The structure of the γ′ extended solid solution in a splat-cooled Ag-50 at.% Cu alloy

The structure of the extended solid solution in electron transparent areas of a splatcooled Ag-50 at.% Cu alloys was examined by transmission electron microscopy. This phase was usually found to be spinodally decomposed at large grain sizes (–1m in diameter), in contrast to X-ray diffraction data indicating that the solid solution was undecomposed. A solidification model for rapidly quenched eutectic alloys is proposed to account for the observed structure of the splat-cooled alloy. A transformation curve for the spinodal decomposition of is also calculated and related to predictions derived from the solidification model.     

Comparison of the long-term strength of α- and α + β- titanium alloys under a layer of chlorides in the temperature range 200–380°C

Conclusions Chlorides have no effect up to a temperature of 300°C on the-Ti alloys studied, but the strength characteristics of the+-Ti alloys fall by 10% (on the basis of 2000 h). In the range 300–380°C the long-term strength under a layer of chlorides on a basis of 2000 h decreases by 2.5 times in the+-Ti alloys, but only by 20—30% in the-alloys. Under the influence of the prolonged action of chlorides (up to 2000 h), the+-Ti alloys suffer corrosion cracking at a lower stress level than do the-alloys. The deformation capacity of the-alloys falls by 2–4 times in the same temperature range, while in the + —alloys it falls practically to zero. A decrease in thickness of the specimen from 5 to 1.5 mm leads to a reduction in the long-term strength limit based on 2000 h by 1.3–1.4 times. A significant size-scale effect appears in- and+-Ti alloys which have a room-temperature yield point of not less than 70 kg/mm². The extent of the fall in long-term strength and deformation capacity with reduction in the thickness of the specimen is somewhat greater in+ —Ti alloys than in-alloys.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 11, No. 5, pp. 67–70, September–October, 1975.     

Microstructure and tensile properties of in situ synthesized (TiBw + TiCp)/Ti6242 composites

In the present work, (TiB_w+ TiC_p)/Ti6242 composites were fabricated via common casting and hot-forging technology utilizing the SHS reaction between titanium and B₄C. The XRD technique was used to identify the phases of composites. The microstructures were characterized by means of OM and TEM. Results from DSC and analysis of phase diagram determine solidification paths of in situsynthesized Ti6242 composites as following stages: -Ti primary phase, monovariant binary eutectic -Ti + TiB, invariant ternary eutectic -Ti + TiB + TiC and phase transformation from -Ti to -Ti. In situsynthesized reinforcements are distributed uniformly in titanium matrix alloy. Reinforcement TiB grows in whisker shape whereas TiC grows in globular or near-globular shape. TiB whiskers were made to align the hot-forging direction after hot-forging. The interfaces between reinforcements and Ti matrix alloy are very clean. There is no any interfacial reaction. Moreover, the mechanical properties improved with the addition of TiB whiskers and TiC particles although some reduction in ductility was observed. Fractographic analysis indicated that the composites failed in tension due to reinforcements cracking. The improvements in the composite properties were rationalized using simple micromechanics principles. The strengthening mechanisms are attributed to the following factors: undertaking load of TiB whiskers and TiC particles, high-density dislocations and refinement of titanium matrix alloy's grain size.     

Ageing characteristics of rapidly solidified Al-Li-Ti alloys at 473 K

The influence of titanium on the precipitate free zones (PFZs) and on the stable phase is investigated in four melt-spun Al-Li-Ti alloys with 2 mass% lithium and 0.10–0.35 mass% titanium. Rapid solidification gives rise to a homogeneous distribution of titanium on the wheel side of the ribbons, and to a cellular distribution of solute atoms in intermediate regions and on the gas side. Heat treatments of up to 1000 h at 473 K do not modify titanium distribution, which remains in solid solution. During ageing, preferential coarsening of particles occurs on cell walls. The growth rate of the PFZs is lower than in other Al-Li alloys and is mainly controlled by the titanium concentration of the alloys. The nucleation and growth of the stable phase is also delayed by the presence of titanium. These results indicate that the effect of titanium in solid solution is to retain vacancies thus reducing lithium diffusion rate.     

Surface engineering and chemical characterization in ion-nitrided titanium and titanium alloys

The chemical and physical characteristics of ion-nitrided surface layers, obtained on - titanium alloys, are examined and correlated both with the working conditions adopted in the ion-nitriding process and with the alloy chemical composition. Besides the influence of the working parameters on the morphology and on the microstructures of the ion-nitrided surface layers, mainly the alloy element distributions both in surface coatings and in the substrate are analysed for five - titanium alloys of industrial use, and for titanium c,p. as reference, ionnitrided at various treatment temperatures. The nitriding process forms, on titanium alloy parts, high-hardness surface layers consisting of TiN ( phase) and Ti₂N ( phase) nitrides and an interstitial solid solution of nitrogen in the close-packed hexagonal lattice of titanium ( phase). The presence and the extent of these phases as well as the ion-nitrided layer morphology are essentially determined by the alloy chemical composition and the working parameters. In particular a low-temperature treatment produces an extended nitrogen diffusion in the matrix beneath a thin continuous nitrided layer, while a high-temperature treatment produces prevalently a continuous nitrided surface layer. The alloy element distribution appears differentiated in the various phases and may be correlated with the different affinity of these elements with nitrogen.     

Cellular microstructure and heterogeneous coarsening of δ′ in rapidly solidified Al-Li-Ti alloys

Four melt-spun Al-Li-Ti alloys with 2 wt% lithium and 0.10 to 0.35 wt% titanium have been obtained and heat-treated at 473 K for up to 1000 h. Rapid solidification gives rise to a matrix with titanium in solid solution which drastically alters the coarsening rate. While TEM studies of samples aged for short times show a homogeneous distribution of metastable phase, as ageing time is increased, and depending on the ribbon section, three different microstructures can be distinguished: (i) on the wheel side, the distribution is homogeneous; (ii) intermediate regions show particles delineating cells with narrow walls; (iii) on the gas side, particles delineate circular cells. A higher titanium content in the cell centres than on cell walls has been determined. The coarsening rate of in microstructure (i) above is slower than in binary Al-Li alloys. Cellular microstructures (ii) and (iii) show the preferential coarsening of particles on the walls, which is faster the higher the titanium concentration. Taking into account the fact that the partition coefficient of titanium in aluminium in the peritectic region is > 1, an explanation of phase evolution is given which leads to the conclusion that the effect of titanium in solid solution is to retain vacancies, restricting lithium diffusion.     

Microstructure, texture, and mechanical properties of continuously cast gamma TiAl

Microstructural and texture analyses have been conducted on two gamma titanium aluminide (-TiAl) sheets produced via the melt overflow rapid solidification technique. For both alloy compositions relatively weak {101} type fiber textures (3 × random) were observed which are indicative of solidification from a prior phase field with rapid growth parallel to the direction. The strengths of the strips were comparable to conventionally processed sheets however tensile ductility was low. The results are discussed with respect to recent studies of the influence of cooling rate on the solidification microstructure of cast titanium aluminides.     

Toughening of titanium alloys by twinning and martensite transformation

Two kinds of titanium alloys, titanium alloy (Ti-10V-2Fe-3Al) and titanium alloy (Ti-5Al-2.5Sn) were used to investigate the toughening mechanisms with new approaches. The results show that Ti-5Al-2.5Sn alloy possesses good combination of strength and ductility as well as satisfied low-cycle fatigue life both at 293 K and 77 K. As for Ti-10V-2Fe-3Al alloy, the microstructure with metastable phase shows lower strength and ductility but higher threshold stress intensity factor (K _th) than solution treated and aged microstructure composed of and phases. The microstructures also show that twinning occurs in deformation of Ti-5Al-2.5Sn alloy at 77 K. Twinning seems to be helpful for improving the low-cycle fatigue life to a great extent at cryogenic temperature. It's also found that owing to stress-assisted martensite transformation in metastable Ti-10V-2Fe-3Al alloy, the fatigue crack propagation path shows a very tortuous way, which decrease the effective stress intensity factor (K _eff) at crack tip, and increase threshold stress intensity factor (K _th).     

The effect of Ag additions on the microstructure of aluminium–lithium alloys

Minor quantities of Ag have been added to Al–Li–Cu–Mg–Zr alloys. Their microstructure has been studied by means of optical metallography, transmission electron microscopy and X-ray diffraction. In the high Li, low Cu:Mg ratio alloys the main phases found were , , S and T1, while fewer T2 and Al7Cu2Fe precipitates were also observed. The addition of up to 0.5 wt% Ag diminishes the and T1 precipitates size. This is attributed to a small increase of Li solubility in the matrix. In the low Li, high Cu:Mg ratio alloy the addition of 0.2 wt% Ag resulted in the precipitation of phase simultaneously with , , S and T1 phases. Due to the low Li concentration an unusual growth of the / precipitates at the expense of the precipitates was also observed. © 1998 Kluwer Academic Publishers     

The role of silicon in the formation of the (Al5Cu6Mg2) σ phase in Al-Cu-Mg alloys

The role of silicon in the precipitation of the phase (Al₅Cu₆Mg₂) has been investigated through comparative studies on Al-3.63Cu-1.67Mg (wt%) and Al-3.63Cu-1.67Mg-0.5Si alloys. Both alloys were extensively examined after solution treating at 525°C for 2.5 h followed by ageing at 265°C for times up to 650 h. Limited studies were also undertaken on both alloys after ageing at 200 and 305°C. Precipitation of was observed in Al-3.62%Cu-1.66%Mg-0.5%Si for all ageing conditions studied but was absent in Si-free Al-3.62%Cu-1.66%Mg. In addition, S and phases were observed in both alloys. The volume fraction of phase in the Si containing alloy was substantially reduced by a pre-age stretch followed by ageing for 24 h at 265°C with S being the dominant precipitate type. The volume fraction of phase in the Si containing alloy was lower after ageing 24 h at 200°C than after 24 h at 265 and 305°C. Peak hardness was higher for the Si free alloy on ageing at 200 and 265°C, but the Si free alloy softened more rapidly, reflecting the more rapid coarsening kinetics of S compared with .     

Observations on the formation of β-Al5FeSi phase in 319 type Al-Si alloys

In Al-Si alloys, the properties are influenced by the shape and distribution of the eutectic silicon particles in the matrix, as also by the iron intermetallics and copper phases that occur upon solidification. The -Al₅FeSi iron intermetallic phase, in particular, is known for its detrimental effect on the properties, and is controlled variously by the iron content and the melt/solidification conditions of the alloy. The formation of the iron intermetallics has been observed in commercial 319 alloy end-chilled castings, obtained from non-treated and treated melts, where the effects of cooling rate, strontium modification and grain refinement have been studied. The volume fraction of -phase formed was seen to increase with the decrease in cooling rate (i.e. with increasing distance from the chill) in the untreated alloy. Sympathetic (preferential) nucleation of the -iron needles was also observed, in which the branching of -needles from a parent needle resulted in the formation of large -needle entities that can cover distances of 1200 m across the matrix surface. The beneficial effect of modification, i.e. strontium addition to the melt, was manifested through its influence on the fragmentation and dissolution of the -needles. The strontium poisons the sites where sympathetic nucleation takes place. Dissolution was accelerated with increasing strontium content, the optimum level being 300 p.p.m. Grain refining, on the other hand, negated the beneficial effect of modification, in that the -needles underwent thickening and the sympathetic nucleation/branching also occurred. The modified alloy was found to possess the lowest volume fraction of -Al₅FeSi phase among the unmodified, modified, grainrefined, and modified/grain-refined alloys.     

The age hardening effect in Ti-6 Al-4 V due to ω and α precipitation in the β grains

The structure at room temperature of a quenched TA6V titanium alloy has been investigated. This structure is + or + according to the treatment temperature; it is always metastable. During ageing the grains decomposed by the reaction + + +; this decomposition was accompanied by a large increase of the 0.2% yield stress. No structural modification was observed in. The and phase of TA6V were separately investigated in the form of single-phase alloys. The hardness of was insensitive to ageing, while was considerably hardened by and; we deduced that the strengthening of the minor phase during ageing is mainly responsible for the hardening of TA6V.     

The martensitic transformation in cold-worked Fe-Mn alloys studied by Mössbauer spectroscopy

The effect of cold-working on 16, 23 and 30 at% Mn iron-manganese alloys (C<0.05 at%) has been examined using Mössbauer spectroscopy and X-ray diffractometry. The induced martensitic transformation , depends on the composition and on the initial structure. It is found that the transformation in presence of the phase occurs only at high deformations.     

Precipitation hardening in Cu 1.81 wt % Be 0.28 wt % Co

The structure of the precipitation hardening alloy Cu 1.81 wt % Be 0.28 wt % Co has been studied as a function of ageing temperature and time, by transmission electron microscopy. The continuous precipitation sequence found is: supersaturated solid solution G.P. zones .The G.P. zone is an ordered platelet precipitate, which is coherent on {100} matrix planes and is nucleated in very high densities (>10²⁴ m^–3). The coherency stress fields, due to the misfit of the G.P. zone and matrix, overlap to produce a net matrix contrast along {110} 10, and give the characteristic tweed structure, which can be described by the kinematical theory of diffraction. The semi-coherent intermediate precipitate is nucleated by the G.P. zones and the transformation is characterized from the changes in the arrowhead structure produced in the electron diffraction patterns. No transformation of to the equilibrium precipitate is found for the ageing times investigated.     

Titanium for aerospace: Rationale and applications

Titanium and titanium alloys are excellent candidates for aerospace applications due to their high strength to weight ratio and excellent corrosion resistance; titanium and its alloys are immune to almost every medium to which they would be exposed in an aerospace environment. Titanium usage is, however, strongly limited by its higher cost relative to competing materials, primarily aluminum alloys and steels. Hence the advantages to using titanium must be balanced against its added cost. The titanium alloys used for aerospace applications, some of the characteristics of these alloys, the rationale for utilizing them, and some specific applications of different types of actual usage will be discussed herein.This is an extension of References 1 and 2, which reviewed applicaiions of alloys. These references will provide more details on applications of the alloys, while the and / alloys are added herein.     

Microstructures of titanium-aluminides produced by laser surface alloying

The microstructures of Ti-Al layers (from 43–80 at %Al) produced by laser surface alloying of titanium substrate with a powder feed technique have been investigated. The laser processing parameters were; 1.8 kW laser power, 3 mm beam diameter, 7 mm s^–1 traverse speed, and values of powder flow rates of aluminium ranging from 0.07–0.11 g s^–1. The microstructures were dendrites of ₂ and interdendritic regions of ₂+ in the Ti-43 at %Al alloy; dendrites of either ₂ or ₂+ with interdendritic in the Ti-50 at %Al alloy; dendrites of ₂+ with interdendritic in the Ti-55 at %Al alloy; single phase in the Ti-60 at %Al alloy and TiAl₃ dendrites and Al solid solution in the interdendritic regions in the Ti-80 at %Al alloy. The microstructures were fine and comparable to those produced by other methods of rapid solidification processing. The microstructures of the Ti-50 and Ti-55 at %Al alloys were in agreement with the existence of the peritectic reactions:L + andL + , in the Ti-Al system.     

Composition and precipitation inhomogeneities in melt-spun Al-Cu and Al-Zn ribbons

Non-uniform distributions of solute content in the supersaturated -Al matrix and inhomogeneous precipitation of metastable/stable phases have been studied in rapidly solidified Al-Cu and Al-Zn ribbons. Double-peaked X-ray diffraction lines from the -Al matrix were observed in three as-quenched Al-Cu alloys and one annealed Al-Zn alloy. For as-quenched Al-Cu ribbons, variations in composition, microstructure and precipitate density across the ribbons result from local differences in cooling rates. These differences originate from recalescence during the solidification. The double-peaked diffraction lines for these alloys correspond to two compositions of the -Al phase with less copper solute in the top-side than in the wheel-side of the ribbons. Subsequently, annealing of the samples leads to inhomogeneous precipitation of the metastable phase across the whole thickness of the ribbons. Such inhomogeneities do not appear in as-quenched Al-Zn alloys which are quenched into a single -Al phase. Splitting of the diffraction lines in the annealed ribbons arises from differences in the composition of two different -Al matrix phases. The -Al phase between the R phase lamellae has a low zinc content, while the -Al phase where high densities of Guinier—Preston zones are retained has a higher zinc concentration.     

Mössbauer study of martensitic transformations in an Fe-29.6% Ni alloy

The application of an external stress may form band shaped strain-induced martensites in the austenite structure of Fe alloys. Mössbauer spectroscopy and transmission electron microscopy techniques were used to clarify certain properties of strain-induced martensite in an Fe-29.6% Ni alloy. The reverse transformation mechanism between thermal plate martensite and the matrix austenite was also studied. Mössbauer spectroscopy made it possible to examine the same area of the austenitic thin foils during the thermal cycles, and the volume fraction changes were determined. The habit plane and orientation relationship of strain-induced martensite were measured from the electron diffraction patterns and the latter parameter was found to be K-S type as with thermal plate martensites of the Fe-Ni alloys. The isomery shifts caused by the deformation and cycling procedures were also calculated for both austenite and martensite structures and the hyperfine magnetic field parameter of Fe-29.6% Ni strain-induced martensite was found to be equal to that of Fe-Ni-C alloys reported earlier.     

An investigation of structural transformation during the solidification of copper-gallium alloys

Metallographic examination, X-ray diffraction, X-ray spectral microanalysis, and mlcrohardness measurements were used to study structural changes during the solidification of copper-gallium alloys. It was shown that the solidification of these alloys in the temperature interval 40–200°C takes place as a result of the formation of the -phase. Long-term holding at 200 and 300°C leads to the formation of ₃-, ₂-, ₁-, and-phases.     

Delayed retardation phenomena of fatigue crack growth in various steels and alloys

The delayed retardation phenomena of fatigue crack growth resulting from a single application of overload were investigated for five steels, two aluminium alloys and a titanium alloy. As long as the small scale yielding condition was satisfied at the overloaded crack tips, the retardation behaviour of these materials was expressed consistently by four parameters; the peak/baseline stress ratio, r, the exponent in the Paris equation, m, the overload-affected zone size, _D, and the crack distance at the minimum rate of crack growth, _B. Then the parameters, _B and _D, characterizing the retardation phenomena for these materials were determined. The retardation of aluminium alloys was stronger than that of the other materials. This is attributed to the lower value of _B/ _D in aluminium alloys than in the other materials. In the case of r=2, the overload-affected zone sizes, _D, were nearly equal to 1.5 ₀ in HT80 steel and aluminium alloys, slightly lower than 1.5 ₀ in SNCM8 steel and much larger than 1.5 ₀ in A553 steel and the titanium alloy, where ₀ is the monotonic plastic zone size calculated according to the Dugdale model. The dependence of retardation on baseline stress intensity, K ₁, appeared somewhat complicated. In the cases of A553 steel and A2017 aluminium alloy the amount of retardation increased with increasing K ₁ value, while in the cases of HT80 steel and Ti-6A1-4V titanium alloy the tendency appeared in the reverse direction. The former behaviour was related to the change in the stress state from plane strain to plane stress at the overloaded crack tips and the latter was related to the threshold of stress intensity.