A fully recrystallized microstructure with a fine grain size can improve the performance of superconducting radio-frequency cavities produced from high-purity Nb (ASTM B393-18 Type 5 Nb) while providing mechanical strength. Recrystallization depends on impurity content, initial microstructure, deformation state, and annealing conditions. To better understand how fine-grained, fully recrystallized microstructures may be produced, the recrystallization behaviors of Type 5 and Type 2 Nb materials were studied. Type 5 Nb specimens were produced with both fine and coarse initial grain sizes. All were cold rolled and then annealed under vacuum for one hour to determine the rolling reductions and temperatures required for recrystallization. The recrystallized fraction exceeded 95% in fine-grained Type 5 Nb rolled to a 30% or greater thickness reduction and then annealed at 800 degrees C or higher. The coarse-grained Type 5 Nb required greater rolling reductions to produce any recrystallized grains at 800 degrees C. The higher interstitial content of the Type 2 Nb required a temperature of 1000 degrees C or higher to reach a recrystallized fraction greater than 95% in specimens rolled to a 60% reduction. Recrystallization is more easily achieved during annealing at a given temperature for a set time by increasing rolling reduction, decreasing interstitial content, and decreasing initial grain size prior to cold rolling. To avoid grain growth following recrystallization, which increases grain size, the minimum time and temperature necessary for complete recrystallization should be applied.

The effects of strain and strain rate on dynamic grain growth (DGG) and subgrain evolution are reported for an interstitial-free steel deformed at 850 ^∘C . Microstructures produced during tension tests at true-strain rates of 10^-4 and 10^-3 s^-1 to true strains ranging from 0.02 to 0.2 were preserved following deformation. These were characterized using electron backscatter diffraction (EBSD), including the application of spherical harmonic transform indexing to produce high-angular-resolution EBSD (HR-EBSD) data. HR-EBSD data resolved the small misorientation angles of subgrain boundaries while imaging much larger data fields than possible with previously available techniques. The resulting data confirmed that steady-state flow stress is inversely proportional to the average subgrain size and that subgrain boundary misorientation angle increases with strain. The following new observations are reported. The rate of DGG increased with respect to time but decreased with respect to strain as strain rate increased. This behavior is rationalized through a simple model using separate rate parameters for the effects of time and strain. Subgrain size was not constant during steady-state deformation, but decreased slowly with increasing strain. Subgrain size distributions and subgrain boundary misorientation angle distributions were measured, and both remained approximately log-normal during steady-state deformation. Subgrain evolution demonstrated no dependence on parent grain size, crystallographic orientation, or Taylor factor. These new data suggest that steady-state flow stress is more likely controlled by the dislocation density internal to subgrains than by the spacing between subgrain boundaries.

The poor formability of AA7075-T6 sheet at room temperature is sufficiently improved at 200 °C to successfully stamp a part of complex geometry. Increased tearing resistance is investigated as a possible source of this improved formability. Tearing energies were measured in AA7075-T6 sheet at room temperature and elevated temperatures of 180, 190, and 200 °C using Kahn-type tear tests. Tests were conducted at displacement rates ranging from 2 to 1024 mm/min. Tear propagation energies increased by a factor of ten from room temperature to 200 °C for all but the fastest displacement rates. Both tear initiation energy and tear propagation energy decrease logarithmically with increasing displacement rate at the elevated temperatures investigated. Improved tearing resistance is associated with a transition from low-ductility fracture at room temperature to ductile fracture at 200 °C and is a likely source of improved formability at this elevated temperature.

The high-temperature plastic-flow behavior of a strain- and strain-rate-hardening material was quantified using a novel, high-throughput technique known as the constant-stress, constant-heating-rate (CSCHR) test. For this purpose, CSCHR experiments were performed on annealed sheet of unalloyed (Type 1) niobium using three constant-stress levels (34, 69, and 103 MPa) and two heating rates (15 and 63 °C/min). To interpret the observations, a suite of relations was derived to enable the extraction of the material coefficients that describe the constitutive behavior (i.e., apparent activation energy Q, strain hardening exponent p, strain-rate sensitivity exponent m) from the CSCHR measurements. Among other things, the relations revealed that the value of Q is a function of the slope of the Arrhenius plot as well as a term dependent on the ratio of p to m. It was also demonstrated that the same material coefficients describing behavior during the continuous heating imposed during CSCHR tests were applicable for quantifying deformation under isothermal, constant-strain-rate conditions within a reasonable engineering accuracy.

Data are presented from tensile tests of commercial Nb-based alloy C103 (Nb-10Hf-1Ti, by wt pct) at temperatures of 1550–1750 °C and true-strain rates of 3× 10^-5 to 3 × 10^-3 s^-1 . Changes in strain rate generated pronounced short-term transients in flow stress. These transients are of the inverse type characteristic of solute drag creep (SDC). C103 produced large tensile elongations of 150–200 pct and strain hardened during plastic deformation. Test data provide an average strain-rate sensitivity of 0.29 and an activation energy for creep of 340 kJ/mol. Short-term transient data indicate a stress dependence for dislocation glide velocity of v̅∝σ ^2.7 and for mobile dislocation density of ρ∝σ ^0.7 . The deformed microstructure contains indistinct subgrains and steep strain gradients. All data indicate deformation by SDC controlled by the diffusion of Hf solute atoms for the range of conditions examined. Data from plastic flow transients suggest that creep rates for C103 available in the literature are likely from the primary creep region. When that is considered, data from the literature and the present study are in good agreement.

X-ray computed tomography (XRCT)X-Ray Computed Tomography was used to characterize fractureFracture paths in AA7075-T6 sheet subjected to ductile tearingTearing at a warm temperature. Tear tests were conducted at 200 °C, and a pulling rate of 64 mm/min until the crack tipTips was approximately half way through the specimen. Three-dimensional XRCT data sets were acquired near each crack tipTips to map the crack, nearby porosity, and intermetallicIntermetallics particles. The XRCT data were analyzed using modern software techniques to segment out each of the important features. The spatial relationships between these features and their anisometry suggest microstructural causes for the anisotropy of tearingTearing resistance measured at warm temperatures; tearingTearing resistance is lower along the sheet rolling direction than it is along the long transverse direction. These connections are discussed, and methodsMethod for improving tearingTearing resistance are proposed.

We present two new methods of processing data from backscattered electron signals in a scanning electron microscope to image grains and subgrains. The first combines data from multiple backscattered electron images acquired at different specimen geometries to (1) better reveal grain boundaries in recrystallized microstructures and (2) distinguish between recrystallized and unrecrystallized regions in partially recrystallized microstructures. The second utilizes spherical harmonic transform indexing of electron backscatter diffraction patterns to produce high angular resolution orientation data that enable the characterization of subgrains. Subgrains are produced during high-temperature plastic deformation and have boundary misorientation angles ranging from a few degrees down to a few hundredths of a degree. We also present an algorithm to automatically segment grains from combined backscattered electron image data or grains and subgrains from high angular resolution electron backscatter diffraction data. Together, these new techniques enable rapid measurements of individual grains and subgrains from large populations.

The tearing resistance of AA7075AA7075-T6 sheet material was measured at room temperature and at 200 °C along different directions. Tearing resistance is characterized by the energy required to completely tear a specimen, with higher energies indicating greater tearing resistance. Specimens were tested at 200 °C for times no longer than would provide a retrogression heat treatmentHeat treatment, from which the full strength of the T6 condition may be recovered by a reaging heat treatmentHeat treatment. Tearing resistance is significantly greater at 200 °C than at room temperature, which promises improved deformation processingProcessing of AA7075AA7075-T6. The tearing resistance at 200 °C varies with direction relative to the rollingRolling direction. The tearing resistance is highest in the L–T orientation and lowest in the T–L orientation, by ASTM E871 specimen orientation designations. The significance of these results to the deformation processingProcessing of AA7075AA7075-T6 sheet at elevated temperatures will be discussed.

A mechanism is proposed for dynamic grain growth (DGG) by subgrain boundaries driving grain-boundary migration. This mechanism is evaluated against data from an interstitial-free steel tested in tension at 850 °C and a true-strain rate of 10^-4 s ^-1 and rapidly quenched to preserve microstructures evolved during deformation. Tensile tests produced steady-state flow, distinct subgrains, and rapid DGG. Static annealing alone produced static grain growth (SGG) that was much slower than DGG. Electron backscatter diffraction (EBSD) provided grain size and orientation measurements. High-resolution electron backscatter diffraction (HR-EBSD) was used to accurately measure subgrain sizes and subgrain boundary misorientations. The average grain size increased linearly with strain during DGG, but the average subgrain size remained constant during straining. The average subgrain boundary misorientation increased with strain, initially rapidly and then slowly. The dihedral angle imposed in grain boundaries by intersecting subgrain boundaries decreased with increasing subgrain boundary misorientation, which supports the proposed mechanism for DGG. The driving pressure for grain-boundary migration from subgrain boundaries is estimated to be approximately one order in magnitude greater than that from dislocation density reduction under the conditions examined.

The tear resistance of AA7075-T6 sheet material was measured at 25 and 200 °C to better understand rupture limited deformation. AA7075-T6 has very limited ductility at room temperature but exhibits increased ductility at 200 °C, at which the stamping of a complex geometry was demonstrated. Tear energies were measured for displacement rates that ranged from 2 mm/min up to 512 mm/min for tests at 200 °C. Tear propagation energies increased by a factor of ten from 25 to 200 °C. Tear propagation energies at 200 °C decreased logarithmically with increasing displacement rate. The direction of tearing influenced tear resistance, with the greater tear resistance along the transverse to the sheet rolling direction. Increased tear resistance at 200 °C is associated with a reduction in flow stress and increased ductility that transitions tearing from shearing fracture at 25 °C to ductile tearing with significant neck development at 200 °C.

Retrogression forming and reaging is a new scientific approach to producing components of high-strength aluminum alloys through warm forming followed by a single reaging heat treatment to restore T6 strength. Retrogression forming was applied to stamp 1.6-mm-thick AA7075-T6 Alclad sheet at 200 °C to a depth of 45 mm without visible defects or splitting. During stamping, the sheet material was subjected to multiple deformation modes common in automotive stamping processes, producing local major strains in excess of 20%. Time and temperature were controlled during retrogression forming using the concept of accumulated reduced retrogression time to enable recovery of the original T6 sheet strength by a single reaging heat treatment after forming. Tensile specimens extracted from the formed parts were tested to evaluate the effects of retrogression forming on strength. Reaging with a previously recommended reaging heat treatment of 120 °C for 24 h produced strengths approximately 5% greater than those of the original T6 temper. Reaging with a simulated paint-bake cycle of 185 °C for 25 min produced strengths nearly equivalent to those of the original T6 temper. Exceeding the maximum reduced time for retrogression during forming produced a strength loss that could not be recovered.

A mechanism is established by which interstitial impurity content increases the high-temperature strength of niobium. A Type 2 niobium sheet material (ASTM B393-18) was mechanically tested in tension from 1473 K to 1773 K (1200 °C to 1500 °C) at constant true strain rates of 10−3 and 10−4 s−1. This material is compared to a Type 1 niobium with lower interstitial impurity levels. The microstructures of both materials are characterized using backscatter electron imaging (BSE) and electron backscatter diffraction (EBSD). The deformation of both materials at these temperatures is dominated by five-power creep and the associated development of subgrains that control strength. Compared to the Type 1 material, the higher interstitial impurity content of the Type 2 niobium produces: 1. higher strength at elevated temperatures, 2. delayed recrystallization, 3. slower grain growth, 4. more inhomogeneous microstructures, and 5. slower recovery that produces smaller subgrains. These result from the fine dispersoids expected from interstitial impurities in the Type 2 niobium, with carbon identified as the most important interstitial element. The finer subgrain size of the Type 2 niobium produces, through five-power creep, a higher strength than the Type 1 niobium at these high temperatures.

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Plastic deformation and microstructure evolution are investigated at elevated temperatures in a low-impurity (ASTM B393-18 Type 1) niobium sheet material. Data from tensile tests are reported for temperatures from 1473 K to 1823 K (1200 °C to 1550 °C) at constant true-strain rates of 10 −3 and 10 −4 s −1 . Microstructures produced by static annealing and by tensile deformation at these temperatures are characterized using backscatter electron imaging. Significant static grain growth is observed and increases with increasing temperature. Young’s elastic modulus data for niobium from the literature are reviewed to recommend modulus values from 293 K to 2300 K (20 °C to 2027 °C) for randomly textured polycrystalline niobium. Steady-state creep analysis of data above 1370 K (1097 °C), half the melting temperature, produces a stress exponent of 5.6 and an activation energy for creep of 454 kJ/mol, which is close to that of lattice self-diffusion. Dislocation substructure is observed following elevated temperature deformation. These data are compared with data from the literature to conclude that low-impurity niobium deforms by five-power creep across the conditions examined.

Retrogression forming and reaging (RFRA) is a new warm-forming process designed to produce automotive structural components from high-strength aluminum alloys. A scientific approach is described to determine appropriate RFRA conditions for AA7075-T6 and is applied to laboratory-scale forming experiments. The concept of reduced time is used with the activation energy of retrogression measured for AA7075-T6 to predict appropriate times and temperatures for retrogression forming. Conditions recommended for AA7075-T6 are retrogression at 200 °C for 3 to 12 min while forming at strain rates of up to 10–1 s−1. The recommended reaging heat treatment to fully restore strength to the T6 condition after retrogression forming is 120 °C for 24 h. These RFRA conditions were successfully applied in laboratory-scale experiments to form AA7075-T6 Alclad sheet and produce a final strength equivalent to the T6 condition. Data from tensile tests provide flow stresses and tensile ductilities across the range of conditions appropriate for RFRA.

A warm forming process with a simultaneous retrogression heat treatment, termed retrogression forming, can achieve good formability in high-strength aluminum alloys and recover their high strength through a single reaging heat treatment after forming. Tensile data from two commercial aluminum alloy sheet materials, AA7075-T6 and AA6013-T6, are presented for conditions suitable to retrogression forming. AA7075-T6 sheet was tested at temperatures from 180 °C to 220 °C and strain rates from 3.2 × 10−3 to 10−1 s−1. AA6013-T6 sheet was tested from 230 °C to 250 °C and 3.2 × 10−3 to 10−1 s−1. Both materials exhibit nearly steady-state flow at these temperatures and produce a modest strain-rate sensitivity of m = 0.039. The activation energies for plastic flow under these conditions are 221 kJ/mol for AA7075-T6 and 253 kJ/mol for AA6013-T6. Test data are used to construct predictive models for flow stress as a function of temperature and strain rate. AA7075-T6 demonstrates an excellent potential for retrogression forming at 200 °C and strain rates up to 10−1 s−1 if time at temperature is held to under 12 minutes. AA6013-T6 exhibits a modest potential for retrogression forming at 240 °C and strain rates up to 10−1 s−1 if time at temperature is held to under 7 minutes.

Retrogression and reaging (RRA) is of interest to the automotive industry for manufacturing components of high-strength aluminum alloys. RRA heat treatments are investigated for AA7075-T6 and AA6013-T6 materials. Retrogression is demonstrated to be a thermally activated process reasonably characterized with a single activation energy. Activation energies for retrogression are measured as 97 ± 7 and 160 ± 30 kJ/mol for AA7075-T6 and AA6013-T6, respectively. Critical retrogression times, tR* and tRmax, are defined and measured across a range of retrogression temperatures. These data are used with the concept of reduced time to predict combinations of temperature and time that produce successful retrogression heat treatments. Recommended retrogression heat treatments are 200 °C for 3 to 12 minutes for AA7075-T6 and 240 °C for 7 minutes for AA6013-T6. Data from reaging heat treatments confirm a significant RRA response in AA6013. Recommended reaging heat treatments are 120 °C for 24 hours for retrogressed AA7075 and 190 °C for 1 hour for retrogressed AA6013. A reaging heat treatment that simulates the automotive paint-bake cycle, 185 °C for 25 minutes, is almost as effective as the recommended reaging heat treatment for AA6013 but is significantly less effective than the recommended reaging heat treatment for AA7075.

Combining a retrogression heat treatment with simultaneous warm forming can increase the formability of peak-aged, high-strength aluminum alloys while allowing peak-aged strength to be recovered through a single reaging heat treatment after forming. This process is termed retrogression-forming-and-reaging (RFRA). This study investigates the applicability of RFRA to AA6013-T6 sheet material. Elevated-temperature tensile tests were performed at temperatures from 230 to 250 °C and strain rates from 3.2 × 10−3 to 10−1 s−1. Tensile tests were followed by reaging with a simulated paint-bake heat treatment. Flow stress at a true strain of 0.10 ranges from 230 MPa (250 °C and 3.2 × 10−3 s−1) to 290 MPa (230 °C and 10−1 s−1), significantly lower than the room-temperature yield strength of 360 MPa in the T6 condition. The average elongation to rupture and reduction in area from elevated-temperature tests are 22% and 56%, respectively, which are similar to the room-temperature values for the T4 condition. Elevated-temperature testing reduced material hardness compared to the original T6 condition. Subsequent reaging with a simulated paint-bake raised hardness to 96% of the T6 condition in un-deformed material, but slightly decreased the hardness of the deformed material. Recommendations for implementing RFRA of AA6013-T6 are presented.

Dynamic grain growth is demonstrated to be much faster than static grain growth in a body-centered-cubic, interstitial-free steel sheet material at 850$$\,^\circ {\rm{C}}$$. Dynamic grain growth occurs during concurrent plastic deformation at elevated temperature, whereas static grain growth occurs during static annealing. Grain growth during steady-state plastic flow in tension at 850$$\,^\circ {\rm{C}}$$ to a true strain of 0.2 at a true-strain rate of $$10^{-4}$$ $${\rm{s}}^{-1}$$ doubled grain size, while static annealing for the same time produced no increase in grain size. This is described as dynamic normal grain growth (DNGG) because no abnormally large grains were observed. The recrystallized microstructure of the steel demonstrated a log-normal distribution of grain sizes. DNGG produced bimodal grain size distributions that deviate from the theoretical expectation of a simple shift to larger sizes during normal growth. The bimodal distributions contained a remnant of small grains that were not consumed during grain growth. DNGG produced a crystallographic texture that is unique from both the recrystallized material and that produced by lattice rotation alone. DNGG strengthened the $$\{111\} \langle 110 \rangle $$ and $$\{111\} \langle 112 \rangle $$ components of the strong $$\gamma $$-fiber component in the original recrystallization texture. Lattice rotation from tensile deformation, by contrast, strengthened the $$\alpha $$-fiber components that intersect the original $$\gamma $$-fiber.