DOI: 10.1595/147106709X463318 Plastic Deformation of ... Plastic Deformation of Polycrystalline

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Transcript of DOI: 10.1595/147106709X463318 Plastic Deformation of ... Plastic Deformation of Polycrystalline

  • 138

    IntroductionIridium is the only refractory f.c.c. metal. Its

    melting point is 2446C, and as one of the plat-inum group metals (pgms), it exhibits excellentmechanical properties and high resistance to corro-sion at elevated temperatures (1, 2). Some featuresof this metal, such as its poor workability (inclina-tion to brittle fracture under mechanical treatment)and intrinsic fracture mode (brittle transcrystallinefracture) are unlike the deformation behaviourobserved in other f.c.c. metals and remain puzzlingfor the materials science community (38).Discussion of the possible mechanisms of defor-mation and fracture in iridium was begun inPlatinum Metals Review more than fifty years ago(9, 10), and continues up to the present. Becauserefining iridium is a very complicated procedure,segregation of impurities at the grain boundaries isconsidered the most likely cause of the weak

    cohesive strength of grain boundaries and, hence,the poor workability of this metal (7, 8, 10, 11, 15).Indeed, it has been shown that high purity iridium can be processed like platinum (16).Polycrystalline iridium free of non-metallic impuri-ties, and its ternary alloy with rhenium andruthenium, demonstrate both transgranular cleav-age and satisfactory plasticity. Traces (~ 10 ppm)of carbon and oxygen in the matrix induce theappearance of intergranular cleavage on the fracture surfaces and, as a result, workability is cat-astrophically diminished (7, 11). The extraction ofdetrimental non-metallic impurities continues tobe a significant problem for pgm refiners (7, 8, 15).

    Some physical parameters of iridium, such asits strong interatomic bonds and high elastic mod-ulus, give a basis for supposing that brittlenessmay be an intrinsic property of this metal (5, 6,2023). For example, formal substitution of the

    Platinum Metals Rev., 2009, 53, (3), 138146

    Plastic Deformation of PolycrystallineIridium at Room TemperatureUNIQUE PROPERTIES OF IRIDIUM, THE SOLE REFRACTORY FACE CENTRED CUBIC METAL

    By Peter Panfilov* and Alexander YermakovLaboratory of Strength, Ural State University, 620083 Ekaterinburg, Russia; *E-mail: peter.panfilov@usu.ru

    and Olga V. Antonova and Vitalii P. PilyuginInstitute of Metal Physics, Ural Division of the Russian Academy of Sciences, 620219 Ekaterinburg, Russia

    Defect structure and its relationship with deformation behaviour at room temperature ofiridium, the sole refractory face centred cubic (f.c.c.) metal, are discussed. Small angleboundaries and pile-ups of curvilinear dislocation segments are the main features of dislocationstructure in polycrystalline iridium at room temperature, while homogeneously distributedrectilinear dislocation segments were the main element of defect structure of iridium singlecrystals at the same conditions. Small angle boundaries and pile-ups of curvilinear dislocationsegments are formed in iridium single crystals under mechanical treatment at elevatedtemperatures ( 800C) only. The evolution of defect structure in polycrystalline iridium andother f.c.c. metals under room temperature deformation occurs by the same process:accumulation of dislocations in the matrix leads to the appearance of both new sub-grainsand new grains up to the fine grain (nanocrystalline) structure. Neither single straightdislocations nor their pile-ups are observed in iridium at room temperature if small angleboundaries have been formed. This feature may be considered as the reason why polycrystallineiridium demonstrates advanced necking (high localised plasticity) and small total elongation.

    DOI: 10.1595/147106709X463318

  • elastic modulus into cleavage criteria equationsleads to the conclusion that iridium is expected tobehave like a brittle substance (6, 20, 24). On theother hand, according to empirical theories ofplastic deformation, it should be deformed likeother f.c.c. metals (13, 20, 25). It is a paradox, butthe highest yield stress and strength of iridium sin-gle crystals under tension (~ 100 MPa and ~ 500MPa, respectively) become similar to those ofother f.c.c. metals when these parameters havebeen normalised on the elastic modulus (20). Thismay also be applied to the concept of dislocationmobility in iridium, as understood by dislocationtheory in the field of continuum mechanics, whichdescribes the motion of single dislocations underexternal stress (normalised on the elastic modu-lus). This apparent contradiction with empiricalknowledge of the deformation and fracture off.c.c. metals merits further discussion among thematerials science community.

    Behaviour of Iridium SingleCrystals

    Experiments have shown that the intrinsicfracture mode of iridium single crystals is brittletransgranular cleavage. However, monocrystallineiridium is also a highly plastic material, as it frac-tures after considerable elongation (up to 80%) atroom temperature and never fails under compres-sion (19, 20, 26, 27). Hence, the brittleness ofiridium in the monocrystalline state is a uniquekind of brittle fracture, since cleavage occurs afterhuge plasticity. Therefore, single crystal iridiummay be classified as a plastic but cleavable solid(28). Study of the geometry of deformation trackson the back surfaces of deformed iridium singlecrystals has shown that octahedral slip is the soledeformation mechanism active at room tempera-ture. In contrast to other f.c.c. metals, no neckingis observed in iridium single crystal samples undertension, in spite of considerable elongation priorto failure. Analysis of the deformation track distri-bution leads to the conclusion that all plasticity ofiridium single crystals is observed during the earlystages of plastic deformation (29).

    Studies by transmission electron microscopy(TEM) have shown that rectilinear dislocation

    segments laid along the crystallographicdirection are the main element of the defect struc-ture in iridium single crystals at room temperature(11, 26, 29). These straight dislocations aregrouped into network-like arrangements, wherethey are distributed almost equidistantly or homo-geneously. The homogeneous distribution ofdislocations takes place in thin foils of f.c.c. met-als in the early stages of plastic deformation, whendislocation density is expected to be small (30).However, the density of dislocations in iridiumsingle crystals may be so high that the foil is nolonger transparent to the electron beam. No smallangle boundaries or other types of inhomoge-neously distributed dislocations are observed insingle crystal iridium deformed at room tempera-ture. Therefore, it may be concluded that thedeformation mechanism of iridium single crystalsat room temperature is octahedral slip of the per-fect dislocation with a Burgers vector (28,29). This observation distinguishes iridium singlecrystals from f.c.c. metals with low and intermedi-ate melting points.

    The evolution of dislocation structure in iridium single crystals stops at the stage of homogeneous distribution of dislocations in thecrystal, which is the first stage of plastic deformation in metallic single crystals (3032). It should be noted that motion of dislocationsnever occurs in thin foils of iridium during room temperature TEM experiments, including in situtension in the column of the microscope. Thesefeatures, together with the high yield stress of iridium single crystals, lead to the conclusion that dislocations have low ability to moveunder external stress in the refractory f.c.c. metalat room temperature. As a result, the dominantdislocation arrangements in iridium single crystalscannot transform into small angle boundariesunder external stress at room temperature. Such behaviour contrasts with f.c.c. metals having lowand intermediate melting points, where thisprocess can occur at room temperature. It may beconsidered as an additional argument for the statement that all plasticity of iridium single crystals is realised during the early stages of plasticdeformation.

    Platinum Metals Rev., 2009, 53, (3) 139

  • High Purity Polycrystalline Iridium High purity polycrystalline iridium (purity

    > 99.9%, including < 0.1% metallic impuritiesand no non-metallic impurities) and iridium alloysalso exhibit brittle transcrystalline fracture as anintrinsic fracture mode under tension, but thetotal elongation of samples having a circularcross-section is less than 10% at room tempera-ture (1719). This does not mean that iridiumwires possess poor plasticity, since advancednecking occurs in the samples even at room tem-perature, in spite of a considerable traverse rateof 1 mm min1. The homologous temperature,THomologous, is defined as Texp (K)/Tmelt (K),where Texp = experimental temperature andTmelt = melting point. For low and intermediatehomologous temperatures (THomologous 0.5),localisation of plasticity induces a visible decreasein the total elongation of wires from 10% downto 5% at THomologous ~ 0.5 when necking to apoint takes place (17). Total elongation begins toincrease as soon as the flowing neck of a multi-neck effect appears at THomologous > 0.5. Thisbehaviour at similar traverse rates and homolo-gous temperatures is observed only in gold wiresand, hence, high purity polycrystalline iridiummay also be considered to be a plastic but cleavable substance.

    It is apparent that the deformation behaviourof polycrystalline iridium on the macroscopic scalehas many features in common with iridium singlecrystals and also many differences. For example,advanced necking in polycrystalline iridium wireand the homogeneous distribution of plasticdeformation in iridium single crystals under ten-sion both point to the high plasticity of iridium.However, this is due to different effects dependingon the microstructure of the material. Currently,no information on the defect structure of poly-crystalline iridium is available in the literature.Therefore, the aim of this work is to carry out aT