C. Ramachandra

Silicide Precipitation in Alloy Ti-6AI-5Zr-0.5Mo-0.25Si

Silicide precipitation in the titanium alloy containing by wt pct 6Al-5Zr-0.5Mo-0.25Si (Alloy 685) has been investigated using electron diffraction. The solutionizing temperature for this alloy is 1323 K. It is observed that no resolvable silicide precipitates are present in the alloy as received, furnace cooled from 1323 K and aged at 823 K, or oil quenched from 1323 K and aged at 823 K. Specimen solutionized at 1323 K for 30 minutes followed by water quenching and aging at 923 K for 24 hours shows fine precipitates at boundaries of α′ platelets. Diffraction analysis shows that the fine precipitates belong to two different types of identifiable hexagonal silicides similar to those observed by Floweret al in the ternary Ti-5Zr-0.5Si alloy to be (TiZr)5Si3 and another of unknown stoichiometry. However, aging of the water quenched specimens for 24 hours at higher temperatures, 973 K and 1073 K, results in the precipitation of a silicide similar to the latter one. It is analyzed that this silicide has lattice parameters a = 0.702 nm and c = 0.368 nm. Since aging at 973 K and 1073 K gives rise to precipitation of the same silicide, it is concluded that this is the stable silicide in Alloy 685 in the temperature range investigated.

Microstructural characterisation of near-α titanium alloy Ti−6Al−4Sn−4Zr−0.70Nb−0.50Mo−0.40Si

Microstructural stability in the near-α titanium alloy (alloy 834) containing Ti-6Al-4Sn-4Zr-0.70Nb-0.50Mo-0.40Si (in weight percent), in the β and(α + β) solution-treated and quenched conditions, has been investigated. The β transus for this alloy is approximately 1333 K. Solution treatment in the β phase field at 1353 K followed by quenching in water at room temperature resulted in the formation of α′ martensite platelets with high dislocation density and stacking faults. Thin films of β are found to be sandwiched between interface phases, which, in turn, are sandwiched at the interplatelet boundaries of lath martensite. The interface phase is a subject of much controversy in the literature. Solution treatment at 1303 K in the(α + β) phase field followed by quenching in water at room temperature resulted in the near-equiaxed primary α and transformed β. Both the β and(α + β) solution-treated specimens were aged in the temperature range of 873 to 973 K. While aging the —treated specimen at 973 K,(α + β)-treated specimen, even at a lower temperature of 873 K for 24 hours, caused precipitation of suicides predominantly at the interplatelet boundaries of martensite laths. Electron diffraction analysis confirmed them to be hexagonal suicide S2 witha = 0.702 nm andc = 0.368 nm. The above difference in the precipitation could be attributed to the partitioning of a higher amount of β- stabilizing elements as well as silicide-forming elements to the transformed β in the(α + β) solution-treated condition. However, ordering of theα′ phase was observed under all of the aging conditions studied. The ordered domains were due to the longer aging times, which cause local increases in the level of theα-stabilizing elements.

Effect of silicide precipitation on tensile properties and fracture of alloy Ti-6Al-5Zr-0.5Mo-0.25Si

The effect of silicon in solid solution and in the form of suicides has been studied on the tensile properties and fracture behavior of alloy Ti-6Al-5Zr-0.5Mo-0.25Si (alloy 685). The heat treatment to hold silicon in solid solution consists of solutionizing at 1323 K for 0.5 hour under vacuum (∼10-5 MPa), followed by water quenching, and the treatment to precipitate suicides involves subsequent aging of the solutionized and water quenched specimens at 1073 K for 24 hours. There is only marginal effect of the aging treatment on strength values; however, the ductility parameters are found to be drastically reduced. There are marked differences in the fracture behavior of the alloy in the as-quenched and the quenched and aged conditions. While the fracture surface of the unaged specimen shows characteristic dimples, there is a large number of facets on the fracture surface of the aged specimen. The facets in the central region are relatively smaller in size than those in the peripheral zone. The central facets show fluted features at higher magnifications; however, the peripheral facets are usually featureless. The faceted fracture in the aged condition is attributed to enhanced tendency for heterogeneous planar slip. The fracture characteristics correlate with the observed differences in the ductility in the two conditions.

Microstructure of β-solution-treated, quenched and aged α+β titanium alloy Ti-6Al-1.6Zr-3.30Mo-0.30Si

Silicide precipitation in the (α+β)-type titanium alloy containing (in weight per cent) 6Al-2Zr-3.30Mo-0.30Si (alloy VT9) has been investigated. The β transus temperature for this alloy is approximately 1243 K. Solution treatment in the β phase field at 1323 K followed by quenching in water at room temperature resulted in the formation of a single-phase martensitic structure. The martensitic structure was confirmed to be orthorhombic (α″) using the X-ray diffraction technique. The water-quenched specimen was subjected to aging treatment over a temperature range 823–1073 K for 24 h. Both X-ray and electron diffraction results of the material aged at 823 K showed the structure to be single-phase h.c.p. martensite. However, aging in the temperature range 873–1073 K for 24 h caused heterogeneous precipitation of silicides. Electron diffraction analysis confirmed that the silicides are of hexagonal structure with a=0.70(2) nm and c=0.36(8) nm. Silicide precipitation is predominantly at the interplatelet boundaries of α′-martensite in the samples aged at 873 and 923 K. However, aging at 973 and 1023 K resulted in precipitation of thin films of β phase sandwiched between the interplatelet boundaries of the martensite. Silicides were mainly at the α-β interfaces or inside the β phase. In the sample aged at 1073 K, apart from thin films of β and silicides, islands of transformed β were observed. The kinetics of silicide precipitation was faster than in near-α alloys. This is attributed to the role of increased molybdenum in the alloy of the present investigation.

Effect of silicides on tensile properties and fracture of alloy Ti-6AI-5Zr-0.5Mo-0.25Si from 300 to 823 K

The tensile properties and fracture behaviour of alloy Ti-6AI-5Zr-0.5Mo-0.25Si (wt%) have been investigated over a wide range of temperature from 300 to 823 K, in the as-water-quenched (WQ) and different aged (473 to 1073 K for 24 h)conditions followingβ-solution-treatment (1323 K for 0.5 h). There is only a limited increase in strength but a drastic reduction in the ductility, at 300 K, due to ageing at ⩾ 923 K. There is strong dynamic strain-ageing (DSA) in the unaged (WQ) state from 623 to 823 K and it is essentially due to silicon in the solid solution. The degree of DSA decreases with the ageing temperature and DSA does not occur in specimens aged at 973 and 1073 K. In general, the ductility of the WQ as well as the aged material increases with test temperature, except in the range of DSA, where the ductility of WQ material is reduced. The mode of fracture of the WQ specimens remains ductile in the lower and higher ranges of test temperature, but changes to quasi-cleavage at intermediate test temperatures. The minimum in the ductility and quasi-cleavage mode of fracture at 773 K, in the WQ material, is due to strong DSA. Three different modes of fracture, namely faceted, ductile, and mixed intergranular and ductile in the lower, intermediate and higher range of test temperature, respectively, are observed also in the aged conditions (at and above 923 K) of the material. The tensile properties and fracture characteristics in the aged conditions are controlled by the silicides.

Orientation relationship between α′ titanium and sulicide S02 in alloy Ti-6Al-5Zr-0.5Mo-0.25Si

Orientation relationships between the silicide S2 and the matrix of alpha-prime platelets are established for the titanium alloy 685 (Ti-6Al-5Zr-0.5Mo-0.25Si), a near-alpha alloy designed for the high-temperature components of jet engines. A stereogram showing the parallel planes of alpha-prime and S2 is presented for the alloy in the water-quenched and aged condition. A table is also presented which lists the parallel planes of the matrix and the silicide along with the misfit parameters. The results obtained are compared with the orientation relationships reported in the literature. 14 references.

Room temperature low cycle fatigue behaviour of titanium aluminide Ti-26.2Al-15.2Nb-0.4Mo

Titanium aluminides, based on the compositions TiAl and Ti{sub 3}Al, are considered promising structural material for aerospace, in particular, for engine components to replace the heavy superalloys. However, these intermetallics are plagued by poor ductility at room temperature. Intense research efforts, over the last one decade, directed to address the ductility problem, met with some success through modification of chemistry, process and microstructural control. The present work is concerned with influence of small addition of Mo on LCF behavior of Ti{sub 3}Al-Nb alloy. Tensile and LCF properties of a quarternary alloy (Ti-26.2Al-15.2Nb-0.4Mo), in {alpha}{sub 2} + {beta}{sub 2} heat treated condition, are evaluated at room temperature and compared with those of ternary alloy Ti-27Al-15Nb.

Nonequilibrium solidification of undercooled melt of Ag-Cu alloy entrained in the primary phase

The solidification structure of undercooled melt of Ag-Cu alloy, entrained in its primary Cu-rich phase, has been investigated. The undercooling procedure consisted of equilibration of a Cu-13 pct Ag alloy in the two-phase liquid-solid region, followed by repeated thermal cycling of the liquid. Slow cooling of the sample in the present work established the ability to undercool the melt up to 70 K below the eutectic temperature of this alloy. The microstructure of the undercooled alloy indicated a complete absence of eutectic reaction on subsequent quenching of the melt directly from the equilibration temperature. The compositional analysis of the constituent phases by electron probe microanalysis (EPMA) technique provided evidence for the massive diffusionless solidification of the undercooled liquid. The X-ray diffraction study and electron microscopic examination indicated evidence for the spinodal transformation of the metastable solid solution phase. The composition of the phases formed on decomposition matched well with the calculated coherent spinodal boundaries in this system. The evolution of the metastable microstructure in the mushy-state quenching process of this alloy is discussed.

Plasma-Melted Nitrogen-Bearing Cast Stainless Steels Microstructure and Tensile Properties

Alloys of Fe-Cr-Ni and Fe-Cr-Mn were plasma arc-melted and chill cast in the form of ingots. Exposure of liquid melts to a nitrogen plasma for the purpose of adding nitrogen to the above alloys was used to dissolve varying amounts (up to 0.32 wt pct) of nitrogen. Carbon and sulfur were varied up to 0.5 and 0.056 wt pct, respectively. The alloys were evaluated for their monotonic behavior. It was observed that while strength and ductility parameters increased considerably with increasing nitrogen and carbon contents, both these parameters deteriorated with sulfur content. The analysis of the present results, along with the data from the literature, suggests that the strength parameters are predominantly chemistry dependent, particularly controlled by the nitrogen and carbon contents. The results also show that the alloys produced by this relatively new technique, plasma arc-melting, are comparable to those produced by other standard techniques.

Plasma-melted nitrogen-bearing cast stainless

Alloys of Fe-Cr-Ni and Fe-Cr-Mn were plasma arc-melted and chill cast in the form of ingots. Exposure of liquid melts to a nitrogen plasma for the purpose of adding nitrogen to the above alloys was used to dissolve varying amounts (up to 0.32 wt pct) of nitrogen. Carbon and sulfur were varied up to 0.5 and 0.056 wt pct, respectively. The alloys were evaluated for their mono- tonic behavior. It was observed that while strength and ductility parameters increased consid- erably with increasing nitrogen and carbon contents, both these parameters deteriorated with sulfur content. The analysis of the present results, along with the data from the literature, sug- gests that the strength parameters are predominantly chemistry dependent, particularly controlled by the nitrogen and carbon contents. The results also show that the alloys produced by this relatively new technique, plasma arc-melting, are comparable to those produced by other stan- dard techniques.