TiAl Mn Research Articles

Self-Propagating High-Temperature Synthesis of a Ti–Al–Mn Alloy

Self-Propagating High-Temperature Synthesis of a Ti–Al–Mn Alloy

Self-Propagiating High-Temperature Synthesis in Ti–Al–Mn System

Self-Propagiating High-Temperature Synthesis in Ti–Al–Mn System

Combustion Synthesis and Phase Formation in the Ti–Al–Mn Alloy

Combustion Synthesis and Phase Formation in the Ti–Al–Mn Alloy

Effect of Nb additions on microstructure and properties of γ-TiAl based alloys fabricated by selective laser melting

Abstract The γ-TiAl based Ti–Al–Mn–Nb alloys with different Nb additions were fabricated by selective laser melting (SLM) on the TC4 substrate. The effects of Nb content on microstructure and properties of the alloys were investigated. The results reveal that the alloys consist of γ-TiAl phase with tetragonal lattice structure and α2-Ti3Al phase with hcp lattice structure, and show a sequential structure change from near full dendrite to near lamellar structure with the increase of Nb addition. Owing to the higher Nb content in γ-TiAl phase and the formation of near lamellar structure, the alloy with 7.0 at.% Nb addition has the best combination of properties among the studied alloys, namely, not only a high hardness of HV 2000, a high strength of 1390 MPa and a plastic deformation of about 24.5%, but also good tribological properties and high-temperature oxidation resistance.

Calculation of phase equilibria in Ti–Al–Mn ternary system involving a new ternary intermetallic compound

The Ti–Al–Mn ternary system is one of the most important systems in the development of low cost titanium alloys. Although several experimental data of this ternary system are available and some assessments are reported, a full set of thermodynamic parameters were not yet published. In this paper, the previous works for the Ti–Al–Mn system and the related sub-binary systems are reviewed, and the Ti–Al–Mn ternary system is assessed by means of the Calphad method using the ternary experimental data. A series of isothermal sections from 1073K to 1603K and the vertical section (25at% Ti) are calculated. It is shown that the present calculated results are in good agreement with most of the experimental results.

Isothermal oxidation behavior of two-phase TiAl–Mn–Mo–C–Y alloys fabricated by different processes

The isothermal oxidation behavior of two-phase Ti–46.6Al–1.4Mn–2Mo–0.3C–0.3Y based alloys in synthetic air at 800 and 900 °C was investigated. The main emphasis was focused on the effect of microstructures obtained by different processes of extrusion, melting and hot rolling, i.e. fully lamellar, nearly lamellar and duplex. On the exposure for 350 h at 800 °C, the growth rate and the spallation behavior of oxide scales of the alloys depended strongly on the fabrication process. The extruded alloy with a fine fully lamellar microstructure showed an excellent oxidation resistance due to the lowest mass gain and strong adhesion of the scale to the substrate, and Y-addition was effective in improving oxidation resistance because of its oxygen-scavenging effect in EPM processing. However, the overall oxidation process was dominantly controlled by the beneficial effect of the Mo-addition in the melted alloy due to the doping effect and formation of Mo-rich Al-depletion layer close to the substrate. The rolled alloy with the duplex microstructure greatly accelerated the oxidation rate because of a high volume fraction of α 2-Ti 3Al. At 900 °C, the melted alloy showed an excellent oxidation resistance during 350 h exposure. The oxidation rate and the spallation resistance of oxide scales did not depend strongly on the microstructure for the extruded or the rolled alloy. The poor spallation resistance of the oxide scales in the extruded alloy was probably related to thicker Mn-containing scales. Neither Mo-rich layer nor Y-rich zone in the oxide scales in the rolled alloy was observed at two exposure temperatures as a result of fast oxidation rate of the scale.

The effect of yttrium on microstructure and dislocation behavior of elemental powder metallurgy processed TiAl-based intermetallics

The effect of yttrium on microstructure and mechanical properties of elemental powder metallurgy (EPM) processed TiAl-based alloys was studied. Y-addition, by forming Y 2O 3 through internal oxidation, improved tensile yield strength of TiAl–Mn–Mo–C alloys based on microstructural refinement and precipitation hardening. The tensile ductility of the alloys was also improved moderated by Y-addition, which was attributed to scavenging of oxygen and activation of ordinary 1/2<110] type dislocations. A profuse generation of the dislocations ahead of the crack tip was also confirmed by an in situ TEM experiment. Y-addition did not seriously affect the cracking characteristics of alloys except for the enlarging plastic zone (PZ) size near the crack tip.

Fabrication of unidirectional porous TiAl–Mn intermetallic compounds by reactive sintering using extruded powder mixtures

The unidirectional porous Ti–45at.%Al–1.6at.%Mn intermetallic compounds having fully lamellar structure were fabricated by reactive sintering method using the extruded mixtures consisting of elemental titanium and aluminum-manganese powders. The average porosity of the specimens is measured to have a distribution in the range from 25% to 35% in volume fraction within the experimental conditions investigated. The porosity of the specimen produced decreased with increasing heating rate in the reactive sintering process. The maximum porosity of 35% can be obtained when the specimen was reactively sintered under the heating rate of 0.17 K/s. The porosity of the reactively sintered specimen can be controlled by a variation of heating rate. From the microstructural observation, we found a directionality of pores oriented parallel to the extrusion direction and suggested that an aligned-structure of pores of the present alloy is closely associated with the extrusion microstructure consisting of elongated feature of constituent mixtures.

Microstructural development of indirect-extruded TiAl–Mn–Mo–C intermetallic alloys during aging

The effect of aging at 800 °C on the microstructure in indirect-extruded TiAl–Mn–Mo–C alloys has been studied by transmission electron microscopy (TEM), high-resolution electron microscopy (HREM), energy dispersive X-ray spectrometry (EDS) techniques and microhardness testing. Indirect extrusion processing, as a means of consolidation, effectively refined the microstructure of elemental powder metallurgy (EPM) TiAl–Mn–Mo–C alloys. The alloys showed a sensitive response to aging heat treatment at 800 °C, the hardness peaking at the 0.3 at.% C-alloy. The morphology of α 2+γ plate colonies was related to the various nucleation conditions for the α→α 2+γ transformations at the prior α grain boundaries. Carbides, P-phase as well as H-phase, formed mainly at the α 2/γ interfaces and less frequently within the γ grain or prior α grain boundaries. The presence of ledges between the P-phase carbides and α 2 phase interfaces indicated that the nucleation stage of the P-phase carbides involved an interface-controlled process.

Effect of chlorine on the tensile behavior of reactive-sintered TiAl–Mn alloys

Ti–43.5 mol%Al–1.6 mol%Mn–2 vol.%TiB 2–(0–600) mass ppm Cl alloys were fabricated by using a reactive sintering process, and tensile behavior of the reactive-sintered TiAl–Mn-based alloys having various chlorine contents was investigated. It was revealed that at room temperature tensile properties are hardly affected by the amount of chlorine. In contrast, at 1073 K, elongation decreased with increasing chlorine content, although yield strength was almost insensitive to the amount of chlorine.

EPM method of synthesizing intermetallics based on Ti

Elemental powder metallurgy (EPM) is a powerful means of synthesizing intermetallics of high melting points. In this study, two intermetallic compounds based on Ti were investigated using different consolidation techniques: TiAl–Mn–Mo–C by hot extrusion and Ti 5Si 3–Nb–C by electro-pressure sintering (EPS). Full density compounds were obtainable in both cases. Aided by interstitial carbon atoms, each intermetallic compound showed attractive mechanical properties such as high tensile yield strength and creep resistance in TiAl–Mn–Mo–C and high fracture toughness and transverse rupture strength in Ti 5Si 3–Nb–C. The roles of carbon atoms, being less than 1 at.% in each material system, were, refinement of the lamellar microstructure and precipitation hardening in the former, and a possible modification of the crystal structure towards easier slip and less thermal anisotropy in the latter.

Improvement in high temperature oxidation resistance of TiAl–Mn intermetallic compound by preoxidation in CO–CO2 gas mixture

The oxidation behaviour of (Ti–44·9Al)–1·6Mn (at.-%) preoxidised at high temperatures in a CO–CO2 gas mixture with a very low equilibrium partial pressure of oxygen was investigated by a cyclic oxidation test at 1000°C in static air. The TiAl–Mn specimens were preoxidised in the temperature range 900–1000°C for various times (10–50 h) in CO–CO2 gas mixtures with various ratios of partial pressure pCO2/pCO , and were subsequently examined by X-ray diffractometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. In the cyclic oxidation test, the TiAl–Mn preoxidised at 975°C for 20 h in CO–CO2 with a pCO2 /pCO of 3 : 7 showed excellent oxidation resistance up to 600 h. This is attributed to the formation of a continuous flat Al2 O3 rich layer beneath the outer mixed oxide layer generated during the CO–CO2 preoxidation. Furthermore, the parabolic rate constant for the early stages of cyclic oxidation was determined to be 1·71 × 10-11 kg2 m-4 s-1 for the above specimen.

An experimental study of phase transformations and a comparison with calculated phase equilibria in Ti–Al–Mn alloys

The solid-state phase transformations and phase equilibria in Ti–Al–Mn alloys containing 5 and 10 at.% Mn (with a Ti:Al atomic ratio of 1.14) have been investigated both experimentally and through CALPHAD methods. The alloys, prepared by plasma arc melting, were studied by differential thermal analysis (DTA) and microstructural characterization of high temperature annealed and water quenched (WQ) samples. On heating, DTA endotherms were observed to occur at temperatures consistent with the microstructural changes identified in annealed and WQ specimens. The 5% Mn alloy is found to transform from ( β+ α+ γ) to ( β+ α) on heating from 1145 to 1330°C. The 10% Mn alloy undergoes the following sequence of phase changes on heating from 1090 to 1330°C: ( β+ γ+(MnAl) 2Ti)→( β+ γ)→( β+ α+ γ)→( β+ α)→ β. Both the α and β phases decompose to a variety of products on WQ. A comparison between calculated and predicted equilibrium phase compositions shows good agreement for the ( α+ β+ γ) tie triangle over a range of temperatures, but the calculated two-phase ( α+ β) region is somewhat displaced from the high temperature experimental phase compositions.

Phase transformations in a welded near-α titanium alloy as a function of weld cooling rate and post-weld heat treatment conditions

In α-β titanium alloys, the high-temperature β phase can decompose in several ways, depending on alloy composition and cooling rate. In the case of welded joints, cooling rates can vary widely as a function of heat input. In the current work, a dilute α-β Ti-Al–Mn alloy was welded over a range of heat inputs using electron beam and gas tungsten-arc welding processes. A major part of the rapidly cooled electron beam weld could be identified as lath-type or “massive” martensite. In the slower cooled gas tungsten-arc welds, the transformation product was a mixture of lamellar α and β phases formed entirely by diffusion. Post-weld heat treatment resulted, in all cases, in an α-β structure that coarsened with annealing temperature and time. Tensile elongation in the as-welded condition was poor on account of a large prior-β grain size and an acicular microstructure. The ductility improved as the structure coarsened on heat treatment. Tensile fracture was always microscopically ductile, but the presence of a grain boundary α layer tended to induce intergranular rupture, especially when a hard, intragranular matrix confined slip to occur in the grain boundary regions.

Consolidation of a gamma TiAl–Mn–Mo alloy by elemental powder metallurgy

Abstract An intermetallic compound based on gamma TiAl–Mn–Mo was consolidated from elemental powders. The process route consisted of powder blending, compacting and final consolidation by hot extrusion or hot forging. Good tensile properties were obtained by the heat treatment given to generate either duplex microstructure or fully lamellar microstructure. In hot forged samples, porosity control was of critical importance. Kirkendall pores in this material were attributed to the formation of transient phases such as TiAl 3 and two preventive methods, heating rate control and extended soaking treatment, were suggested and experimentally verified. Beta phase was found in the regions rich with Mo and could be removed by controlling the heat treatment schedule. Microstructural evolution during heat treatment of elemental powder metallurgy (EPM) TiAl alloys was studied by X-ray diffraction, optical microscopy and micro-hardness measurement and was explained in terms of a phase diagram. Overall this work proposes EPM as a viable method for economic production of gamma TiAl alloys.

Effect of microstructural features on the fracture toughness of a welded alpha-beta TiAlMn alloy

The fracture toughness of weld fusion zones in a dilute, medium-strength alpha-beta TiAlMn alloy has been studied in relation to their microstructures. Welding was performed over a range of energy inputs using different techniques. The weldments were subjected to three different annealing treatments. It has been shown that the fracture toughness of all weld metals is greater than that of the parent material. The data also reveal that there is a further improvement in fracture toughness on postweld heat treatment. The results are discussed in terms of the structural changes occurring during welding and subsequent annealing.

Influence of chlorine on the oxidation behavior of TiAlMn intermetallic compound

Abstract Excellent oxidation resistance of reactive-sintered titanium aluminides was reviewed and the mechanism of forming protective Al 2 O 3 film was studied. TiAlMn containing more than 200 mass ppm chlorine from raw titanium powder showed very little mass gain after oxidation from 1173 to 1398 K in air. The chlorine effect was found obviously in composition of α 2 + γ and γ regions in the TiAl phase diagram. The chlorine effect was found even in a plasma arcmelted TiAlMn alloy with chloride covering or oxidized in air containing a small amount of chlorine gas. Also, other halogen elements as fluorine and bromine had the same effect as chlorine. It was clear, in particular by surface analyses that chlorine existed in TiO 2 near the oxide/metal interface in the early stage of oxidation and decreased oxygen ion vacancies in TiO 2 . Then TiO 2 growth was interrupted and protective Al 2 O 3 scale was formed on the titanium aluminide.

Microstructure, Mechanical Properties, and High-Temperature Oxidation Resistance of Boronized γ - TiAl(Mn)

ABSTRACTTo improve the wear and high-temperature oxidation resistances of reactively sintered γ -TiAl(Mn) intermetallic compounds, they were boronized in the temperature range of 900 to 1100 °C for 5 to 11 hours with powder mixtures of B4C and Na2B4O7. It was found that the coating layer consisted of three sublayers, i.e., outer, middle, and inner sublayers. The outer, middle, and inner sublayers were identified as a mixture of TiB2 and TiO2, a mixture of Al2O3 and TiO2, and Ti-rich TiAl based compounds, respectively. The coating layer significantly improved the surface hardness and the wear and high-temperature oxidation resistances. The highest surface hardness (Hv ≈ 2720) was obtained at 900 °C for 11 hours. This surface hardness is much higher than that of TiAl(Mn) (Hv ≈ 400). In addition, the specimens boronized at both 1000 and 1050 °C for 9 hours showed an excellent high-temperature oxidation resistance. It is believed that the boronizing on the TiAl(Mn) intermetallics is very effective to improve the wear and high-temperature oxidation resistances.

反応焼結ＴｉＡｌ（Ｍｎ）の常温引張特性に及ぼす反応焼結前の押出比の影響

反応焼結ＴｉＡｌ（Ｍｎ）の常温引張特性に及ぼす反応焼結前の押出比の影響