Dislocation structures of monocrystalline copper during corrosion fatigue in 0.1 M perchloric acid

We have studied the dislocation structures of single crystals of copper cycled in 0.1 M perchloric acid and under different polarization potentials. TEM samples were cut from representative specimens both after saturation and after fracture. Although much higher strain localization was observed in the crystals cycled at anodic potentials, the dislocation structures observed were very similar to those of specimens tested at cathodic potentials and in air. For a plastic shear strain amplitude of 2×10⁻³, regular loop patches and dipolar walls were observed. For a higher strain amplitude of 4×10⁻³, dipolar walls associated with secondary slip were found in addition to regular primary walls. We believe this structure to be associated with breaking the primary persistent slip bands (PSB's) into truncated groups, of which the truncations were lined up along the traces of the secondary slip plane. Dislocation structures observed after final fracture of the crystals were different from those formed by cycling just into saturation; dipolar walls formed first and cell structures developed later. Thus, we confirmed the transition of dipolar walls into cells, reported by other investigators. Moreover, the transition of loop patches into “rungs” structure, the embryo of the PSB, was also observed in the late stages of life. BENDA YAN, formerly with the University of Pennsylvania     

Strength and plastic flow in “in situ” TiC reinforced aluminum composites

The deformation behavior of TiC particulate-reinforced aluminum composites (Al-TiC _p ) was investigated in this work using pure aluminum as the reference matrix material. Uniaxial compression tests were carried out at 293 and 623 K and at two strain rates (3.7×10⁻⁴ and 3.7×10⁻³ s⁻¹). Yield strengths of up to 127 MPa were found in composites containing 10 vol pct TiC particulates, which were almost 4 times the yield strength of pure Al. In addition, at 623 K, relatively small reductions in yield strength were found, suggesting that this property was rather insensitive to temperature for the temperatures investigated in this work. Nevertheless, at 623 K, increasing the rate of straining from 3.7×10⁻⁴ s⁻¹ to 3.7×10⁻³ s⁻¹ lowered the yield strength, particularly in 10 vol pct TiC _p -Al composites. Two stages of work hardening were identified in pure Al and a 10 vol pct TiC _p composite during plastic flow through the modified version of the Hollomon equation (σ = Kɛ ⁿ ± Δ). In particular, the work-hardening exponents found in pure Al shifted from high to low values as the extent of plastic strain was increased while the opposite was true for the 10 vol pct TiC _p composite. Finally, at 623 K, dynamic recovery mechanisms became dominant at plastic strain levels >0.2 in 10 vol pct TiC _p -Al composites, with the effect being minor at room temperature.     

Fatigue deformation of silver single crystals

The defect microstructure at saturation of silver single crystals subjected to constant plastic strain amplitude, γ_p, fatigue tests in the range γ_p = ±0.0025 to ±0.025 was studied by transmission electron microscopy. The rate of fatigue hardening and the saturation stress increase with increasing plastic strain amplitude. The dislocation microstructure at saturation changes from an open band structure at small plastic strain amplitudes to a closed cell structure at large plastic strain amplitude. The defect microstructure at saturation can be characterized by the density and distribution of point defect clusters and the dislocation wall spacing along the primary Burgers vector direction. The saturation stress is inversely proportional to the dislocation wall spacing along the primary Burgers vector direction and to the spacing of the point defect clusters on the primary slip plane. There is no significant difference between the fatigue hardening, the cyclic stress-strain curve and the dislocation microstructure at saturation of silver and copper single crystals.     

Mechanisms of Hf dopant incorporation during the early stage of chemical vapor deposition aluminide coating growth under continuous doping conditions

A laboratory-scale chemical vapor deposition (CVD) reactor was used to perform “continuous” Hf doping experiments while the surface of a single-crystal Ni alloy was being aluminized to form an aluminide (β-NiAl) coating matrix for 45 minutes at 1150 °C. The continuous doping procedure, in which HfCl₄ and AlCl₃ were simultaneously introduced with H₂, required a high HfCl₄/AlCl₃ ratio (>∼0.6) to cause the precipitation of Hf-rich particles (∼0.1 μm) at grain boundaries of the coating layer, with the overall Hf concentration of ∼0.05 to 0.25 wt pct measured in the coating layer by glow-discharge mass spectroscopy (GDMS). Below this ratio, Hf did not incorporate as a dopant into the growing coating layer from the gas phase, as the coating matrix appeared to be “saturated” with other refractory elements partitioned from the alloy substrate. In comparison, the Hf concentration in the aluminide coating layer formed on pure Ni was in the range of ∼0.1 wt pct, which was close to the solubility of Hf estimated for bulk NiAl. Interestingly, the segregation of Hf and the formation of a thin γ′-Ni₃Al layer (∼0.5 μm) at the coating surface were consistently observed for both the alloy and pure-Ni substrates. The formation of the thin γ′-Ni₃Al layer was attributed to an increase in the elastic strain of the β-NiAl phase, associated with the segregation of Hf as well as other refractory alloying elements at the coating surface. This phenomenon also implied that the coating layer was actually growing at the interface between the γ′-Ni₃Al layer and the β-NiAl coating matrix, not at the gas/coating interface, during the early stage of the coating growth.     

Effects of Strain State and Strain Rate on Deformation-Induced Transformation in 304 Stainless Steel: Part I. Magnetic Measurements and Mechanical Behavior

The γ→α transformation in 304 stainless steel can be induced by plastic deformation at room temperature. The kinetics of strain-induced transformations have been modeled recently by Olson and Cohen. We used magnetic techniques to monitor the progress of the γ→α transformation in 304 stainless steel sheet loaded in uniaxial and biaxial tension at both low (10^-3 per second) and high (10³ per second) strain rates. We found that using the von Mises effective strain criterion gives a reasonable correlation of transformation kinetics under general strain states. The principal effect of increased strain rate was observed at strains greater than 0.25. The temperature increase resulting from adiabatic heating was sufficient to suppress the γ→α transformation substantially at high rates. The consequences of the γ→α transformation on mechanical behavior were noted in uniaxial and biaxial tension. Uniaxial tension tests were conducted at temperatures ranging from 50 to -80°C. We found that both the strain hardening and transformation rates increased with decreasing temperature. However, the martensite transformation saturates at ≈85 pct volume fraction α. This can occur at strains less than 0.3 for conditions where the transformation is rapid. Once saturation occurs, the work hardening rate decreases rapidly and premature local plastic instability results. In biaxial tension, the same tendency toward plastic instability associated with high transformation rates provides a rationale for the low biaxial ductility of 304 stainless steel.     

Evolution of Slip Morphology and Fatigue Crack Initiation in Surface Grains of Ni200

The evolution of slipbands into fatigue cracks in surface grains of commercially pure Ni, Ni200 (99.35Ni-0.4Fe-0.25Cu, in wt pct), was studied at ambient temperature. Round-bar specimens with electropolished surfaces were fatigued under displacement-controlled, fully reversed conditions at four strain amplitudes under a nominal strain rate of 1 × 10⁻³ s⁻¹. Low-cycle fatigue tests were periodically interrupted to characterize the slip morphology at various fatigue cycles using scanning electron microscopy. The results showed that the distribution of slip in Ni200 varied considerably in individual surface grains at a given strain amplitude. Some grains were deformed more severely and exhibited more intense slipbands than others, while some surface grains showed the absence of slip lines with no evidence of plastic deformation. The evolutions of slipband width and spacing in deformed surface grains were followed as a function of fatigue cycles in order to assess the slipband morphology at the onset of fatigue crack initiation.     

The influence of crystallographic orientation and strain rate on the high-temperature low-cyclic fatigue property of a nickel-base single-crystal superalloy

Fully reversed low-cyclic fatigue (LCF) tests were conducted on [001], [012], [-112], [011], and [-114] oriented single crystals of nickel-based superalloy DD3 with different cyclic strain rates at 950°C. The cyclic strain rates were chosen as 1.0×10⁻², 1.33×10⁻³ and 0.33×10⁻³ s⁻¹. The octahedral slip systems were confirmed to be activated on all the specimens. The experimental result shows that the fatigue behavior depends on the crystallographic orientation and cyclic strain rate. Except [001] orientation specimens, it is found from the scanning electron microscopy (SEM) examination that there are typical fatigue striations on the fracture surfaces. These fatigue striations are made up of cracks. The width of the fatigue striations depends on the crystallographic orientation and varies with the total strain range. A simple linear relationship exists between the width and total shear strain range modified by an orientation and strain rate parameter. The nonconformity to the Schmid law of tensile/compressive flow stress and plastic behavior existed at 950°C, and an orientation and strain rate modified Lall-Chin-Pope (LCP) model was derived for the nonconformity. The influence of crystallographic orientation and cyclic strain rate on the LCF behavior can be predicted satisfactorily by the model. In terms of an orientation and strain rate modified total strain range, a model for fatigue life was proposed and used successfully to correlate the fatigue lives studied in this article.     

Cyclic strain hardening in polycrystalline copper

Polycrystals of pure copper were cyclically deformed, at room temperature under symmetric tension-compression fatigue at constant total strain amplitude control with an approximate constant plastic strain rate of 10⁻⁴. The relationship between the saturation stress amplitude and strain amplitude over a range of plastic strain from 2 × 10⁻⁷ to 10⁻² reveals three regions of cyclic hardening. A quasi-plateau, where the stresses show a slow constant increase, was observed in the intermediate region extending in the plastic strain range γ_pl, of 1.5 × 10⁻⁵ to 7.5 × 10⁻⁴. In this region, persistent slip bands (PSBs) which consist of “ladder” structures, similar to the case of single crystals, were found in the bulk of the fatigued polycrystals. The fatigue limit was found to be Δσ_s/2 = 73 MPa which corresponds to the plastic strain Δϵ_pl/2 = 1.5 × 10⁻⁵ where PSBs do not form.     

The effect of strain rate and depressed temperature on the low cycle deformation behavior of alpha iron

The low cycle deformation saturation stress in Ferrovac-E a-iron was studied using diametral plastic strain (0.001 ≤ Δε_dp/2 ≤ 0.0135) as the control variable. Increasing strain rate (6 × 10^•5 s^•1 • 4 × 10^•3 s^•1) and decreasing temperature (295 to 173 K) increased the saturation stress levels. The cyclic work hardening coefficient decreased from 0.18 at 295 K to 0.10 at 173 K, which is consistent with previous studies of monotonie deformation. The temperature dependence of both the saturation stress and the strain rate sensitivity, as measured during cyclic deformation, were similar to that measured during monotonic tensile tests. The temperature dependence of the dislocation velocity indexm* was in good agreement with published values from high cycle fatigue and monotonie tensile tests. Thus the same deformation mechanisms are believed to occur in both monotonie and large plastic cyclic deformation (Δε_dp/2 ≥ 0.001) of a-iron.     

Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution (γ′_N) based on the γ′-Fe₄N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at. pct) and 〈110〉 streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above the 20 at. pct γ′-Fe₄N-phase value and leads to a lattice parameter that is greater than that of the γ′-Fe₄N phase. The subsurface of the layer is composed of a supersaturated solid solution based on γ-austenite, which is an expanded austenite, γ _N. Its morphology shows the basketweave or “tweedlike” contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The ε martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of ε martensite in the subsurface of the layer indicate that nitrogen lowers the stacking fault energy of austenite.     

A point defect model for fatigue crack initiation in Ni3Al+B single crystals

Transmission electron microscopy of boron-doped Ni₃Al single crystals, oriented for single slip and cyclically deformed at room temperature, revealed a high density of dislocation dipoles and point defect clusters. Observations of circular perfect dislocation loops, Frank loops, vacancy tetrahedra and spherical voids provide evidence of vacancy condensation during fatigue cycling at room temperature. It is suggested that lattice misfit develops between persistent slip bands (PSB) and matrix as a result of the generation and coalescence of excess vacancies in PSBs. The misfit strain at PSB/matrix interfaces is considered to increase with increasing cumulative plastic strain. Together with SEM observations of surface topography, it is suggested that fatigue damage in Ni₃Al single crystals is initiated by the formation of microvoids (microcracks) at PSB/matrix interfaces. The microvoids (microcracks) break down the coherency of the PSB/matrix interfaces and thereby relieve the accumulated misfit strain at the interfaces. A model of fatigue crack initiation based upon a surface energy criterion is proposed.     

Strengthening by high densities of nanometer-size precipitates: Oxides in Ni

We have measured the yield strengths of Ni samples having high densities of nanometer-size precipitates. Surface layers containing NiO or γ-Al₂O₃ precipitates were formed in Ni specimens by ion implanting O alone or O and Al, with subsequent annealing. The yield strengths of the layers were obtained through nanoindentation in conjunction with finite-element simulations. The yield strengths of the Ni alloys were combined with earlier data for O-implanted Al and compared to predictions of a recent treatment of the Orowan mechanism, in which dislocations loop around precipitates and by-pass them. The strengths vary with changes in precipitate microstructure, as predicted, and conform to the theory in absolute magnitude to within a factor of 1.5. This agreement extends over broad microstructural ranges: precipitate sizes from ∼1 to 20 nm, volume fractions from 0.05 to 0.30, densities from 4×10¹⁶/cm³ to as high as ∼10²⁰/cm³, edge-to-edge spacings as small as 1.4 nm, two precipitated phases, and two metal matrices with shear moduli differing by a factor of 3. Ion implantation increases near-surface yield strengths to as high as 5 GPa, suggesting that this treatment may be useful for hardening the surfaces of Ni components in micro-electromechanical systems.     

A synchrotron X-ray diffraction study of the local residual strains around a single inclusion in an AI/W metal-matrix composite

An X-ray technique for local measurements of the internal residual stress near inclusions in metalmatrix composites (MMCs) is presented. The technique utilizes medium- to high-energy monochromatic X-rays from a synchrotron source and a combination of slits on the entry and exit sides of the sample in order to determine the strains from small volumes deep within the composite sample. The strains of the individual matrix grains are sampled and averaged, allowing for a much improved spatial resolution. An analyzer is used in order to avoid well-known systematic errors related to geometry and stability of the beam. First results are obtained on a model system consisting of a 500 μm continuous W fiber imbedded in an A1 matrix. Two specimens were investigated with typical Al grain sizes of 1 mm and 30 μm. With a gage volume of 10×10×170 μm³, we obtained count rates on the order of 5000 cps and an accuracy in the strain measurements of Δε≤5×10⁻⁵. For both specimens, we found no variations of the radial and transverse strain components with the distance to the fiber, indicating either a complete debonding of the system, a very weak interface, or interface deterioration due to chemical reaction. Intragranular strain fluctuations on the order of ε=±10⁻⁴ were found to build up close to the grain boundaries. For the specimen with the smaller Al grain size, sampling data from approximately 15 grains at the same distance to the fiber was sufficient for averaging out the intergranular fluctuations. Finally, we observe effects from the conventional metallurgic sample preparation up to 400 μm from the surface, emphasizing the relevance of bulk techniques.     

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were performed at a strain amplitude of ±0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of γ′ and carbides); and microstructure B, in a double-aged condition with γ′ of 18-nm diameter and M₂₃C₆ carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered γ′ that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 × 10^-5 to 3 × 10^-3 s^-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.     

Auger electron analysis of the initial oxidation of titanium aluminides based on Ti-48Al

The surface of a Ti-48 at. Pct Al alloy was examined by Auger electron microscopy to study oxidation at room temperature. On exposure to air at room temperature, both Al and Ti oxides were observed together with an abundance of C. The amount of C was always larger in the two-phase α₂ + γ region compared to the single-phase γ region. The Ti oxides formed on the surface of they grains were primarily Ti₂O₃ rather than TiO₂. On depth profiling with Ar⁺ ion sputtering, lower oxide states of Ti were found. This was attributed to either the Ar⁺ ion sputtering or the fact that the inner layers of oxide represented oxides of Ti in their lower valence states. The A1₂O₃ was stable and did not exhibit any transient oxidation states. The dominant oxidation product on the surface of sputtered single-phase γ grains after an 84-hour exposure in the ultrahigh vacuum Auger chamber at room temperature is A1₂O₃. A depletion of C and O occurred beneath the oxide surface in some γ grains. The chemical shift between the Al L_2,3MM and A1₂O₃ L_2,3(A1)M_(O)M_(O) peaks in the Auger spectrum of A1₂O₃ formed on the γ phase in TiAl was found to be 11 eV. Y.T. Peng, Graduate Student, Formerly with the Materials Science and Engineering Program, University of Texas at Arlington, Arlington, TX 76019,     

Microstructural evolution of the nickel platinum-aluminide bond coat on electron-beam physical-vapor deposition thermal-barrier coatings during high-temperature service

The microstructural evolution of a (Ni,Pt)-aluminide bond coat underneath the ZrO₂-based thermal-barrier coating (TBC) topcoat system on a René N5 Ni-based superalloy turbine blade during prolonged high-temperature service has been characterized using transmission electron microscopy (TEM). The as-deposited bond coat has a spatially varying microstructure, which consists of an outer layer of single-phase β-(Ni,Pt)Al, a middle layer of a β-(Ni,Pt)Al matrix containing a high number density of μ-phase precipitates, and an inner layer containing a γ/γ′ matrix and numerous μ- and σ-phase precipitates. During service, microstructural changes in the hotter sections of the blade are more extensive than those in the cooler parts, as expected. As a result of interdiffusion, the inner layer grows into the γ/γ′ substrate, with the formation of some M₂₃C₆ precipitates, and the β matrix in the middle layer is transformed into a two-phase mixture of β and γ′. Corresponding changes occur in the morphologies and volume fractions of the various precipitate phases present in the bond coat. The single-phase β material in the outer layer retains its basic structure, except that the compositional changes due to diffusion between the bond coat and turbine blade cause a martensitic transformation to occur in the hottest sections during the final cooling of the blade. The distribution of various elements in the different layers has also been analyzed, as has growth of the thermally grown oxide (TGO) at the bond coat/TBC interface.     

Coarsening of Al<Subscript>3</Subscript>Sc precipitates in an Al-0.28 wt pct Sc alloy

The Ostwald ripening of Al₃Sc precipitates in an Al-0.28 wt pct Sc alloy during aging at 673, 698, and 723 K has been examined by measuring the average size of precipitates by transmission electron microscopy (TEM) and the reduction in Sc concentration in the Al matrix with aging time, t, by electrical resistivity. The coarsening kinetics of Al₃Sc precipitates obey the t ^1/3 time law, as predicted by the Lifshitz-Slyozov-Wagner (LSW) theory. The kinetics of the reduction of Sc concentration with t are consistent with the predicted t ^−1/3 time law. Application of the LSW theory has enabled independent calculation of the Al/Al₃Sc interface energy, γ, and volume diffusion coefficient, D, of Sc in Al during coarsening of precipitates. The Gibbs-Thompson equation has been used to give a value of γ using coarsening data obtained from TEM and electrical resistivity measurements. The value of γ estimated from the LSW theory is 218 mJ m⁻², which is nearly identical to 230 mJ m⁻² from the Gibbs-Thompson equation. The pre-exponential factor and activation energy for diffusion of Sc in Al are determined to be (7.2±6.0)×10⁻⁴ m² s⁻¹ and 176±9 kJ mol⁻¹, respectively. The values are in agreement with those for diffusion of Sc in Al obtained from tracer diffusion measurements.     

Internal displacement reactions in multicomponent oxides. Part I. Line compounds with narrow homogeneity range

As a model of an internal displacement reaction involving a ternary oxide “line” compound, the following reaction was studied at 1273 K as a function of time, t: Both polycrystalline and single-crystal materials were used as the starting NiTiO₃ oxide. During the reaction, the Ni in the oxide compound is displaced by Fe and it precipitates as a γ-(Ni-Fe) alloy. The reaction preserves the starting ilmenite structure. The product oxide has a constant Ti concentration across the reaction zone, with variation in the concentration of Fe and Ni, consistent with ilmenite composition. In the case of single-crystal NiTiO₃ as the starting oxide, the γ alloy has a “layered” structure and the layer separation is suggestive of Liesegang-type precipitation. In the case of polycrystalline NiTiO₃ as the starting oxide, the alloy precipitates mainly along grain boundaries, with some particles inside the grains. A concentration gradient exists in the alloy across the reaction zone and the composition is >95 at. pct Ni at the reaction front. The parabolic rate constant for the reaction is k _p =1.3 × 10⁻¹² m² s⁻¹ and is nearly the same for both single-crystal and polycrystalline oxides.     

Microstructure–Wear Resistance Correlation and Wear Mechanisms of Spark Plasma Sintered Cu-Pb Nanocomposites

The dispersion of a softer phase in a metallic matrix reduces the coefficient of friction (COF), often at the expense of an increased wear rate at the tribological contact. To address this issue, unlubricated fretting wear tests were performed on spark plasma sintered Cu-Pb nanocomposites against bearing steel. The sintering temperature and the Pb content as well as the fretting parameters were judiciously selected and varied to investigate the role of microstructure (grain size, second-phase content) on the wear resistance properties of Cu-Pb nanocomposites. A combination of the lowest wear rate (~1.5 × 10^?6 mm³/Nm) and a modest COF (~0.4) was achieved for Cu-15 wt pct Pb nanocomposites. The lower wear rate of Cu-Pb nanocomposites with respect to unreinforced Cu is attributed to high hardness (~2 to 3.5 GPa) of the matrix, Cu₂O/Fe₂O₃-rich oxide layer formation at tribological interface, and exuding of softer Pb particles. The wear properties are discussed in reference to the characteristics of transfer layer on worn surface as well as subsurface damage probed using focused ion beam microscopy. Interestingly, the flash temperature has been found to have insignificant effect on the observed oxidative wear, and alternative mechanisms are proposed. Importantly, the wear resistance properties of the nanocomposites reveal a weak Hall–Petch-like relationship with grain size of nanocrystalline Cu.     

Role of Plastic Deformation on Elevated Temperature Tribological Behavior of an Al-Mg Alloy (AA5083): A Friction Mapping Approach

Friction maps have been developed to explain the behavior of aluminum alloys under dynamic tribological conditions generated by the simultaneous effects of temperature and strain rate. A specially designed tribometer was used to measure the coefficient of friction (COF) of AA5083 strips subjected to sliding with a simultaneous application of tensile strain in the temperature range of 693 K to 818 K (420 °C to 545 °C) and strain rates between 5 × 10⁻³ s⁻¹ and 4 × 10⁻² s⁻¹. The mechanisms of plastic deformation, namely, diffusional flow, grain boundary sliding (GBS), and solute drag (SD), and their operation ranges were identified. Relationships between the bulk deformation mechanism and COF were represented in a unified map by superimposing the regions of dominant deformation mechanisms on the COF map. The change in COF (from 1.0 at 693 K (420 °C) and 1 × 10⁻² s⁻¹ to 2.1 at 818 K (545 °C) and 4 × 10⁻² s⁻¹) was found to be largest in the temperature–strain rate region, where GBS was the dominant deformation mechanism, as a result of increased surface roughness. The role of bulk deformation mechanisms on the evolution of the surface oxide layer damage was also examined.