Ultrahigh strength and ductility in newly developed materials with coherent nanolamellar architectures

Nano-lamellar materials with ultrahigh strengths and unusual physical properties are of technological importance for structural applications. However, these materials generally suffer from low tensile ductility, which severely limits their practical utility. Here we show that markedly enhanced tensile ductility can be achieved in coherent nano-lamellar alloys, which exhibit an unprecedented combination of over 2 GPa yield strength and 16% uniform tensile ductility. The ultrahigh strength originates mainly from the lamellar boundary strengthening, whereas the large ductility correlates to a progressive work-hardening mechanism regulated by the unique nano-lamellar architecture. The coherent lamellar boundaries facilitate the dislocation transmission, which eliminates the stress concentrations at the boundaries. Meanwhile, deformation-induced hierarchical stacking-fault networks and associated high-density Lomer-Cottrell locks enhance the work hardening response, leading to unusually large tensile ductilities. The coherent nano-lamellar strategy can potentially be applied to many other alloys and open new avenues for designing ultrastrong yet ductile materials for technological applications.

Triple Junction Motion – A New Recovery Mechanism in Metals Deformed to Large Strains

A phenomenologically new recovery mechanism – triple junction motion is presented. This recovery mechanism is found to be the dominant one at low and medium temperatures in highly strained aluminum, which has a very fine microstructure, composed of lamellae with the thickness of a few hundred nanometers. Triple junction motion leads to removal of thin lamellae and to a consequent increase of the thickness of neighboring lamellae. This recovery mechanism therefore increases the average lamellar boundary spacing and causes a gradual transition from a lamellar structure to a more equiaxed structure preceding recrystallization.

Microstructure and Mechanical Properties of Al-0.5 at.% X (=Si, Ag, Mg) Alloys Highly Deformed by ARB Process

Effect of solid solution elements on microstructure evolution and mechanical properties was investigated using a high purity Al (purity 99.99%) and Al-0.5 at.% X ( X = Si, Ag, Mg ) alloys deformed by accumulative roll bonding (ARB) process up to 7 cycles (equivalent strain of 5.6) at ambient temperature. The ARB-processed high purity Al showed the equiaxed microstructure having mean grain size of 750 nm. On the other hand, the microstructure of the ARB-processed Al-0.5at.%X alloys showed lamellar boundary structures elongated along RD. The mean lamellar boundary spacing significantly differed depending on the alloying elements, which suggested that solute atoms had a significant effect on microstructure evolution. The difference in the grain size was regarded to be caused by the difference in recovery processes in the alloys. The tensile strength of the alloys increased with increasing the number of ARB cycles. In the Al-Si and Al-Ag alloys, the post-uniform elongation increased with increasing the number of the ARB cycles. On the other hand, the elongation of the Al-Mg hardly changed during the ARB process.

Low Temperature Recrystallization of High Purity Iron Severely Deformed by ARB Process

Recrystallization behavior of SPD processed high purity iron was studied. The 99.95% iron sheet was deformed by the accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. Subsequently, the ARB-processed specimens were annealed for 1.8ks at various temperatures from 300°C to 500°C. The microstructures of these specimens were characterized by TEM and SEM/EBSP. The microstructure of the as-ARB-processed specimens showed the lamellar boundary structure elongated along RD, which was the typical microstructure of the ARB-processed materials. The mean interval of the lamellar boundaries was about 100 nm. After annealing at 400°C, the ARB specimen showed a partially recrystallized microstructure composed of equiaxed grains with grain size larger than 10 5m and the recovered lamellar boundary structure. After annealing above 500°C, the microstructures were filled with equiaxed recrystallized grains. These results suggest that conventional discontinuous recrystallization characterized by nucleation and growth occurs during annealing at annealing temperature above 400 °C. In previous work reported about the annealing behavior of the low carbon IF steel ARB processed, the continuous recrystallization occurred during annealing at annealing temperature above 600 °C. The recrystallization temperature of the pure iron was much lower than the IF steel and the recrystallization process were significantly different. This difference was suggested to be caused by inhomogeneous microstructure in the pure iron ARB-processed.

Microstructural Evolution in 36%Ni Austenitic Steel during the ARB Process

A 36mass% Ni austenitic steel was deformed to equivalent strains of 0.8 to 4.8 by the accumulative roll-bonding (ARB) process at 500°C, with slight lubrication. We analyzed the microstructure and crystallographic analysis by employing the electron back-scatter pattern (EBSP) technique in a field emission gun (FEG) SEM. After several ARB cycles, ultrafine lamellar boundary structures elongated in the rolling direction (RD) formed uniformly in the material. Observations indicated that the mean spacing of high-angle lamellar boundaries determined from the EBSP results decreased exponentially as a function of equivalent strain. The fraction of high-angle boundaries (HABs) increased, thus the average misorientation of the boundaries increased with increasing strain. In the six-cycle ARB-processed specimen, the mean spacing of the uniform lamellar boundaries was 150 nm, the fraction of HABs was 75%, and the average misorientation was 32°. The ultrafine lamellar boundary structure in the 36%Ni austenitic steel was finer and straighter than in ferritic steel (IF steel) deformed under similar conditions, probably because recovery occurs more easily in ferritic steel than austenitic steel.

Martensite Transformation of Ultrafine Grained Austenite in Fe-28.5at%Ni Alloy

Martensite transformation of the ultrafine grained (UFG) austenite fabricated by the accumulative roll bonding (ARB) process was studied. The Fe-28.5at.%Ni alloy sheet was severely deformed in austenite state by the ARB process up to 5 cycles. The ARB processed sheet had the ultrafine lamellar boundary structure. The mean lamellar spacing was 230 nm in the 5 cycles specimen. The sheets ARB processed by various cycles were cooled down to 77 K to cause the martensite transformation. Martensite transformation starting (Ms) temperature decreased with increasing the number of the ARB process. The Ms temperature of the ultrafine lamellar austenite in the 5 cycles specimen was 225 K, which was lower than that (247 K) of the conventionally recrystallized specimen with mean grain size of 22 μm. The martensite having characteristic morphologies appeared from the UFG austenite, although the martensite transformed from the coarse-grained specimen showed typical plate (or lenticular) morphology. The strength of the nano-martensite transformed from the UFG austenite was about 1.5 times higher than that of the UFG austenite, and it reached to 970 MPa.

Microstructural Evolution in Pure Copper Severely Deformed by the ARB Process

An oxygen free high conductivity (OFHC) copper (99.99%) was intensely deformed by the accumulative roll-bonding (ARB) process up to equivalent strain of 4.8 at ambient temperature. The microstructure evolution during the ARB process was explained by grain subdivision. The deformed specimens revealed dislocation cell structures at low strain and elongated ultra fine grains separated by high angle boundaries at high strain. The spacing of the high angle lamellar boundary exponentially decreased as a function of strain. The fractions of high angle boundaries (HAB) and the low angle boundaries (LAB) were nearly equal even at strain of 3.2, which was significantly different from the ARB processed Al alloys and ferritic steel where the HAB fraction was above 70% at the same strain. TEM observations indicated a mixed microstructure of dislocation boundaries and cell walls with dislocation tangle at low strain of 1.6, and small recrystallized grains partly appeared above strain of 3.2. As a result, the LAB fraction due to partial recrystallization was high even at strain of 4.8. The occurrence of recrystallization is attributed to high purity of the OFHC copper, the accumulated dislocation density, and the adiabatic heating during the ARB process of one-pass large reduction without lubrication.

Effect of Strain on Deformation Microstructure and Subsequent Annealing Behavior of IF Steel Heavily Deformed by ARB Process

Ultra low-carbon interstitial free (IF) steel having ferrite (b.c.c.) single phase was deformed to various equivalent strains ranging from 0.8 to 5.6 by the accumulative roll bonding (ARB) process at 500°C. The microstructure and crystallographic feature of the deformed specimens were characterized mainly by FE-SEM/EBSD analysis. Grain subdivision during the plastic deformation up to very high strain was clarified quantitatively. After heavy deformation above 4.0 of strain, the specimens showed the lamellar boundary structure uniformly, in which the mean spacing of the lamellar boundaries was about 200nm and more than 80% of the boundaries were high-angle ones. Annealing behavior of the ARB processed IF steel strongly depended on the strain. The specimens deformed to medium strains exhibited discontinuous recrystallization characterized by nucleation and growth, while the specimens deformed above strain of 4.0 showed continuous recrystallization. The recrystallization behaviors are discussed on the basis of the microstructural and crystallographic parameters quantitatively measured in the as-deformed samples.