The Dynamic Mechanical Behavior and Microstructural Evolution of Commercial Pure Titanium

Fundamental understanding of high strain rate deformation behavior of materials is critical in designing new alloys for wide-ranging applications including military, automobile, spacecraft, and industrial applications. High entropy alloys, consisting of multiple elements in (near) equimolar proportions, represent a new paradigm in structural alloy design providing ample opportunity for achieving excellent performance in high strain rate applications by proper selection of constituent elements and/or thermomechanical processing. This dissertation is focused on fundamental understanding of high strain-rate deformation behavior of several high entropy alloy systems with widely varying microstructures. Ballistic impact testing of face centered cubic Al0.1CoCrFeNi high entropy alloy showed failure by ductile hole growth. The deformed microstructure showed extensive micro-banding and micro-twinning at low velocities while adiabatic shear bands and dynamic recrystallization were seen at higher velocities. The Al0.7CoCrFeNi and AlCoCrFeNi2.1 eutectic high entropy alloys, with BCC and FCC phases in lamellar morphology, showed failure by discing. A network of cracks coupled with small and inhomogeneous plastic deformation led to the brittle mode of failure in these eutectic alloys. Phase-specific mechanical behavior using small-scale techniques revealed higher strength and strain rate sensitivity for the B2 phase compared to the L12 phase. The interphase boundary demonstrated good stability without any cracks at high compressive strain rates. The Al0.3CoCrFeNi high entropy alloy with bimodal microstructure demonstrated an excellent combination of strength and ductility. Ballistic impact testing of Al0.3CoCrFeNi alloy showed failure by ductile hole growth and demonstrated superior performance compared to all the other high entropy alloy systems studied. The failure mechanism was dominated by micro-banding, micro-twining, and adiabatic shear localization. Comparison of all the high entropy alloy systems with currently used state-of-the-art rolled homogenous armor (RHA) steel showed a strong dependence of failure modes on microstructural features.

As a model material, commercial pure titanium was rolled to plates with different dislocation boundaries. The dynamic mechanical response of Ti specimen was analyzed during impacted with Split Hopkinson Pressure Bar (SHPB) at different strain rates, and microstructure evolution was investigated using optical microscopy and transmission electron microscopy. It was found that adiabatic shear sensitivity was decreased with increasing strain rates for all as-annealed, 25% and 50% cold rolled states. To the contrary, for 70% cold rolled state the adiabatic shear sensitivity was increased with increasing strain rates. The microstructure of adiabatic shear bands (ASBs) were developed from elongation morphology to fine equiaxed grains in the specimens of 25% cold rolled state, and ASBs became broader with increasing strain rate.

Statistical analysis of dislocations and dislocation boundaries from EBSD data

In this paper, effects of initial micro-structures on deformation behaviors of commercial pure titanium were elaborated by investigating the evolution of dislocation boundary and its adiabatic shear sensitivity. At the low to medium stain rates, the main plastic deformation mechanism of as-annealed commercial pure titanium is dislocation slipping. Meanwhile, geometrically necessary boundaries (GNBs) with different directions are generated and crossed with each other. However, new dislocation boundaries are formed in as-cold rolled plates, which are parallel to the initial ones induced by cold rolling. When the strain rate is up to 1000 s-1, the initial dislocation boundary playes an adverse role in the adiabatic shear sensitivity of commercial pure titanium. The adiabatic shear band is the high-speed deformation characteristic micro-structure in commercial pure titanium. In addition, dynamic recrystallized grains are generated in the center of an adiabatic shear band, which is consistent with the sub-grain rotation mechanism.

The deformation microstructures of commercially pure aluminum deformed by plane strain compression to 50 pct thickness reduction at temperatures between 100 °C and 300 °C, under two strain rates, 5 × 10−2 s−1 and 5 × 10−4 s−1, have been characterized by transmission electron microscopy. As the deformation temperature increases, the deformation microstructure gradually changes from a checkerboard pattern into an equiaxed subgrain structure with increasing subgrain size. The fraction of geometrically necessary boundaries (GNBs) found in warm-worked aluminum is much less than that found at room temperature. The average misorientation of dislocation boundaries appears to be independent of deformation temperature and strain rate. The constancy of the average misorientations is a combined effect of the variation of the fractions of GNBs and incidental dislocation boundaries (IDBs) and the variation of the average misorientations of GNBs and IDBs. Scaling theory can apply to both boundary misorientations and subgrain sizes that formed at different temperatures and strain rates. Subgrain size distributions for different temperatures and strain rates all resemble a lognormal distribution.

Role of {332}<113> twinning on adiabatic shear behavior during dynamic loading in β-type Ti–15Mo alloy

The technology of high speed machining on high strength steel is applied widely in manufacture industry. The formation of serrated chips which is produced by the adiabatic shear behavior in high speed machining is also an emphasis in research. First of all, the experimental study of orthogonal cutting in different cutting conditions is investigated for H13 hot work die steel. Then, the morphology of chips and microstructure characterization in adiabatic shear bands of H13 hot work die steel were observed and analyzed by microhardness tester, optical microscope and SEM. The results of these investigations show that a critical cutting speed induced the adiabatic shear behavior. Two types of adiabatic shear band, i.e. deformed band and transformed band are formed in cutting process. The morphology of chips gradually changes along with the appearance and evolution of adiabatic shear bands. Microstructure surveying by SEM and Hardness measurement were performed and the debate of formation in adiabatic shear band is discussed in detail. It is important to know reasons of chip formation better during high speed machining. It also provides a theoretic basis for practical cutting manufacture.

An examination of adiabatic shearing behavior in ZK60 alloy with different states of heat treatment

Formation of adiabatic shearing band for high-strength Ti-5553 alloy: A dramatic thermoplastic microstructural evolution

The effect of strain on the adiabatic shear behaviour of AZ31 magnesium alloy under high strain rate compression was studied using split Hopkinson pressure bar (SHPB). The microstructure of the specimen was characterised using optical microscopy and electron backscatter diffraction. The results indicate that the adiabatic shear sensitivity increased with the strain. The microstructure evolution of adiabatic shear deformation has been investigated. Firstly, a large number of twins and dislocations are formed and accumulate in the early stage of deformation. Subsequently, they transform into dynamic recrystallised grains, forming an adiabatic shear band (ASB) and ultimately leading to crack formation. The dynamic recrystallisation mechanism in the ASB involves twinning-induced dynamic recrystallisation (TDRX) and rotational dynamic recrystallisation (RDRX). This study has analysed the ASB mechanism, which provides a foundation for material selection and the design of magnesium alloys.

Adiabatic shear localization induced by dynamic recrystallization in an FCC high entropy alloy

High strain rate deformation behavior, texture and microstructural evolution, characterization of adiabatic shear bands, and constitutive models in electron beam melted Ti-6Al-4V under dynamic compression loadings

The formation and evolution of adiabatic shear behaviors, as well as the corresponding mechanical properties of a near-Ti-6Al-3Nb-2Zr-1Mo (Ti-6321) alloy during dynamic compression process, were systematically investigated by the split Hopkinson pressure bar (SHPB) compression tests in this paper. Ti-6321 samples containing three types of microstructures, i.e., equiaxed microstructure, duplex microstructure and Widmanstätten microstructure, were prepared to investigate the relationship between microstructures and dynamic mechanical behaviors under different strain rates in a range from 1000 s-1 to 3000 s-1. It was found by the dynamic strain-stress relation that the Ti-6321 alloys containing equiaxed microstructure, duplex microstructure and Widmanstätten microstructure all exhibited a strong strain-hardening effect. The samples containing equiaxed microstructure exhibited a larger flow stress than samples containing duplex microstructure and Widmanstätten microstructure. The adiabatic shearing behaviors in Ti-6321 alloy are significantly influenced by different types of microstructures. The formation of adiabatic shearing bands occurs in equiaxed microstructure when the strain rate is increased to 2000 s-1. The adiabatic shear bands are formed in duplex microstructure when the strain rate reaches 3000 s-1. However, the initiation of adiabatic shear bands is found in Widmanstätten microstructure under the strain rate of 1000 s-1. The Widmanstätten microstructure shows a larger sensitivity to adiabatic shearing than the equiaxed microstructure and duplex microstructure samples.

Deformation and fracture behaviors of Ti-6Al-4V-0.1B alloy with Widmanstätten, equiaxed and bimodal microstructures were investigated by Split Hopkinson Pressure Bar (SHPB) under high strain rates of 2100-3200 s-1. The results showed that the equiaxed and bimodal structures had a higher bearing capacity at high strain rates than that of the Widmanstätten structure. With the same microstructure, the increase of strain rate gave rise to an improved uniform plastic deformation. According to an observation on the deformed microstructure, it was found that adiabatic shear behavior was the main reason for failure and fracture of the alloy. The formation and propagation of adiabatic shear bands (ASBs) was the precursor for the failure and fracture of the material. Cavities at the interface between TiB phase and the matrix readily formed due to the uncoordinated deformation, which are not the dominate reason for the failure and fracture.

A Split Hopkinson Pressure Bar system was employed to investigate the compressive dynamic mechanical behaviors of Ti-10V-2Fe-3Al (Ti-1023) alloy with lamellar microstructure, over a broad strain rates ranging from 1500/s to 5100/s. The results reveal that the strain rate has a significant effect on the flow stress of Ti-1023 alloy, and there exists serious thermal softening as the strain rate exceeds 3200/s. The critical strain rate of fracture for this alloy is 2300/s. The microstructure examination indicated that adiabatic shear bands (ASBs) bifurcate more intensely with the increasing of strain rate. Micro-voids nucleate either in the ASB or interface between shear band and matrix bulk. Finally, fracture of this alloy proceeds through the nucleation, growth and coalescence of these voids and cracks along the ASBs.