The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

The effect of high temperature plastic flow on the superconducting transition in A15 compounds

Low temperature heat capacity measurements have been made on hot isostatically pressed Nb₃Sn and arc cast, homogenized, and A15 transformation annealed V-23.3 at. pct Ga and V-18.1 at. pct Ga. The heat capacity measurements display the superconducting transition temperature (T _c), and have been used to evaluate the effects of plastic deformation on theT _c range. Plastic deformation to the extent of 15 pct compression at 1525 and 1650 °C does not alter the superconducting transition in polycrystalline Nb₃Sn. Plastic deformation to the extent of 15 pct compression at 1050 and 1200 °C does not alter the superconducting transition in polycrystalline V-23.3Ga, transformed to A15 structure with a fifty-hour 1150 °C anneal. The same statement can be made for A15 V-18.1Ga, deformed at 1050 °C after a fifty-hour, 1150 °C anneal. With onlyfive hours annealing at 1150 °C, A15 V-23.2Ga displays a lowering and broadening of theT _c range upon 1050 and 1200 °C plastic deformation. With onlyfive hours annealing at 1150 °C, A15 V-18.1Ga displays a broad, but increasedT _c range after plastic deformation at 1050 °C.     

Structure and deformation behavior ofT 1 precipitate plates in an Al- 2Li- 1 Cu alloy

A high-resolution TEM study was performed in order to determine the structure, transformation mechanisms, and deformation behavior ofT ₁ precipitate plates in an Al-2Li-lCu alloy aged to peak strength. It is shown that a possible structure for theT ₁ plates may be an A₁BA₂C... stacking of close-packed planes with the A, planes mostly Al, the B and C planes containing a mixture of Cu and Al, and the A₂ planes mostly Li. Furthermore, the structural transformation necessary to achieve the A₁BA₂C ... stacking from the ABC ... matrix planes may be accomplished by the nu-cleation and propagation of a pair of Shockley partial dislocations on every third and fourth {111}_α matrix plane. After straining 3 pct, theT ₁ plates are sheared by dislocations, which cause a disruption in atomic order within the precipitates. Experimental evidence also indicates that the binding between Li and Cu in Al-Li-Cu alloys may be a major factor in determining microstructural evolution in this system, and comparison of diffraction data from hexagonal precipitates on {111}_α planes in a variety of Al alloys indicates that they may have similar structures. Formerly a Staff Scientist, ALCOA Laboratories.     

The role of plastic deformation on the competitive microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy

The role of plastic deformation prior to artificial aging on the microstructural evolution and mechanical properties of a novel Al–Li–Cu–X alloy designated AF/C 458 was investigated. Induced plastic deformation ranged from a non-stretched or 0% stretch condition to an 8% stretch, with intermediate stretches of 2%, 4% and 6%. Tensile properties, fractography and quantitative precipitate analysis were acquired from specimens that were water quenched from a solution heat treatment, immediately stretched and artificially aged at 150°C. Fractography was investigated through scanning electron microscopy (SEM). Quantitative transmission electron microscopy (TEM) determined the variation in precipitate type, number density, size and volume fraction of the major strengthening precipitates Al₂CuLi (T₁), Al₂Cu (θ″/θ^′) and Al₃Li (δ^′).Age hardening curves for each level of mechanical stretch illustrated the enhanced aging kinetics of plastically deformed material. Quantitative TEM indicated that increasing amounts of pre-age stretch were found to greatly affect the competitive precipitation kinetics of T₁ and θ″/θ^′ in AF/C 458 augmenting the volume fraction of fine matrix T₁ plates and dramatically decreasing the volume fraction of θ″/θ^′ for isochronal treatments. A quantitative microstructural comparison of specimens exhibiting a given strength demonstrated that the imposed level of cold work dictated the density, size and volume fraction of the competing precipitates. The tensile data indicated a trend of increasing ductility for equivalent yield strengths with the increasing amount of pre-age mechanical stretch and therefore shorter artificial aging times. The quantitative precipitate data were used with a computer simulation for yield strength determination. The theoretical simulation reported calculated yield strengths in good accord with experimental results and can thus be used to predict the optimum microstructural configuration for high strength.     

Effect of hot-rolling reduction on shape of sulfide inclusions and fracture toughness of AISI 4340 ultrahigh strength steel

Commercial AISI 4340 ultrahigh strength steels with hot-rolling reductions of 80 to 98 pct have been studied to determine the effect of the shape of sulfide inclusions on plane-strain fracture toughness(K _IC ) of the ultrahigh strength low alloy steels. The significant conclusions are as follows: decreasing the hot-rolling reduction from 98 to 80 pct for the steels modified the shape of sulfide inclusions from the stringer (average aspect ratio = 17.5) to the ellipse (average aspect ratio = 3.8). This improved theK _IC in the longitudinal testing orientation by about 20 MPa · m^1/2 at similar strength levels. This could be due to the fact that the ellipsed sulfide-inclusions separate from the matrix during plastic deformation, producing large voids. During testing these act to blunt and arrest cracks propagating across the specimen which would normally cause failure. The decrease in the hot-rolling reduction also developed theK _IC in the transverse testing orientation by about 17 MPa · m^1/2 at increased ductility and Charpy impact energy levels. This can be attributed to the fact that lamellate fracture, which occurs in a brittle manner along the interfaces of the sulfide-inclusion/matrix at the crack tip, is considerably suppressed by modifying the shape of the inclusions from the stringer to the ellipse.     

The aging characteristics of an AI-2 Pct Li-3 Pct Cu-0.12 Pct Zr alloy at 190 °C

The aging characteristics of an Al-2 pct Li-3 pct Cu-0.12 pct Zr alloy at 190 °C have been examined using transmission electron microscopy. Specifically, the development of intragranular microstructures, and the interactions among the various precipitate species have been documented. Theδ′ phase (Al₃Li) is shown to be present, not only as small spherical particles, but also as a shell aroundβ′ (Al₃Zr) and as a film on the broad coherent faces of the θ′ (Al₂Cu). At peakage (18 hours), theδ′ is associated predominantly with theθ′. However, overaging leads to dissolution of theθ′ and a concomitant increase in the volume fraction of theT ₁. This latter phase can cut/grow through the δ’ and plays an increasing role in determining theδ′ distribution after prolonged aging. Finally, T₂ is found to precipitate within the matrix in the overaged condition. TheT ₂ precipitates display icosahedral symmetry. No evidence for the presence of theδ phase has been obtained.     

Discussion of “effects of tensile stress on microstructural change of eutectoid Zn-AI alloy”

In a recent contribution,[1] Zhu and Orozco presented a phase transformation of the ternary alloy Zn-20.2 wt pct Al-1.8 wt pct Cu, studied under tensile stress by using X-ray diffraction and scanning electron microscopy techniques. The authors report the existence of three phases in the alloy at room temperature after furnace cooling,α,ε, and a newη _′ ^T instead of the zinc-rich solid solutionη, as appears in the phase diagrams. The reported parameters for this hcp metastable phase are^[1,2] a = 0.2663 andc = 0.4873 nm; these values are close to the parameters of pure zinc,^[3] witha = 0.2664 nm andc = 0.4946 nm. The difference betweenη _′ ^T and zinc in thea parameter is around 0.03 pct, and it is 1.47 pet for thec parameter. When zinc is saturated with aluminum in the Zn-AI alloys, thea parameter shrinks^[3] to 0.2660 nm. It is possible to see that the value ofa of theη _′ ^T phase lies in-between the values of pure zinc and zinc-aluminum solid solution. The solubility of Al and Cu in Zn^[4] at 100 °C is 0.3 wt pct Cu and 0.06 wt pct Al. The covalent radius of Cu (0.117 nm) is smaller than the covalent radius of Al (0.118 nm) and Zn (0.125 nm), so the introduction of Cu in the zinc structure can result in a reduction of thec parameter. These values suggest that the metastable phaseη _′ ^T could be the hcp zincrich solid solution with low aluminum and copper contents. The article of Zhu and Goodwin,^[5] cited by Zhu and Orozco in their Reference 14, is related not to the eutectoid alloy, as they argue, but to an alloy with 27 wt pct Al, and no reports about the transformation ofε intoT′ were found. The presence of the metastable e phase (CuZn₄, sometimes called CuZn₅) at room temperature and its transformation to the stable phaseT′ (rhombohedral intermetallic phase, Al₄Cu₃Zn) have been observed by other authors.^[6,7] Y.H. ZHU and E. OROZCO:Metall Mater. Trans. A, 1995, vol. 26A, pp. 2611-15.     

The dependence of the fracture toughness of mi steel on temperature and crack velocity

The dependence of the dynamic plane-strain fracture toughness,K _Id, on temperature and crack velocity was measured for propagating cracks in 1020 steel. The dynamics of crack propagation in double-cantilevered specimens was recorded using electroresistivity techniques. The fracture surface energy was found by comparing the crack propagation to solutions of crack motion in wedged-open cantilevered specimens. The_KId behavior was investigated over a range of temperatures from —196° to —50°C and crack velocities of 3 × 10^-3 to 5 × 10^-2 of √E/p. The rate and temperature dependence ofK _Id over the range ofT and υ_c investigated is well described by:1/K _ld ²= υ₀ are experimental constants. A dynamic value ofK _Id was 70 pct ofK _Ic at the same temperature, although in the temperature and crack velocity range investigated the specific fracture surface energy varies by a factor of 6. The temperatureT _T =B/A in(υ _o/υ_c) for which1/K _Id ² = 0 is similar to Charpy impact transition temperature values whenυ _c = 3 × 10^-3√.E/p. If the plane-strain stress condition could be maintained, thenT _T would define a brittle-ductile transition temperature for dynamic plane-strain fracture toughness. The constantsA andB are interpreted by understanding the plastic energy dissipated by a moving crack. Formerly with Brown University, Providence, R. I.     

The dependence of the lower yield strength in iron and steel on grain size and temperature

Experimental results obtained from the literature on the dependence of the lower yield strength on grain size and temperature in α-Fe at temperatures between 90° and 373°K are shown to substantiate the validity of the relation, σ _aF = σ _i +V _c/NθKl ²)^1/m . Values ofm, the dislocation velocity-stress exponent, and its temperature dependence show reasonable agreement with results obtained from stress relaxation, double strain-rate cyclic and premacroyield tests in polycrystalline iron and Ti-stabilized iron after homogeneous and inhomogeneous deformation. An apparent activation enthalpy of 0.73 ev was determined and an enthalpy of 0.86 ev was computed which agree well with values reported for homogeneous deformation in α-Fe. The results obtained indicate that the dislocation mechanisms controlling inhomogeneous and homogeneous deformation in α-Fe at low temperatures are the same. As temperature is lowered, the marked increase in lower yield strength in α-Fe is related to a strong decrease in θ, the average dislocation velocity at unit effective stress.     

Deformation mechanisms in commercial TI-5Al-2.5 Sn (0.5 At. pct Oeq) alloy at intermediate and high temperatures (0.3-0.6 tm)

The plastic flow of the commercial α-alloy Ti 5A1-2.5 Sn (0.5 at. pct Oeq) of 11 to 19 μm grain size was investigated in tension over the temperature range of 600 to 117.3 K (CPH structure) and strain rates of 3x10^-5 to 3 per s employing both constant strain rate and strain rate cycling tests. Dynamic strain aging (due to substitutional solutes) occurred in the temperature range of 600 to 850 K (0.31 to 0.44T _m)with an activation energy of 22.5 KcalJmole (94.1 KjJmol) derived from the start of serrations in the stress-strain curves. At temperatures above 850 K (0.46 to 0.60T _m), the deformation could be described by Dorn’s general equation for diffusion controlled mechanisms with an activation energy of 50 KcalJmole (209 KJJmol). M. D?NER was formerly with Department of Metallurgical Engineering and Materials Science, University of Kentucky, Lexington, Ky. 40506     

Microstructure,tensile deformation,and fracture in aged ti 10V-2Fe-3Al

In the /3-Ti alloy Ti-10V-2Fe-3Al a variety ofα-andω-aged microstructures with different yield stresses was established by combinations of forging and heat treatment. Tensile tests have shown that plastic deformation and fracture are strongly influenced by the morphology, size, and volume fraction of the different types of a-phase (primary a, secondaryα, grain boundaryα), as well as by the-phase. A detailed microscopical study revealed several deformation and fracture modes. It appears that at several sites stress and strain concentrations and subsequent void nucleation can occur and that the quantitative combinations of the differentα-types determine which sites are active. The dominant deformation mode for the (α +gb) solution treated andα-aged conditions was a strain localization in theα-aged matrix leading to voids at the interface between aged matrix and primary a-phase. In case of theβ-solution treated andα-aged microstructures the grain boundaryα leads to a strain localization in the softα-film and to void nucleation at grain boundary triple points at low macroscopic strains. Based on the above mechanisms it is discussed in detail how varying size, volume fraction, and morphology of theα-phase affect the ductility. The embrittling effect ofω-particles can be largely reduced by a grain refinement.     

Effect of hydrogen on the low-temperature creep of a submicrocrystalline Ti-6Al-4V alloy

The effect of alloying with 0.002–0.24 wt % H on the creep of the Ti-6Al-4V alloy at a temperature of ∼0.15T _m (T _m is the melting temperature) is studied. The formation of a submicrocrystalline structure in the alloy is found to increase the stress-rupture strength and the hydrogen embrittlement resistance. Possible causes of the increase in the deformation localization resistance of the alloy in the presence of hydrogen in a solid solution are discussed.     

Elevated temperature fracture of RS/PM alloy 8009: part i. fracture mechanics behavior

Increasing temperature and decreasing loading rate degrade the plane strain initiation (K _ICifrom theJ integral) and growth (tearing modulus,T _R) fracture toughnesses of RS/PM 8009 (Al-8.5Fe-1.3V-1.7Si, wt pct).K _ICidecreases with increasing temperature from 25 ‡C to 175 ‡C (33 to 15 MPa√m for an extrusion and 28 to 11 MPa√m for hot cross-rolled plate) and further declines to 10 MPaVm at 316 ‡C without a minimum.T _Ris greater than zero at all temperatures and is minimized at 200 ‡C. A four order-of-magnitude decrease in loading rate, at 175 ‡C, results in a 2.5-fold decrease inK _ICiand a 5-fold reduction inT _R.K _ICiandT _Rare anisotropic for extruded 8009 but are isotropic for cross-rolled plate. Cross rolling does not improve the magnitude or adverse temperature dependence of toughness. Delamination occurs along oxide-decorated particle boundaries for extruded but not cross-rolled 8009. Delamination toughening plays no role in the temperature dependence ofK _ICi, however,T _Ris increased by this mechanism. Macroscopic work softening and flow localization do not occur for notch-root deformation; such uniaxial tensile phenomena may not be directly relevant to crack-tip fracture. Micromechanical modeling, employing temperature-dependent flow strength, modulus, and constrained fracture strain, reasonably predicts the temperature dependencies ofK _ICiandT _Rfor 8009. While E and σ_ys decrease with increasing temperature for all aluminum alloys, the strain to nucleate crack-tip damage dominates the fracture toughness of 8009 and decreases with increasing temperature for a range of constraint. Damage mechanisms for this novel behavior are evaluated in Part II. Formerly Graduate Student Department of Materials Science and Engineering, University of Virginia     

Precipitation-strengthened austenitic Fe−Mn−Ti alloys

The precipitation of intermetallic compounds in the Fe−20Mn−2Ti and Fe−28Mn−2Ti alloy systems has been investigated over the temperature range 700 to 900°C by hardness measurements, optical and scanning electron microscopy, and X-ray diffraction. In both systems only the equilibrium Laves phase was observed. The precipitate was identified as C14(MgZn₂) type hexagonal Laves phase with a chemical composition close to Fe₂ (Ti, Mn). In an as-annealed sample precipitation occurred in a heterogeneous manner, predominantly along grain boundaries. The effect of a cold deformation between the solution annealing and aging processes was also investigated. In addition to a high density of dislocations, martensitic phases were induced by deformation: a γ→∈ transformation occurred in the Fe−28Mn−2Ti alloy while a γ→α′ transformation was predominant in the Fe−20Mn−2Ti alloy. Subsequent aging was conducted at temperatures above theA _f . A large number of very fine precipitates formed randomly in the matrix after a short aging period. This cold work plus aging treatment resulted in an increase in yield strength. The enhancement of mechanical properties is due to the randomly distributed precipitates combined with the high defect density and fine substructure.     

Analysis of low-temperature intermetallic growth in copper-tin diffusion couples

A multiphase diffusion model was constructed and used to analyze the growth of the ε- and η-phase intermetallic layers at a plane Cu-Sn interface in a semi-infinite diffusion couple. Experimental measurements of intermetallic layer growth were used to compute the interdiffusivities in theε andη phases and the positions of the interfaces as a function of time. The results suggest that interdiffusion in the ε phase(≈D _ε) is well fit by an Arrhenius expression with D₀ = 5.48 × 10⁻⁹ m²/s andQ = 61.9 kJ/mole, while that in the η phase (≈D_η) has D₀ = 1.84 × 10⁻⁹ m²/s andQ = 53.9 kJ/mole. These values are in reasonable numerical agreement with previous results. The higher interdiffusivity in theη phase has the consequence that theη phase predominates in the intermetallic bilayer. However, the lower activation energy for interdiffusion in theη phase has the result that theε phase fills an increasing fraction of the intermetallic layer at higher temperature: at 20 °C, the predicted ε-phase thickness is ≈10 pct of that ofη, while at 200 °C, its thickness is 66 pct of that ofη. In the absence of a strong Kirkendall effect, the original Cu-Sn interface is located within theη-phase layer after diffusion. It lies near the midpoint of theη-phase layer at higher temperature (220 °C) and, hence, appears to shift toward the Sn side of the couple. The results are compared to experimental observations on intermetallic growth at solder-Cu interfaces.     

Rolling contact deformation of 1100 aluminum disks

The plastic deformation produced by pure, two dimensional, rolling contacts has been studied by subjecting 1100 aluminum disks to repeated contacts with well-defined relative peak contact pressures in the range 2 ≤P ₀/k _c ≤ 6.8. Two microstructural conditions are examined: as-received (warm worked) and annealed, displaying cyclic softening and cyclic hardening, respectively. Measurements of the distortion of wire markers imbedded in the rims, microhardness values of the plastically deformed layer, and changes in disk radius and width are reported. These are used to evaluate the plastic circumferential, radial, and axial displacements of the rim surface and the depth of the plastically deformed layer. These features are compared with the classical, elastic-quasi plastic analysis of rolling, and with recent elastic-plastic finite element calculations. The results show that the rim deformation state approaches plane strain when the disk width-to-Hertzian half contact width-ratioB/w ≥ 200. The presence of a solid lubricant has no detectable influence on the character of the plane strain deformation. The measurements of the per cycle forward (circumferential) displacements for the two conditions are self-consistent and agree with the finite element calculations when the resistance to plastic deformation is attributed to the instantaneous cyclic yield stress, but not when the resistance is identified with the initial monotonie yield stress. At the same time, the extent of the plastic zone is 5× greater than predicted by the analyses. These and other results can be rationalized by drawing on the special features of the resistance to cyclic deformation. They support the view that the deformation produced by theN ^th rolling contact is governed by the shape of the stress-strain hysteresis loop after the corresponding number of stress-strain cycles which depends on the cycle strain amplitude, degree of reversibility, and the strain path imposed by the contact loading at different depths.     

Plastic work of fatigue crack propagation in steels and aluminum alloys

The plastic work per unit area of fatigue crack propagation,U, is one of the parameters controlling the rate of fatigue crack propagation,dc/dN. The equation,dc/dN = A ΔK ⁴/(σf_y ²μ U), was previously shown to fit the data for 7 iron and aluminum base alloys for the range of thedc/dN vs ΔK curve where the Paris relation is valid. Values ofU are now available for 6 additional alloys covering a much wider range of σ_y 42 to 868 MN/m². For the total populationA = (2.8 ± 0.9) X 10^-3 where 2.8 is the mean and 0.9 is the standard deviation. In this equation, σ_y is the 0.2 pct offset cyclic yield stress and μ is the shear modulus. The parameterU is related to microstructure and should be of interest to the metallurgist. Generally,U varies oppositely to σ_y due to decrease in the plastic zone size; however, the plastic strain amplitude and degree of localization of the plastic strain in the plastic zone are also important.     

Deformation and failure of Zr<Subscript>57</Subscript>Nb<Subscript>5</Subscript>Al<Subscript>10</Subscript>Cu<Subscript>15.4</Subscript>Ni<Subscript>12.6</Subscript>/W particle composites under quasi-static and dynamic compression

We have investigated the mechanical behavior of a composite material consisting of a Zr₅₇Nb₅Al₁₀Cu_15.4Ni_12.6 metallic glass matrix with 60 vol pct tungsten particles under uniaxial compression over a range of strain rates from 10⁻⁴ to 10⁴ s⁻¹. In contrast to the behavior of single-phase metallic glasses, the failure strength of the composite increases with increasing strain rate. The composite shows substantially greater plastic deformation than the unreinforced glass under both quasi-static and dynamic loading. Under quasi-static loading, the composite specimens do not fail even at nominal plastic strains in excess of 30 pct. Under dynamic loading, fracture of the composite specimens is induced by shear bands at plastic strains of approximately 20 to 30 pct. We observed evidence of shear localization in the composite on two distinct length scales. Multiple shear bands with thicknesses less than 1 μm form under both quasi-static and dynamic loading. The large plastic deformation developed in the composite specimens is due to the ability of the tungsten particles both to initiate these shear bands and to restrict their propagation. In addition, the dynamic specimens also show shear bands with thicknesses on the order of 50 μm; the tungsten particles inside these shear bands are extensively deformed. We propose that thermal softening of the tungsten particles results in a lowered constraint for shear band development, leading to earlier failure under dynamic loading.     

Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10⁻⁴ s⁻¹ and 10⁻¹ s⁻¹ over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theM _s ^σ temperature. At temperatures much greater thanM _s ^σ , austenite twinning will occur, while at temperatures close toM _s ^σ , bcc martensite will form.     

The precipitation of Ni3Nb phases in a Ni−Fe−Cr−Nb alloy

The age hardening of a Ni?Fe?Cr?Nb alloy containing 4.85 wt pct Nb has been studied using transmission electron microscopy. The major hardening phase in this alloy isγ*, DO₂₂-ordered Ni₃Nb, which precipitates as a fine dispersion of square plates. It is shown that nucleation ofγ* plates may be dependent upon matrix excess vacancy concentration, but nucleation ofγ* plates is also observed at dislocations and extrinsic stacking faults. Theγ* phase is metastable with respect to the orthorhombic Ni₃Nb phase, β, which precipitates by either a cellular or an intragranular reaction, depending upon the aging temperature. It is proposed that the intragranular nucleation of β laths proceeds by the growth of stacking faults from withinγ* plates; theγ* plates subsequently dissolve in favor of the β laths. Room temperature deformation of theγ* dispersion is shown to produce faults within theγ* plates; possible dislocation reactions occurring during this deformation are discussed.