) and (5 wt pct Mo + 5 wt pct W), respectively. While σ and x were the main secondary phases found in the W-free alloy, replacement of Mo by W was found to promote the formation of Laves-related phases with high Mo + W content. The complex crystallographic nature of Laves-related precipitates was exemplified through the formation of intergrowing C14 Laves, μ, and C phases, all with closely related crystal structures. There was no difference in chemical composition between the three phases. Prolonged aging resulted in intragranular precipitation of different intermetallic phases, as well as formation of nitrogen bearing phases, π and Cr₂N, adjacent to previously formed intermetallic precipitates. The content of Mo + W was found to decrease with increasing aging time for all secondary phases.     

Structural Characterization of Sputter-Deposited 304 Stainless Steel+10 wt pct Al Coatings

An SS304?+?10?wt pct Al (with a nominal composition of Fe-18Cr-8Ni-10Al by wt pct and corresponding to Fe-17Cr-6Ni-17Al by at. pct) coating was deposited on a 304-type austenitic stainless steel (Fe-18Cr-8Ni by wt pct) substrate by the magnetron sputter-deposition technique using two targets: 304-type stainless steel (SS304) and Al. The as-deposited coatings were characterized by X-ray diffraction, transmission electron microscopy, and three-dimensional (3-D) atom probe techniques. The coating consists of columnar grains with ?? ferrite with the body-centered cubic (bcc) (A2) structure and precipitates with a B2 structure. It also has a deposition-induced layered structure with two alternative layers (of 3.2 nm wavelength): one rich in Fe and Cr, and the other enriched with Al and Ni. The layer with high Ni and Al contents has a B2 structure. Direct confirmation of the presence of B2 phase in the coating was obtained by electron diffraction and 3-D atom probe techniques.     

Microstructure-property relationships of two AI-3Li-2Cu-0.2Zr-XCd alloys

The microstructure and tensile properties of two A1-3 wt pct Li-2 wt pct Cu-0.2 wt pct Zr alloys, one Cd-free and one containing 0.2 wt pct Cd, have been investigated. The Cd-free alloy remained unrecrystallized for all solutionizing treatments studied, whereas a special treatment had to be developed to prevent recrystallization during solutionizing of the 0.2 wt pct Cd alloy. In combination with cadmium, zirconium either enters into, or nucleates on, the course Al₇Cu₂Fe and T₂ phases during high temperature annealing. This reduces the volume fraction of small coherent Al₃Zr particles in the matrix which normally inhibits recrystallization. Consequently, a low temperature anneal to precipitate Al₃Zr is necessary prior to high temperature solutionizing in order to prevent recrystallization in the Cd-containing alloy. Unlike its effect in lower lithium, higher copper content aluminum alloys, cadmium does not significantly affect the nucleation of the strengthening precipitates. If anything, cadmium has a detrimental effect on the age hardening response of this alloy, since it increases the formation of coarse Al-Cu-Li equilibrium phases at grain and subgrain boundaries and thus removes some of the copper and lithium from participating in the formation of the strengthening precipitates T₁ and δ′. Subgrain boundary fracture occurred during tensile tests of both alloys in the unrecrystallized condition; however, transgranular fracture occurred in tests of the partially recrystallized 0.2 wt pct Cd alloy. Both types of fractures are believed due to a form of strain localization associated with precipitate free zones and shearable precipitates. Formerly with the Fracture and Fatigue Research Laboratory, Georgia Institute of Technology, Atlanta, GA     

Calorimetrically Measured Enthalpies for

The enthalpy for the direct reaction of H₂ (g) with Zr has been measured by calorimetry at moderate, 323 K, and elevated, 928 K, temperatures over a large range of H contents. The elevated temperature enthalpies were determined for solution in the α phase, for the (α + β) phases, the β phase, the(β + γ) phases, and they phase. Simultaneously, the equilibrium pressures were measured. A combination of ΔG_H = 1/2RT In p _{H 2} , values and the enthalpies gives the corresponding entropies. At 323 K, where equilibrium pressures cannot be measured, the enthalpy for the reaction 1/2H₂ (g) + Zr/1.5 → ZrH_1.5 /1.5 was determined as -87 ± 1.5 kJ/mol H. The enthalpy for reaction of H₂ (g) and Zr (2.5 wt pct Nb) has been determined calorimetrically at 323 K and is found to decrease in exothermicity from —86 to —78 kJ/mol H in the range of (H/Zr) values from 0 to 1.5. Enthalpies of reaction were also measured at the same tem- perature for a two-phase alloy consisting of Zr-rich and Zr₂Ni phases with an overall stoichi- ometry of Zr_0.85Ni_0.15. Formerly Graduate Student, Formerly Graduate Student,     

Determination of the Fe-Ni and Fe-Ni-P phase diagrams at low temperatures (700 to 300 °C)

The α + γ two-phase fields of the Fe-Ni and Fe-Ni (P saturated) phase diagrams have been determined in the composition range 0 to 60 wt pet Ni and in the temperature range 700 to 300 °C. The solubility of Ni in (FeNi)₃P was measured in the same temperature range. Homogeneous alloys were austenitized and quenched to form α₂, martensite, then heat treated to formα (ferrite) + γ (austenite). The compositions of the α and γ phases were determined with electron microprobe and scanning transmission electron microscope techniques. Retrograde solubility for the α/(α + γ) solvus line was demonstrated exper-imentally. P was shown to significantly decrease the size of the α + γ two-phase field. The maximum solubility of Ni in α is 6.1 ± 0.5 wt pct at 475 °C and 7.8± 0.5 wt pct at 450 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams, respectively. The solubility of Ni in α is 4.2 ± 0.5 wt pct Ni and 4.9 ± 0.5 wt pct Ni at 300 °C in the Fe-Ni and Fe-Ni (P saturated) phase diagrams. Ternary Fe-Ni-P isothermal sections were constructed between 700 and 300 °C. Formerly Research Assistant in Department of Metallurgy & Materials Engineering, Lehigh University, Bethlehem, PA.     

A study of the weldability and weld related microstructure of cabot alloy 214

The weldability and weld metal microstructure of Cabot Alloy 214 have been investigated with a variety of experimental and analytical techniques. These include Varestraint hot crack testing, hot ductility testing, pulsed Nd:YAG laser welding, scanning and analytical electron microscopy, electron microprobe analysis, and X-ray diffraction. A heat of Alloy 214 containing intentionally alloyed B (0.003 wt pct) and Zr (0.07 wt pct) was much more sensitive to both fusion zone hot cracking as quantified by the Varestraint test and to simulated heat-affected-zone (HAZ) cracking as quantified by hot ductility testing than a heat of Alloy 214 containing no intentionally added B (0.0002 wt pct) or Zr (0.02 wt pct). Scanning electron microscopy of the high B and Zr alloy showed the presence of dendritically-shaped, Zr-rich constituents in interdendritic regions in the gas-tungsten-arc (GTA) welds. Electron microprobe analysis of these welds revealed a segregation pattern of Cr, Al, Mn, and Zr enrichment in interdendritic regions and Ni and Fe enrichment in dendrite core regions. Analytical electron microscopy revealed the presence of ZrX (X = B, C, N, O), M₂₃C₆, andγ′ in the fusion zone of GTA weld specimens,γ′ was also found in the as-received base metal and in the GTA weld HAZ. X-ray diffraction analysis of extractions from the high B and Zr GTA weld metal also indicated the presence of a ZrX-type constituent. The results of this study are in qualitative agreement with earlier work performed on alloys such as NIMONIC 90 and INCONEL 718^∗ relative to the detrimental effect of B and Zr additions on fusion zone and HAZ hot cracking susceptibility. Formerly with Sandia National Laboratories, Albuquerque, NM     

Phase equilibria of the ternary Ni-Cr-Zr system and interfacial reactions in the Ni-Cr/Zr couples

The phase equilibria of the ternary Ni-Cr-Zr system and interfacial reactions in the Ni-Cr/Zr couples at 900 °C were determined. Fifty alloys were prepared from pure Ni, Cr, and Zr. The alloys were metallographically analyzed. Both X-ray diffraction and electron-probe microanalysis (EPMA) were carried out for structural identification and compositional analysis of phases formed in these alloys. At 900 °C, the Cr-Ni₁₀Zr₇ two-phase region divides the system into two halves. ZrCr₂(C14) exists in the Zr-Cr-Ni₁₀Zr₇ half, and the ZrCr₂ (C15) phase has an extensive ternary solubility. In the Cr-Ni₁₀Zr₇-Ni half, except for the Ni₇Zr₂ phase, most of the binary Zr-Ni compounds are in equilibrium with either Cr or Ni phase. Reaction-couple techniques were used for the interfacial reaction study. The reaction path was Zr/NiZr₂/NiZr/Ni₁₀Zr₇/Ni₂₁Zr₈/Cr/Ni-Cr alloy in the Ni-Cr/Zr couples examined in this study. The results indicate that Ni is the fastest-diffusing species, while Cr is the slowest one.     

Microstructure and its development in Cu-Al-Ni alloys

The microstructure of as-cast Cu-AI-Ni alloys, based on copper containing 9 to 10 wt pct Al and up to 5 wt pct Ni, has been examined. The development of the microstructure on continuous cooling has also been investigated. For alloys with 9.2 to 9.3 wt pct Al, and less than 1 wt pct Ni, the as-cast microstructure consists of proeutectoid α solid solution, α + γ₂ eutectoid, and martensitic β. If the nickel content is more than 2.5 wt pct, the α + γ₂ eutectoid is replaced by α + β ₂ ^′ eutectoid, and no martensitic β is observed in the as-cast alloys. The morphologies of the β ₂ ^′ and γ₂ eutectoid phases are similar; both have the Kurdjumov-Sachs (K-S) orientation relationship with the a phase. Two eutectoid reactions, involving β to α + γ₂ and β to α + β′₂, have been observed in an alloy containing 9.7 wt pct Al and 2.7 wt pct Ni. When both eutectoid reactions occur, the Nishiyama-Wassermann (N-W) orientation relationship exists between γ₂ or β ₂ ^′ and the α phase. During continuous cooling, proeutectoid α solid solution is the first phase to precipitate from the high-temperature β phase. The β to α + β ₂ ^′ eutectoid reaction starts at higher temperatures than the β to α + γ₂ reaction. Tempering of the as-cast alloys results in the elimination of the martensitic β. Y.S. SUN formerly Research Associate with the Manchester Materials Science Centre.     

The eutectoid region of the Zr−Ga system

The zirconium-rich portion of the Zr?Ga phase diagram was determined by the optical examination of microstructures of isothermally annealed and quenched alloys. A deviation from binary equilibrium, was observed even though careful selection of materials and techniques held impurities to a minimum and produced alloys with a purity of at least 99.9 pct. The slopes of the α-β boundaries are depressed and the range of solubility of the solid solution phases is restricted when compared to the phase diagrams of other Group IIIB elements, apparently as a result of the large difference in atomic size between zirconium and gallium. Thea ₀ andc ₀ lattice constants of cph zirconium are contracted and the axial ratio is expanded by the addition of gallium. The change inc/a at 1 at. pct was very close to the change observed in Zr-In alloys, in agreement with general dependence of these properties in zirconium alloys upon electron to atom ratio. A eutectoid reaction occurs at 860°C with β solid solution (1.8 at. pct Ga) decomposing into α solid solution (0.8 at. pct Ga) and Zr₃Ga. Cast microstructures suggest a eutectic reaction in which liquid (21.0 at. pct Ga) decomposes into β (8.0 at. pct Ga) and Zr₅Ga₃. It is proposed that intermediate phases are formed at 25.0 at. pct Ga (Zr₃Ga), 37.5 at. pct Ga (Zr₅Ga₃), and 50.0 at. pct Ga (ZrGa) although the exact composition was not determined.     

Beta decomposition of (HFxZr1_x)80Nb20 ternary alloys

The processes of beta decomposition have been examined in ternary alloys of the form (Hf_xZr_1-x)₈₀Nb₂₀ to determine the influence of Hf additions to a basic Zr₈₀Nb₂₀ composition. In the chill cast condition, Hf additions have been found to decrease the temperature coefficient of electrical resistivity from a value of-0.0015 pct/K for the binary Zr8oNb2o alloy to a value of-0.011 pct/K for a (Hf_xZr_1-x)₈₀Nb₂₀ ternary alloy. This change is explained in terms of the bcc lattice instability typical of Ti, Zr, and Hf alloys. The Hf additions enhance the kinetics of ω-phase precipitation during aging at 648 K. The aging of a (Hf₀₅Zr₉₅)₈₀Nb₂₀ alloy for 12 h results in the precipitation of a high volume fraction of cuboidal shaped co-phase particles. A phase separation which results in the formation of solute lean discs (β₁) occurs together with the precipitation of the ω-phase. These discs formed both randomly within the matrix and heterogeneously along dislocations and at grain boundaries.     

A pressure-composition-temperature study of Zr-Nb-H system

The solubility of hydrogen was determined in the (Zr + 5 wt pct Nb)-H₂, (Zr + 10 wt pct Nb)-H₂, and (Zr + 20 wt pct Nb)-H₂ systems as a function of composition, temperature (700° to 950°C) and hydrogen equilibrium pressure (0.5 to 760 mm Hg). The position of boundariesβ - (β + δ) and(β + δ)-δ were determined in each of the above three systems. Niobium significantly reduces the solubility of hydrogen in theβ andδ phases and increases the equilibrium hydrogen pressure for any fixed concentration. The equilibrium pressure-temperature relations in the two phase region (β + δ) were derived and the heat of formation ofδ-hydride from saturatedβ-Zr, ΔH _β → δ, were determined. The value of ΔH _β → δ increases up to about 5 wt pct Nb after which the effect of niobium seems to be insignificant. The maximum hydrogen pick-up of zirconium at room temperature decreases with increasing niobium content of the alloy.     

Alloying mechanism of beta stabilizers in a TiAl alloy

The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti₅₂Al₄₈-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti₅₂Al₄₈-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti₅₂Al₄₈-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti₅₂Al₄₈-1V, Ti₅₂Al₄₈-Cr, and Ti₅₂Al₄₈-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2 (W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti₅₂Al₄₈-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.     

Phase transformations in the Zr-rich part of the Zr-Fe system resulting from heat treatment and plastic deformation

Phases and phase transformations occurring in the Zr-rich part of the Zr-Fe system during heat treatment and plastic deformation were identified by means of Mössbauer spectroscopy and X-ray diffraction analysis. Low iron alloys (<0.02 wt pct Fe) undergo a complete β → α_m reaction (martensitic type) on quenching. For higher iron content alloys (0.02 to 0.25 wt pct Fe) the β → α_m ransformation is accompanied by formation of metastable intermediate phase designated θ. The iron concentration of θ-phase is much higher than that of αZr(Fe). During the aging process, at the outset of the equilibrium state, the θ-phase disappears by transforming to Zr₂Fe intermetallic. Cold rolling of quenched(α _m + θ) specimens leads to formation of the athermal ω-phase. Presence of the intermediate θ-phase seems to be a prerequisite for the athermal ω-phase formation. A decrease in specific volume (ΔV_θ < 0) accompanying the α→ θ transition was suggested as a possible mechanism of the α →θ → ω transformation. Mössbauer parameters for the thermal and athermal ω -phase were determined. Presence of θ and athermal ω -phases were identified by Mössbauer spectroscopy only, being undetectable by X-ray diffraction, because of their minute quantities. Solubility of iron in the α Zr(Fe) solid solution was determined in the range of temperatures 713 to 943 K (440 to 670 °C).     

The solubility and solution behavior of antimony and tin in α-lron and the effects of nickel and chromium additions

The solid solubilities of Sn and Sb in α-Fe have been determined by means of lattice parameter measurements. The Sb solubility ranges from a maximum of 11 wt pct (5.4 at. pct) at 1000°C down to 5.3 wt pct (2.5 at. pet) at 600°C; the Sn solubility ranges from a maximum of 17.7 wt pct (9.2 at. pet) at 900°C to 6.5 wt pct (3.2 at. pet) at 600°C. These solubilities are remarkably large in view of the large sizes of the Sb and Sn atoms in relation to the Fe atom. It was not possible to rationalize the variation of the α-phase lattice parameter with Sb or Sn content from the point of view of atomic diameter or atomic volume. The addition of 1 wt pct Ni lowers the Sb solubility at 600°C from 5.3 to 3.5 wt pct (2.5 to 1.6 at. pet); the effect of Cr on the Sb solubility appears to be small. The addition of 1 wt pct Ni or 1 wt pct Cr lowers the Sn solubility from 6.5 to 5.2 wt pct (3.2 to 2.5 at. pet). It was found that a substantial amount of Ni substitutes for Fe in both the FeSb phase and the Fe₅Sn₃ phase. Formerly Research Fellow, Department of Metallurgy and Materials Science and LRSM, University of Pennsylvania     

Structure and mechanical properties of boron-doped cubic zirconium trialuminides

Cubic (L1₂) ternary zirconium trialuminides macroalloyed with Cu(Al₅CuZr₂), Mn(Al₆₆Mn₉Zr₂₅), and Cr(Al₆₇Cr₈Zr₂₅) (atomic percent) and doped with 50 and 100 ppm boron were fabricated by induction melting. Their as-cast microstructures are characterized by a small amount of porosity (1 to 2 pct) and second phase (2 to 3 pct). Boron seems to slightly enhance porosity (up to 3.3 pct) in Al₅CuZr₂ +100 ppm B alloy, and it also promotes some compositional inhomo-geneity in Al₆₆Mn₉Zr₂₅ alloy. Vickers microhardness and compressive properties at room temperature (RT), peak strength temperature (500 °C to 600 °C) and 900 °C were investigated. Microcracking development was also investigated in Al₅CuZr₂ +100 ppm boron alloy exhibiting a stepped load-deflection curve. Vickers microhardness strongly depends on load, similarly to boron-free cubic ternary zirconium and titanium trialuminides, and increases in a systematic way with increasing boron content which seems to indicate a solid solution strengthening effect. At RT, 0.2 pct offset yield strength is not increased by the boron doping in most of the alloys studied except for Al₆₆Mn₉Zr₂₅ + 50 ppm B alloy. Permanent deformation (apparent ductility) at ultimate compressive strength is not enhanced by boron doping. In Al₅CuZr₂ +100 ppm B alloy microcracks start nucleating and proliferating in the elastic region of load-deflection curve in characteristic “bursts” accompanied by a “click” sound and the appearance of a discernible step on the load-deflection curve. Pre-existing pores are observed to be active centers of microcracking.     

Microstructural Evolution of the Interface Between Pure Titanium and Low Melting Point Zr-Ti-Ni(Cu) Filler Metals

Low melting point Zr-based filler metals with melting point depressants (MPDs) such as Cu and Ni elements are used for titanium brazing. However, the phase transition of the filler metals in the titanium joint needs to be explained, since the main element of Zr in the filler metals differs from that of the parent titanium alloys. In addition, since the MPDs easily form brittle intermetallics, that deteriorate joint properties, the phase evolution they cause needs to be studied. Zr-based filler metals having Cu content from 0 to 12 at. pct and Ni content from 12 to 24 at. pct with a melting temperature range of 1062 K to 1082 K (789 °C to 809 °C) were wetting-tested on a titanium plate to investigate the phase transformation and evolution at the interface between the titanium plate and the filler metals. In the interface, the alloys system with Zr, Zr₂Ni, and (Ti,Zr)₂Ni phases was easily changed to a Ti-based alloy system with Ti, Ti₂Ni, and (Ti,Zr)₂Ni phases, by the local melting of parent titanium. The dissolution depths of the parent metal were increased with increasing Ni content in the filler metals because Ni has a faster diffusion rate than Cu. Instead, slow diffusion of Cu into titanium substrate leads to the accumulation of Cu at the molten zone of the interface, which could form undesirable Ti _x Cu _y intermetallics. This study confirmed that Zr-based filler metals are compatible with the parent titanium metal with the minimum content of MPDs.