An electron microscopic and diffraction study of Cu-9 wt pct Al alloy

An electron microscopic investigation of Cu-9 wt pct Al alloy low thermal treated at 250°C for 30 min was performed in the deformed and non-deformed condition. For this composition and ordering treatment the alloy exhibits the higher increase in strength. It was found that the images formed in the non-deformed alloy, reveal the presence of ordered domains of an average size of 80?, being the amount of order dependent of whether or not quenched-in vacancies are present. It is probable that the domains nucleate preferentially at stacking faults in the deformed condition. A periodic antiphase structure was determined from computations and comparison with electron diffraction data. The superlattice cell is based on the LI₂ type, tetragonal face centered with a period approximately three times the lattice parameter of the matrix, having three variants of orientation within an ordered region.     

Dislocation structures in disordered and ordered Ni3Fe

Transmission electron microscopy (TEM) methods have been used to examine the dislocation structures in thin foils of Ni₃Fe in four different states, corresponding to disordered and deformed; fully ordered and deformed; deformed when disordered and afterwards fully ordered; and deformed when disordered, afterwards fully ordered and additionally deformed. The study has been carried out on single crystals deformed at room temperature. In the disordered alloy slip is coarse and group motion of dislocations is prevailing, as is confirmed by the abundance of planar dislocation arrays. The dislocation structure of such a disordered deformed crystal remains unchanged after additional ordering by annealing 1000 h at 460°C. No rearrangement of the unit dislocations into superlattice dislocations is observed. The dislocations are preserved and since they are unit dislocations they are sessile. The additional deformation of this disordered deformed and afterwards fully ordered crystal proceeds by the glide of superlattice dislocations. They originate in the high internal stresses of the preserved unit dislocations. Cross slip from (111) onto (11̄1) planes is very frequent, whereas cross slip from (111) onto (010) planes is rather rare. The structure of the superlattice dislocations in fully ordered and deformed specimens consists of dipoles and bundles of dipoles of near edge orientation. Superlattice dislocations of near screw orientation are rarely observed, since they cross slip from (111) onto (11̄1) planes and annihilate in most cases. The experimental results on ordered Ni₃Fe samples differ characteristically from those reported in the literature on other alloys having the L1₂ long range ordered structure (e.g. Ni₃Al, Cu₃Au, Ni₃Ga).     

Some Effects of Parent Phase Aging on the Martensitic Transformation in a Cu- AI- Ni Shape Memory Alloy

Transmission electron microscopy observations have been carried out for a Cu-14 pct Al-4 pct Ni (wt pct) alloy aged in the thin foil state in an electron microscope. It was found that large cuboidal precipitates of theγ ₂ phase and many small domains of a highly ordered phase form in the DO₃ matrix during aging. The small ordered domains form preferentially on matrix antiphase boundaries as well as within the antiphase domains. The formation ofγ ₂ and the highly ordered phase, both of which are rich in alloy content, depletes the matrix of solute and thus raises the transformation temperaturesM _s andM _f. The small domains of the highly ordered phase prevent the propagation and reversion of martensite plates, leading to higherM _s-M_f andA _fins-A_f temperature intervals.     

Local Composition Migration Induced Microstructural Evolution and Mechanical Properties of Non-equiatomic Fe40Cr25Ni15 Al15Co5 Medium-Entropy Alloy

A newly designed composition of non-equiatomic Fe₄₀Cr₂₅Ni₁₅Al₁₅Co₅ medium-entropy alloy (MEA) was produced by induction melting (IM). The as-cast alloy was found to consist of a two-phase microstructure of BCC (2.87 ± 0.01 Å) and ordered B2 (2.88 ± 0.02 Å) type phases. The structures of these phases were confirmed through X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. It was observed that the Ni-Al-enriched ordered B2 phase of cuboidal shapes (~ 100 to 200 nm) is homogeneously distributed in Fe-Cr-rich BCC matrix with a cube-on-cube orientation relationship. The formation of the columnar dendrites (width 50 to 100 μm) was identified through optical microscopy (OM). The structural and microstructural stability of the alloy was investigated by heat-treating the alloy through different schedules. Heat-treated samples at different temperatures (< 1273 K) exhibit a similar type of two-phase microstructure with columnar dendrites. However, compositional rearrangement takes place during long time exposure to develop polymorphically related phases. The alloy was observed to possess a high compressive yield strength and hardness, i.e., ~ 1047 MPa and 391 ± 9 HV, respectively, at room temperature. Heat-treated samples at 600 °C and 900 °C (873 K and 1173 K) showed an increase in yield strength and ultimate strength with a significant increase in plasticity due to the increase in volume fraction of B2 phase and softening of the BCC matrix phase. The thermal stability and high strength of this alloy may open new avenues for high-temperature applications.

     

High resolution electron microscopic study of the ordering processes in Ni4Mo alloy

We have studied the ordering processes of a Ni-20.1at.%Mo alloy using the many-beam imaging technique with the aid of an image processing method. No microdomain is detected in high resolution electron micrographs of the alloy quenched from 1000°C, but lattice modulations which probably be attributed to concentration waves appear parallel to the {210} planes at the initial ordering stage. It is revealed that the observed contrasts of D1_a-type and D0₂₂-type microdomains are apparently formed with superpositions of different pairs of the {210} lattice modulations. When the alloy is annealed at 650°C for 192 h, Ni₂Mo domains appear in between the Ni₄Mo domains, and their composition is determined to be the stoichiometry (32at.%Mo) by the energy dispersive X-ray (EDX) analysis. The superstructure images of Ni₄Mo and Ni₂Mo are discussed in comparison with computer simulations by the multislice method.     

Precipitation-hardening in Cu-3.6 wt PCT Ti

The room temperature tensile properties of age-hardened Cu-3.6 wt pct Ti were investigated. The resulting structures of both single- and double-aged samples were examined by transmission electron microscopy. Emphasis was placed on the correlation of structure with mechanical properties. Isothermal aging at 400°C resulted in an increase in yield strength from that of 40 ksi in the solution-treated alloy to a maximum of 98 ksi after aging for 10,000 min (167 hr). The structure associated with maximum strength was that of an aligned disordered cubic coherent precipitate. Examination of deformed specimens showed that slip was heterogeneous both within a grain and along a given shear band. Aging at 550°C resulted in the same aligned cubic coherent precipitate as at 10 min, but continued-isothermal holding caused the growth of a weaker recrystallized structure and thus a decrease in strength. It was concluded that overaging in this system was due entirely to the consumption of the continuous precipitate by the discontinuous. This recrystallized constituent was found to be composed of a copper-rich solid solution and Cu3Ti arranged in alternate parallel lamellae. The hcp Cu₃Ti precipitate had its close-packed plane parallel to the {Ill} plane of the matrix, and edge dislocations were found to take up the misfit between these two structures. A series of single- and multiple-aging treatments gave three completely recrystallized structures with approximately the same yield strength. However, the total elongation of these structures decreased as the final aging temperature was reduced. This was attributed to an ordering reaction in the Cu₃Ti plates at lower temperatures. Direct evidence of a change in deformation characteristics was obtained by examining specimens deformed to the same extent, but having different degrees of order induced by varying the final aging temperature. The disordered plates were shown to plastically deform along with the matrix, but the ordered plates fractured. Formerly Graduate Student, Department of Metallurgy and Materials Sciences, New York University, Bronx, N. Y. This paper is based upon a thesis submitted by H. T. MICHELS in partial fulfillment of the requirements of the degree of Doctor of Philosophy at New York University.     

Laser surface alloying of nickel by molybdenum and aluminium— microstructural studies

It is now well established that a considerable improvement in the mechanical and chemical properties of the near surface regions of materials may be achieved by the use of high energy laser beams. By manipulating the laser power density and the time of interaction of the laser beam with an appropriately coated work piece, it is possible to achieve a surface chemistry that would have an improved resistance to wear, fatigue and corrosion failures. The change in chemistry at the surface is attained through the process of melting and mixing of the coating and a thin layer of the substrate. Solidification of this molten region at the surface results, due to an interplay of various forces, in the development of very complex microstructures. To analyse these in a piece of nickel that had been coated with a mixture of molybdenum and aluminium powders and then treated with a continuous wave CO₂ laser, extensive transmission electron microscopy was done on a thin foil obtained from near the bottom region of the recast pool. The foil was found to have a cellular microstructure comprising cells of the Ni₃Al (γ′) ordered phase. In the intercellular regions, two dispersed phases, namely, martensitic Ll₀ phase and Ni₂Al₃, and a contiguous phase, identified as a close derivative of the equilibrium δ-NiMo phase were found. A gradient in the degree of order could be observed within the γ′ cells. In the central portion of the cells, rapid solidification resulted, by the process of sequential ordering, in the development of a partially ordered alloy with very fine domains. In the peripheral regions of the cells, the alloy solidified by direct ordering into larger domains. This paper discusses some of these results.     

The effect of ordering on the hydrogen embrittlement susceptibility of Ni2Cr

Isothermal annealing at 500 °C for various lengths of time after rapid quenching from 900 °C results in different degrees of ordering in Ni₂Cr. Tensile specimens of disordered and ordered Ni₂Cr were subjected to hydrogen embrittling and nonembrittling environments before and during tensile testing. The hydrogen embrittlement susceptibility was determined by reduction-in-area losses (RA loss) after failure. The results of the tensile tests indicated that the disordered alloy and highly ordered alloy were the most susceptible to hydrogen embrittlement. The RA loss in each case was approximately 70 pct. The tests revealed a minimum susceptibility to hydrogen embrittlement between 40 and 50 pct order. The fracture surfaces of the specimens, as examined by scanning electron microscopy, showed that the extent of embrittlement is correlated with the amount of intergranular failure. The mechanisms for failure in Ni₂Cr appear to depend upon the extent of aging in the alloy.     

Phase transition in an Fe-23.2Al-4.1Ni alloy

The phase transition in an Fe-23.2 at. pct Al-4.1 at. pct Ni alloy has been investigated by means of transmission electron microscopy. In the as-quenched condition, the microstructure of the alloy is a mixture of (A2 + B2) phases. When the as-quenched alloy is aged at temperatures ranging from 500 °C to 1050 °C, the phase transition sequence is found to be (A2 + B2∗) → (B2 + B2∗) → B2 → A2, where B2∗ is also a B2-type phase. It is worthwhile to note that the coexistence of two kinds of ordered B2 phase has not previously been observed by other workers in the Fe-Al-Ni ternary alloy system.     

High resolution electron microscopy of the early decomposition stage of AlLi alloys

The early decomposition stages of Al-7.8at.%Li, Al-10.4at.%Li and Al-11.8at.%Li alloys were studied by high resolution electron microscopy. In all the above alloys, it was confirmed that in the as-quenched stage small ordered domains of Ll₂ structure were present surrounded by the disordered matrix. From this microstructure, it was concluded that the alloy was already decomposed as in the as-quenched stage. No clear evidence was found for congruent ordering which was proposed to occur prior to spinodal decomposition. Examination of the sublattices of the individual ordered domains also failed to give convincing evidence for congruent ordering. The present observations suggest that the kinetics of spinodal decomposition of this system is too fast to be detected by TEM. In order to slow down the decomposition kinetics, solution treated samples were quenched to just above the solvus line for δ′ and then to room temperature. By this heat treatment, a solution treated sample with muck weaker order spots could be obtained. However, this microstructure could not be interpreted as congruently ordered either based on the weak Ll₂ spots. As there is a limitation to the quenching speed, it is suggested that ordering and spinodal decomposition progress concomitantly during the observable range of time.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensiticR transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain=0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.     

Study of the Ni<Subscript>41.3</Subscript>Ti<Subscript>38.7</Subscript>Nb<Subscript>20</Subscript> wide transformation hysteresis shape-memory alloy

The shape-memory characteristics in the Ni_41.3Ti_38.7Nb₂₀ alloy have been investigated by means of cryogenic tensile tests and differential scanning calorimetry measurement. The martensite start temperature M _s could be adjusted to around the liquid nitrogen temperature by controlling the cooling condition. The reverse transformation start temperature A′ _s rose to about 70 °C after the specimens were deformed to 16 pct at different temperatures, where the initial states of the specimens were pure austenite phase, martensite phase, or duplex phase. The shape-memory effect and the reverse transformation temperatures were studied on the specimens deformed at (M _s +30 °C). It was found that once the specimens deformed to 16 pct, a transformation hysteresis width around 200 °C could be attained and the shape recovery ratio could remain at about 50 pct. The Ni_41.3Ti_38.7Nb₂₀ alloy is a promising candidate for the cryogenic engineering applications around the liquid nitrogen temperature. The experimental results also indicated that the transformation temperature interval of the stress-induced martensite is smaller by about one order of magnitude than that of the thermal-induced martensite.     

Plastic straining effects on the microstructure of a Ti-rich NiTi shape memory alloy

A Ni-52 at. pct Ti shape memory alloy, cold drawn to 30 pct, was annealed at 1173 K for 1 hour, water quenched, and then subjected to differential scanning calorimetry (DSC). No evidence of the premartensitic R transformation was found during either the forward or the reverse transformation. Microstructurally, it was found that the alloy possessed a relatively large volume fraction (∼0.05) of coarse second-phase brittle particles. These precipitates acted as preferential sites for martensite plate nucleation and gave rise to a “starlike” morphology. The tensile and compressive properties of the alloy in the as-received condition were also investigated. The alloy exhibited relatively good ductility (fracture strain = 0.28), which was attributed to its inherent ability to relieve or delay the development of plastic instabilities through rapid strain hardening. In addition, X-ray diffraction (XRD) of deformed specimens indicated the presence of an extraintensity peak corresponding to the B2 phase (110)_B2 when the alloy was plastically deformed in compression. Accordingly, it is suggested that plastic deformation induces the reverse transformation to the B2 phase in highly stressed local regions. Transmission electron microscopy (TEM) of deformed martensite structures showed slip lines probably due to dislocation slip, as well as variant interpenetration. Besides, optical and scanning microscopy of regions adjacent to the fractured surfaces indicated that fine martensite plates and/or “apparent” new grains develop at regions of prior stress intensification (former crack-tip regions) during crack propagation.     

Work hardening in the Cu3Au superlattice

Single crystals of a fully-ordered Cu₃Au alloy were deformed in compression at room temperature on a single glide system to a very high degree of plastic strain in Stage II. Transmission electron microscopy observations showed a very high density of superlattice screw dislocations which had cross-slipped onto {100} type planes. These cross-slipped superlattice dislocations were for the most part arranged as plus-minus pairs,i.e. multipoles. It is this large density of cross-slipped superlattice dislocations which is believed responsible for the high rate of work hardening observed in ordered Cu₃Au alloys. J. CZERNICHOW, formerly Research Assistant, Engineering Materials Group and Department of Mechanical Engineering, University of Maryland, College Park, Md.     

High resolution study of ordering reactions in gold-chromium alloys

The ordering reactions in Au₄Cr and Au₃Cr alloys have been investigated using high resolution dark field and lattice imaging techniques. In all cases the ordered structures can be described in terms of compositional modulations as occur during spinodal decomposition. In alloys quenched from above the critical temperature sro microdomains of size 10 to 15Å have been resolved with structures represented by modulations in composition along <420>. After aging the Au₄Cr alloy belowT _c the Dla lro phase appears as ordered domains of size 50 to 100Å showing composition modulations with wavelength of 5d<531>. These domains grow in size up to 500Å becoming finally the D1a fully ordered structure. A new observation has been mdde in the Au₃Cr alloy,viz that the lro structure is nonstoichiometricL1₀ withλ _[001] = 4Å. The transformation from sro in this alloy occurs with the formation of a transitional ordered phase, (similar toDO ₂₂, which can be described in terms of composition modulations along [001], with wavelength equal to thec parameter (8Å) of theDO ₂₂ structure. Lattice imaging of the two variants of superlattice planes in ordered Au₄Cr enabled the atomic arrangement in the area of translational and rotational antiphase boundaries to be determined.     

Combined Recrystallization and Precipitation in a Cu-9Ni-6Sn Alloy

An attempt has been made to investigate the structural changes occurring in a heavily cold-deformed Cu-9Ni-6Sn alloy consequent upon aging at two different temperatures of 350°C and 600°C. At the lower temperature of aging a fine continuous ordered precipitate forms first in the highly dislocated matrix, followed by combined discontinuous recrystallization and precipitation of the equilibrium (Cu, Ni)₃ Sn particles. Aging at the higher temperature seems to produce recrystallization of the deformed matrix first, followed by discontinuous precipitation. In this connection the role of deformation bands in nucleating the discontinuous reaction fronts has been discussed. The crystallographic texture of the fully recrystallized alloy has been found to be very much the same as that of the cold-deformed alloy and can be characterized as {110}«112» plus {110}«001». Aging beyond the primary recrystallization stage seems to alter the relative predominance of these texture components.     

High resolution electron microscopic studies of Ni-9.3 and 14.7 at.%Ti alloys

The early stages of decomposition in Ni-9.3 and 14.7 at.% Ti alloys are studied by means of high resolution electron microscopy. The microstructure of the as-quenched Ni-9.3 at.% Ti alloy shows contrast of broad circular regions, and local lattice images show some difference in lattice spacing suggesting compositional modulations. Electron and optical diffraction patterns indicate faintly ordered regions. After ageing for 15 min at 570°C, the ordered regions, approx. 50 Å in diameter, appear as clear lattice images surrounded by dark areas of the f.c.c. γ-solid solution. The Ni-14.7at.% Ti alloy aged for 15 min at 570°C shows ordered lattice images in wide areas of the matrix, apart from some large needle-like precipitates of the equilibrium η-phase formed during the homogenization at 1200°C. As the atomic arrangement in the ordered regions corresponds to the (110) projection of the L1₂ structure, the image is interpreted as the structure image of ordered L1₂, Ni₃(Ti_xNi_1−x). The lattice spacings locally change in the ordered regions, suggesting some modulation in composition and degree of order.     

Effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy

The effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy (alloy C-276) has been investigated by means of tensile tests in air and with hydrogen-charging in 1N-H₂SO₄ solution. The annealed specimen has exhibited intergranular fracture by hydrogen-charging, resulting in a marked reduction in tensile elongation and ultimate tensile strength. The mode of fracture was changed by aging at 773 K, and the transgranular fracture has been found to be dominant in the aged specimens. The susceptibility to hydrogen embrittlement, as identified by the test method used in this study, seems to be reduced by short-term aging, though it turns out to be increased again by further aging. The fractured boundaries have been characterized using electron channeling pattern (ECP) analysis of adjacent grains. It is found that the misorientation of grain boundaries plays an important role in fracture, and ∑3 boundaries, twin boundaries in a face-centered cubic (fcc) lattice, are most likely to fracture in the aged specimens. Transmission electron microscopy (TEM) observation has shown that a short-range ordering reaction from a disordered fcc lattice into an ordered Ni₂(Cr, Mo) (Pt₂Mo type) super-lattice takes place by aging, and hence, superdislocation triplets with APB (antiphase boundary) become predominant when deformed. It is also seen that in the aged specimens, deformation twinning is another mode of deformation, and this leads to the transgranular fracture at twin boundaries by hydrogen-charging. These results suggest that a change in the mode of deformation after aging plays a major role in fracture due to hydrogen embrittlement as a consequence of the heterogeneous interaction between slip dislocations and twin boundaries.     

Ultrafine-Grained Al-Mg-Sc Alloy via Friction-Stir Processing

Friction-stir processing (FSP) of twin-roll cast (TRC) Al-Mg-Sc alloy resulted into ultrafine-grained microstructure. The alloy was processed in as-received and aged (563 K [290 °C], 22 hours) conditions and at three different tool rotation rates: 800, 400, and 325 rpm. The microstructural features were characterized using electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The grain size varied from 0.89 μm to 0.39 μm depending on the processing and initial thermo-mechanical conditions of the alloy. The TRC alloy processed at 325 rpm in aged condition had all the grains less than 1 μm, and 95 pct of grains had high-angle grain boundaries (HAGBs). In all the cases, the fraction of HAGBs were more than 80 pct. The variation of misorientation angle distribution was similar to the theoretical MacKenzie distribution for cubic crystal materials. Grain size analysis at different sections and locations on the transverse section of the dynamically recrystallized zone showed a homogeneous and equiaxed microstructure. The average dispersoid (Al₃(Sc,Zr)) size was ~8.0 nm in diameter obtained using high-resolution TEM. Grain size reduction was observed with increase in Zener–Hollomon parameter. It was shown that under the current microstructural and deformation conditions, dynamic recrystallization via particle-stimulated nucleation might not be possible during FSP.     

Phase transformations in Ti2Ni3 precipitates

The morphology and crystallography of transformation products of Ti₂Ni₃ in an aged Ti-52.0 at. pct Ni alloy have been studied by means of optical and electron microscopy and electron diffraction. Band-shaped surface relief was observed in optical micrographs at room temperature. Needle and antiphase-like domains were observed in electron micrographs at room temperature. The former correspond to the band-shaped relief seen in optical micrographs. The crystal structure of Ti₂Ni₃ at room temperature was confirmed to be monoclinic by electron diffraction. The needle-like domains and matrix are twin-related with respect to the {120} plane of the monoclinic structure. Upon heating, first the needle-like domains (low temperature phase) shrank and disappeared at about 50 °C, and then the antiphase-like domains (intermediate phase) disappeared at about 100 °C. Upon subsequent cooling, antiphase-like domains appeared and then needle-like domains appeared. These transformations are reversible upon thermal cycling. The antiphase-like domains in the intermediate phase disappeared with the growth of the needle-like domains and new antiphase-like domains appeared within the needles. The sequence of transformation events in the Ti₂Ni₃ phase was deduced by electron microscopy and diffraction to be as follows: parent phase (tetragonal) → intermediate phase characterized by antiphase-like domain morphology (orthorhombic) → low temperature phase characterized by needle-like domain morphology (monoclinic). In spite of some differences in crystallographic and morphological changes, the above phenomena are much like theR-phase transition associated with CDW’s in TiNiFe alloys. Formerly with the University of Illinois.