Microstructure and tensile properties of Ni3 (Si,Ti) alloys containing second phase dispersions

Abstract

The alloying behaviour, microstructure, and high temperature mechanical properties of quaternary polycrystalline Ni₃ (Si,Ti), which was alloyed with transition elements V, Nb, Zr, and Hf beyond their maximum solubility limits, were investigated. The solubility limits of the quaternary elements in the L1₂ Ni₃ (Si,Ti) phase were determined to be ranked in the sequence of Nb > V > Hf > Zr, and correlated with the size misfit parameter between Si and the quaternary element X, and with the difference in formation enthalpy between Ni₃ Si and Ni₃ X. The second phases (dispersions) formed beyond the solubility limit were identified as a face centred cubic type Ni solid solution for the V containing Ni₃ (Si,Ti) alloy and Ni₃ X type compounds of the Nb, Zr, and Hf containing Ni₃ (Si,Ti) alloys. The second phase dispersions in the L1₂ phase matrix resulted in strengthening over a wide range of temperatures. The high temperature tensile elongation was improved by the introduction of the second phase dispersions. Among the quaternary Ni₃ (Si,Ti) alloys observed in the present study, the Nb containing Ni₃ (Si,Ti) alloy with the Nb containing second phase dispersion was shown to have the most favourable mechanical properties.     

Short communication Effects of tantalum addition on transformation behaviour of (Ni51 Ti49 )1-x Tax and Ni50 Ti50-y Tay shape memory alloys

Abstract

The effect of Ta content on the transformation characteristics of Ni–Ti–Ta ternary alloys has been studied. In (Ni₅₁ Ti₄₉ )_1-x Ta_x type alloys, the phase transformation temperatures increase with Ta content, especially when the Ta content is less than 4 at.-%. In Ni₅₀ Ti_50-x Ta_x type alloys, the phase transformation temperatures decrease as Ta content increases. The martensite start temperature is less sensitive to changes in Ni content in ternary Ni–Ti–Ta alloys than that in Ni–Ti binary alloys. The phase transformation temperatures of Ni–Ti–Ta ternary alloys are mainly controlled by the Ni/Ti ratio in the Ni–Ti matrix.     

Mechanical properties of Ni3(Si,Ti) polycrystals alloyed with substitutional additions

The mechanical properties of the L1₂-type Ni₃(Si, Ti) polycrystals, which were alloyed with 1–2 at% of various transition metals and also doped with boron, were investigated over a wide range of temperatures. The addition of Hf enhanced the levels of yield stress whereas the addition of Cr, Mn and Fe reduced the levels of the yield stress over a wide range of temperatures. Ni₃(Si, Ti) alloyed with Cr, Mn and Fe showed a shallow minimum in the yield stress-temperature curves. This result was correlated with the modification of the micro-cross-slip process by the additives. At low temperatures, the addition of Hf and Nb slightly reduced the elongation, while the addition of Cr, Mn and Fe improved elongation. This elongation behaviour was interpreted as the alloying effect on the intergranular cohesive strength of L1₂ ordered alloys. At high temperatures, the elongation of Ni₃(Si, Ti) alloyed with Hf showed a particularly high value. This elongation behaviour is discussed based on the alloying effect on the competition between dynamic recrystallization and cavitation at grain boundaries. The fracture surfaces exhibited a variety of fracture patterns, depending on temperature and the alloy, and were primarily well correlated with the elongation behaviour. The ductilities of most of the alloys at high temperatures were reduced by the tests in air.     

The effect of Nb addition on microstructure and mechanical properties of Ni3(Si,Ti) alloy

The alloying behavior, the microstructure and the high-temperature tensile properties of the Nb-added Ni₃(Si,Ti) alloys were investigated. The solubility limit of Nb element in the L1₂ Ni₃(Si,Ti) phase at 1273 K was shown to be approximately 2.7 at%, and thermodynamically discussed. The second-phases (dispersions) formed beyond the solubility limits were identified as D0_a-type Ni₃Nb and Ni₁₂Si₄TiNb₂ compounds, and contained some lattice defects with incoherency with the L1₂ Ni₃(Si,Ti) matrix. The Nb-containing second-phase dispersions in the L1₂ Ni₃(Si,Ti) phase matrix resulted in strengthening over a wide range of temperature, and also an improvement of the high-temperature tensile elongation.     

The effect of titanium addition on the microstructure,mechanical and tribological properties of Ni3Si alloys prepared by powder metallurgy method

Ni₃Si alloy with different content of titanium was fabricated by powder metallurgy method. The microstructures, hardness and tribological properties of the alloys were investigation. The results showed that pure Ni₃Si alloy was composed of β₁‐Ni₃Si phase and γ‐Ni₃₁Si₁₂ phase, and Ni₃Ti phase formed with titanium addition. The hardness of the alloy decreased with the increasing titanium content. The friction coefficient of pure Ni₃Si alloy increased with the increasing load, while the friction coefficient of the alloy with titanium addition decreased. The wear rates of the alloys were all increased with increasing load, and the alloy with 5 % titanium addition had the best wear resistance properties. The wear mechanisms of the alloys were abrasive wear at low load, and the wear mechanisms changed to oxidative wear at high load.     

Aging behaviour of cobalt free chromium containing maraging steels

Abstract

This paper reports an investigation of the aging behaviour of two Co free Cr containing maraging steels (Fe–1·0Si–11·2Cr–1·3Mo–9·1Ni–1·2Al–1·0Ti and Fe–0·8Si–17·2Cr–6·1Ni–0·4Al–0·9Ti, all at.-%), using hardness measurements, electron microscopy of replicas and thin foils, atom probe field ion microscopy (APFIM), and thermochemical calculations. Two different families of intermetallic phases (Ti₆Si₇Ni₁₆G phase and η Ni₃Ti) have been found to contribute to age hardening. The composition and morphology of these precipitates were studied in deformed and undeformed alloys after aging at 420–570°C for various times. In addition, reverted austenite has been found in the aged structure. Results obtained using APFIM are compared with equilibrium thermodynamic calculations and previous APFIM studies of conventional Cr free low Al and Si maraging steels having higher Mo contents.

MST/1558     

Environmental effect on mechanical properties of recrystallized L12-type Ni3(Si,Ti) intermetallics

The environmental effect on the mechanical properties of boron-doped and undoped Ni₃(Si, Ti) polycrystals was investigated by tensile testing in air from room temperature to 1073 K, and the results were compared with those obtained previously by tensile testing in vacuum. The environmental effect for the Ni₃(Si, Ti) alloys was significant at ambient temperatures whereas that for the boron-doped Ni₃(Si, Ti) alloys was considerable at elevated temperatures. When these samples at associated temperatures were tensile tested in air and also at low strain rate, intergranular fracture was dominant. It was suggested that the environmental embrittlements at low and high temperatures were due to hydrogen and oxygen absorbed from the air, respectively, and were caused by the weakening of the grain-boundary cohesion. It was proposed that boron competing with hydrogen, for site occupation or for its effectiveness at grain boundaries, has the effect of suppressing hydrogen embrittlement, whereas it was suggested that the low-melting phases, consisting of boron and oxygen (and/or constituent atoms), may be responsible for the ductility loss in the boron-doped Ni₃(Si, Ti) alloys.     

Mechanical properties of the Ni3(Si,Ti) alloys doped with carbon and beryllium

The Ni₃(Si, Ti) alloys doped with small amounts of carbon and beryllium were tensile tested in two environments, vacuum and air, over a wide range of test temperatures. The yield stresses of the carbon-doped alloys were almost identical to the undoped alloys while those of the beryllium-doped alloys were slightly higher than the undoped Ni₃(Si, Ti) alloys. The doping with carbon enhanced the elongation and ultimate tensile strength (UTS) whereas doping with beryllium reduced the elongation over the entire temperature range tested. The fracture patterns were primarily associated with the ductility behaviour. As the elongation (or UTS) increased, the fracture pattern changed from the intergranular to the transgranular fracture patterns. No environmental embrittlement of the ductility of the carbon-doped alloys was found at ambient temperatures but it was evident at elevated temperatures. Ductilities were reduced at high temperatures when the carbon-doped alloys were tensile tested in air. At high temperatures the environmental embrittlement observed is suggested to be due to the penetration of (free) oxygen into the grain boundaries causing the ductility loss in the carbondoped alloys.     

Microstructural characterisation and mechanical properties of Ni–Ge alloys containing Ni3Ge

Abstract

Six Ni–Ge alloys were prepared and equilibrated at 1000°C. The microstructures, compositions of phases, solubilities in various phases, volume fractions of phases, lattice parameters of Ni₃ Ge, and microhardness values of phases were determined. Two alloys contained Ni₃Ge and α, one alloy was single phase Ni₃Ge, and three alloys contained Ni₃Ge and Ni₅Ge₃. The volume fractions of second phase were 6 and 33%α and 37 and 54%Ni₅Ge₃. The amount of second phase in the third Ni₃Ge–Ni₅Ge₃ alloy was too low to be determined accurately. The two phase alloys containing 6%α, 37%Ni₅Ge₃, or 54%Ni₅Ge₃ were tested in compression. A yield strength maximum was observed in the three alloys. The deformation behaviour of the alloy containing 6%α was similar to that of single phase Ni₃Ge: crack formation along Ni₃Ge grain boundaries and very low plastic strain were revealed. In alloys containing Ni₃Ge and an appreciable percentage of Ni₅Ge₃, crack formation was not observed along Ni₃Ge grain boundaries. Instead, at low temperatures, deformation and crack growth occurred within Ni₅Ge₃ grains and, at high temperatures, decohesion occurred along the Ni₃Ge/Ni₅Ge₃ interphase. The plastic strain in the alloy containing 54%Ni₅Ge₃ increased to about 40%.

MST/1724     

Precipitation of second phases in aged Ni rich NiTiRe shape memory alloy

Abstract

This study investigated the effect of aging on the structure and precipitation of second phases of Ni₅₂Ti_47·7Re_0·3 shape memory alloys. The alloy was solutionised at 1000°C for 24 h before aging at various temperatures ranging from 300 to 600°C for 3 h. The matrix phase in both solutionised and aged specimens was martensite. Ti₂Ni phase was also present in the microstructure of both solutionised and aged specimens and its volume fraction decreased as the aging temperature increased. Ni₄Ti₃ phase began in appearance by increasing aging temperature to 400°C. Ni₄Ti₃ precipitates had lenticular and non-geometry shapes. Aging at 600°C led to precipitation of Ni₃Ti phase in the microstructure. This precipitated phase formed in white blocky shapes. Ti/Ni ratio increased and/or Ni content decreased in the matrix with increasing in aging temperature.     

The effect of Si additions on the sintering and sintered microstructure and mechanical properties of Ti-3Ni alloy

Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti-Ni alloys because small additions of Si lower the solidus of Ti-Ni alloys appreciably (>200 °C by 1 wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti-3Ni alloy at 1300 °C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti₅Si₃ form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti₅Si₃ phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si.     

Reversible structural relaxation of amorphous Ni70TM5Si10B15 (TM = V,Cr, Mn,Fe, Co,Ni) alloys

The structural relaxation of amorphous Ni₇TM₅Si₁₀B₁₅ (TM = V, Cr, Mn, Fe, Co, Ni) alloy was investigated by the electrical resistance measurement under isochronal annealing. The reference Ni₇₅Si₁₀B₁₅ alloy showed no reversible changes in electrical resistance. All alloys other than TM = Ni exhibited a reversible change which is peculiar to the chemical short-range ordering. Only in TM = Cr alloy was the reversible part predominantly observed after pre-annealing, and in other alloys the irreversible change was also seen. The degree of reversible change below 623 K was in the order Fe > V > Cr > Co > Mn. This origin is discussed on the basis of Ni-TM correlation, which is considered to play an important role in ordering, and TM-metalloid correlation, which is considered to hinder the formation of an ordered phase.     

Mechanical properties of recrystallized L12-type Ni3(Si,Ti) intermetallics

The mechanical properties of the Ni₃(Si, Ti) alloys undoped and doped with 50 p.p.m. boron, both of which were polycrystalline specimens prepared by recrystallization, were investigated by tensile testing. The yield stress was found to increase with increasing test temperature to a maximum at 800 K, followed by a decrease. The tensile elongation was highest at room temperature and tended to decrease with increasing temperature for both alloys, but was consistently higher in the boron-doped Ni₃(Si, Ti) alloys than in the undoped ones over all the test temperatures. The change in the ultimate tensile stress (UTS) with temperature was similar to that of tensile elongation. The transgranular fracture became dominant as the elongation increased, regardless of the alloys and the testing temperature. Thus, this work again verified that the alloying method proposed by the present authors is useful for improving the grain-boundary cohesion of L1₂-type ordered alloys.     

Equilibria involving P- and (σ-phases in Ni–Cr–Al–Mo system

Abstract

An investigation is reported of phase equilibria in alloys of the Ni–Cr–AI–Mo system containing 60 and 50 at. –%Ni annealed at 1523 K. The experimental methods used mainly were electron microscopy, electron probe microanalysis, and X–ray diffraction. Four quaternary alloys were used, with compositions (at.–%) lying on a line between Ni₆₀Al₄₀ and Ni₆₀Cr₂₀Mo₂₀,and one alloy containing 50Ni–28Cr–10AI-12Mo.The composition range was chosen to include the P–and σ –phases based on the Ni–Cr–Mo system. The phases involved in equilibria at 1523 K in the 60 at. –%Ni alloy series were γ (Ni–base solid solution), β (based on NiAI), Mo–base solid solution, and P; the 50 at. –%Ni alloy contained γ, β and σ. Partial isothermal sections of the quaternary system at 60 and 50 at. –%Ni have been determined. Eutectics containing either γ + P or γ + β + P were present

in the as–solidified 60 at. –%Ni alloys, while a γ + β + σ eutectic was present in the 50 at–%Ni alloy; some compositional data for eutectic phases were obtained by analytical transmission electron microscopy. High hardness values (from ~ 500 to 670 HV) were obtained in the as–solidified alloys.

MST/265     

Alloy design for Al addition on microstructure and mechanical properties of Ni3(Si,Ti) alloy

The effect of Al addition on the microstructure and tensile properties of Ni₃(Si,Ti) alloys with an L1₂ ordered structure, which were fabricated through thermomechanical processing from arc-melted ingots, was investigated. Al was added to a Ni₃(Si,Ti) alloy by using two methods such that Al substituted for (1) only Ti and (2) both Ni and Ti along a Ni₃(Si,Ti)-Ni₃Al pseudo-binary line. In the case of the alloys prepared by the former method, the addition of more than 4 at.% Al resulted in a two-phase microstructure consisting of disordered fcc Ni solid solution dispersions in the L1₂ matrix, while in the case of the alloys prepared by the latter method, the addition of 4 at.% Al retained the L1₂ single-phase microstructure. In the case of the 4 at.% Al-added alloys, the room-temperature tensile properties were similar and independent of the alloying methods, whereas the high-temperature yield stress was higher in the alloys prepared by the latter method than in the case of the alloys prepared by the former method. These results suggest that a single-phase microstructure consisting of an entire L1₂ structure is favorable for obtaining high-temperature tensile properties.     

Microstructure and thermal stability of Al–Cr–X (X=Ni,Mo, Si) powders obtained by centrifugal atomisation

Abstract

The microstructure and the thermal stability of rapid solidified centrifugally atomised AI–3Cr–X (X=1Ni, 3Ni, 0·3Mo, 1Si, or 3Si, at.-%) powders were studied. Three main types of microstructure were observed in the powders: cellular, globular, and rosettelike. Some powders exhibited a mixture of these. In the atomised state the alloys usually had two phases, intermetallic Al₁₃Cr₂ and α-Al solid solution. Thermal stability was studied for a range of temperatures from 20 to 500°C. The phase Al₃Ni appeared in the nickel containing alloys and grew upon heat treatment. The molybdenum containing alloy did not show any noticeable change upon heat treatment. With respect to the silicon containing alloys, the intermetallic Al₁₃Cr₂ transformed into Al₁₃Cr₄Si₄ at high temperatures. On the basis of bibliographic information a nucleation map was calculated relating the prevalence of the intermetallic Al₁₃Cr₂ phase and the α-Al phase to the particle diameter and the chromium concentration of powder obtained by centrifugal atomisation.

MST/3273     

Analysis of tensile strength and microstructure of Ni–SiC composites prepared by electroforming

Ni–SiC metal matrix composites with two kinds of SiC content were prepared by electroforming in a nickel sulphamate bath. Tensile strength and microstructure of the composites before and after heat treatment were investigated. The maximum of tensile strength was obtained after heat treatment at 300 °C × 24 h. The values were 641 N/mm² and 701 N/mm² respectively. The complete reaction between nickel and SiC particles can produce shrinkage pores in the interface. The volume of shrinkage pores was equal to 8% of the volume of SiC particles in the composites. The interfacial reaction products were composed of Ni₃Si and a little amount of Ni₃₁Si₁₂ after heat treatment at 600 °C × 24 h. The fracture evolution went though microcracks initiation, growth and coalescence. Cracking of the matrix, debonding of Ni–SiC interfaces and cracking of particles were three types of cracking modes for Ni–SiC composites.     

Dissimilar joining of Ti3Al-based alloy to Ni-based superalloy by arc welding technology using gradient filler alloys

In this study, Ti–Al–Nb, Ti–Ni–Nb and Ni–Cr–Nb system alloys were designed and incorporated in order to construct a gradient structure at the surface of the joined Ti₃Al base material. And the Ti₃Al-based alloy and Ni-based superalloy were successfully joined together using gas tungsten arc (GTA) welding technology. The microstructure evolution, mechanical properties and fractured behaviors of the joints were investigated. The gradient structure remarkably decreased the formation tendency of brittle phases within the joints compared with a single filler alloy and thus improved the joint strength effectively. The average room-temperature tensile strength of the Ti₃Al/In718 dissimilar joint reached 353 MPa, and the strength value at 873 K was 245 MPa. At the Ti–Ni–Nb/Ni–Cr–Nb interface, some Ni₃(Nb, Ti) + (Nb, Ti)Cr₂ and TiNi₃ phases were detected in the Ti–Ni–Nb matrix. It was believed that their presence decreased the room-temperature strength of the Ti–Ni–Nb alloy but improved its high-temperature strength.     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃,is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.

When microstructure and composition are optimized (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss + Nb₃Al + Nb₅Si₃) three-phase alloys is improved by the addition of titanium.