Inspired by the gradient structures of biological materials, researchers have explored compositional and structural gradients for about 40 years as an approach to enhance the properties of engineering materials, including metals and metallic alloys. The synthesis of various gradient nanostructured materials, such as gradient nanograined, nanolaminated nd nanotwinned metals and alloys, has provided new opportunities to understand gradient-related mechanical behaviour. These emerging gradient materials often exhibit unprecedented mechanical properties, such as strength–ductility synergy, extraordinary strain hardening, enhanced fracture and fatigue resistance, and remarkable resistance to wear and corrosion, which are not found in materials with homogeneous or random microstructures. This Review critically assesses the state of the art in the field of gradient nanostructured metallic materials, covering topics ranging from the fabrication and characterization of mechanical properties to underlying deformation mechanisms. We discuss various deformation behaviours induced by structural gradients, including stress and strain gradients, the accumulation and interaction of new dislocation structures, and unique interfacial behaviour, as well as providing insight into future directions for the development of gradient structured materials. Gradient nanostructured metals and alloys are an emerging class of materials that exhibit a combination of excellent mechanical properties that are not possessed by their homogeneous counterparts. This Review assesses the fabrication, characterization and deformation behaviour of these materials, as well as the challenges and future directions of the field.

Mechanical properties and deformation mechanisms of gradient nanostructured metals and alloys

Microstructure, residual stress and tensile properties control of wire-arc additive manufactured 2319 aluminum alloy with laser shock peening

High-performance integrated additive manufacturing with laser shock peening (LSP), is an innovative selective laser melting (SLM) method to improve mechanical properties, and refine microstructure in the surface layer of metallic components. Phase, residual stress distribution, surface micro-hardness, tensile properties and microstructural evolution of SLMed and SLM-LSPed specimens in horizontal and vertical directions were examined. In particular, typical microstructural features in the surface layer were characterized by transmission electron microscopy (TEM) observations. Results indicated that surface micro-hardness subjected to massive LSP treatment had significantly improved, tensile residual stress was transformed into compressive residual stress by LSP-induced plastic deformation, and both SLMed specimens in two directions exhibited a good combination of the ultimate tensile strength (UTS) and ductility. Meanwhile, high-density dislocations and a large number of mechanical twins were generated in the coarse α′ martensites by laser shock wave (LSW), and gradually evolved into refined α′ martensites. Furthermore, according to the included angle between LSW and the deposited plane, two kinds of LSW-induced atomic diffusion processes at the interfaces between both adjacent deposited layers were presented, and the influence mechanisms of the included angle between LSW and the deposited plane on tensile properties of both SLM-LSPed specimens were revealed. The hybrid additive manufacturing technology combined SLM with LSP realizes the high-efficiency and high-quality integrated manufacturing of the formed metallic components for practical applications.

High-performance integrated additive manufacturing with laser shock peening –induced microstructural evolution and improvement in mechanical properties of Ti6Al4V alloy components

Microstructural characterisation of metallic shot peened and laser shock peened Ti–6Al–4V

Crystallography and Crystal Defects

Definitive evidence from both electron backscattered diffraction and transmission electron microscopy is shown for the existence of twinning as a rare deformation twinning mode in coarse-grained commercial purity titanium after room temperature ballistic impact testing at 103 s−1. Non-Schmid-based twin-type selection is demonstrated for and the conjugate deformation twinning modes in this material within grains where the c-axis is closely aligned to the loading direction. Limited Schmid-based twin variant selection is shown for and deformation twinning modes in this material. The occurrence of high area fractions of twinning has relevance for high strain rate plasticity modelling of grains of textured titanium, compressed parallel to their c-axes.

https://www.tandfonline.com/doi/pdf/10.1080/14786435.2015.1051157

{1124} Deformation Twinning in Commercial Purity Titanium at Room Temperature

Determining the local orientation of crystals in engineering and geological materials has become routine with the advent of modern crystallographic mapping techniques. These techniques enable many thousands of orientation measurements to be made, directing attention towards how such orientation data are best studied. Here, we provide a guide to the visualization of misorientation data in three-dimensional vector spaces, reduced by crystal symmetry, to reveal crystallographic orientation relationships. Domains for all point group symmetries are presented and an analysis methodology is developed and applied to identify crystallographic relationships, indicated by clusters in the misorientation space, in examples from materials science and geology. This analysis aids the determination of active deformation mechanisms and evaluation of cluster centres and spread enables more accurate description of transformation processes supporting arguments regarding provenance.

/pdf/on-three-dimensional-misorientation-spaces-195o7o60zl.pdf

On three-dimensional misorientation spaces.

Determining the crystallographic orientation of hexagonal crystal structure materials with surface acoustic wave velocity measurements.

Spatially resolved acoustic spectroscopy (SRAS) is a laser ultrasonic technique that shows qualitative contrast between grains of different orientation, illustrating the sensitivity of acoustic waves to the material structure. The technique has been improved significantly on determining the full orientation of multigrain cubic metals, by comparing the measured surface acoustic wave (SAW) velocity to a pre-calculated model. In this paper we demonstrate the ability of this technique to determine the orientation of hexagonal structure crystals, such as magnesium and titanium based alloys. Because of the isotropy of the SAW velocity on the basal plane (0001) of hexagonal crystals, the slowness surface is shown as a circle. As the plane moves from (0001) towards (110) or towards (100), the slowness surface gradually turns into an oval. These acoustic properties increase the difficulty in orientation determination. The orientation results of a grade 1 commercially pure titanium by SRAS is presented, with comparison with electron backscattered diffraction (EBSD) results. Due to the nature of SAWs on hexagonal structure crystals, only the results of Euler angles 1 and 2 are discussed. The error between SRAS and EBSD is also investigated.

https://iopscience.iop.org/article/10.1088/1742-6596/684/1/012001/pdf

Steven J. Lainé

Papers

Microstructural characterisation of metallic shot peened and laser shock peened Ti–6Al–4V

{1124} Deformation Twinning in Commercial Purity Titanium at Room Temperature

On three-dimensional misorientation spaces.

Determining the crystallographic orientation of hexagonal crystal structure materials with surface acoustic wave velocity measurements.

Crystallographic Orientation Determination of Hexagonal Structure Crystals by Laser Ultrasonic Technique