Microstructural Characterization and Shape Memory Response of Ni-Rich NiTiHf and NiTiZr High Temperature Shape Memory Alloys

NiTiHf and NiTiZr high temperature shape memory alloys (HTSMAs) have drawn a great deal of attention as cheaper alternatives to Pt, Pd and Au alloyed NiTi-based HTSMAs while NiTiZr alloys also providing at least 20% weight reduction then its NiTiHf counterparts with the same stoichiometry. (Ti + Hf/Zr)-rich compositions were already reported to have high thermal hysteresis, poor dimensional and thermal stability due to their low matrix strength hampering their practical applications. However, Ni-rich compositions of NiTiHf alloys were shown to have very promising shape memory responses recently due to generation of fine Ni-rich particles after proper heat treatments not only strengthening the matrix but also leading to relatively high transformation temperatures. Comparable studies have not been performed on Ni-rich NiTiZr compositions. Furthermore, very few published work are present on these new Ni-rich NiTiHf and NiTiZr systems. Hence many critical characteristics still remains unknown and further investigation is necessary to reveal the effect of precipitation on the microstructures and its subsequent effect on the transformation characteristics and shape memory responses. The present study focuses on the extensive microstructural and thermo-mechanical property characterizations of the Ni-rich NiTiHf and NiTiZr HTSMAs in order to develop the fundamental knowledge necessary for the optimization and development of reliable, cheap, lightweight HTSMAs operating up to 300 °C with improved thermal and dimensional stability. Several different compositions of Ni-rich NiTiHf and NiTiZr HTSMAs are systematically precipitation heat treated for the microstructural control and then subjected to multi-scale microstructural and thermo-mechanical characterizations to achieve this goal. Differential scanning calorimetry measurements are conducted on the aged samples to reveal the transformation characteristics and furthermore generate the time-temperature-transformation temperature (TTT) diagrams of the individual alloy systems. The shape memory response and characteristics of the alloys are investigated through load-biased thermal cycling and superelasticity tests. The microstructures of the aged samples are extensively characterized using transmission electron microscopy (TEM) to build up microstructure-property relationships as well as providing deeper understanding of precipitate crystal structure, composition and morphology. Such an experimental approach is crucial for the development of new ternary alloy compositions and for the careful control of the microstructure to obtain desired properties. The outcomes of the present study is expected to help to reveal the potential of these alloys to be utilized in a wide range of applications at elevated temperatures in aerospace, automotive and oil-gas industries

On the use of multiple layer thicknesses within laser powder bed fusion and the effect on mechanical properties

Laser Powder Bed fusion is capable of rapid production of parts, from conception, compared with traditional manufacturing methods. This said, the time taken to fabricate a single part can still be significant – typically many hours. Processing thicker layers, and hence fewer total layers, in the Laser Powder Bed Fusion process, is an effective way to reduce build times. However, mechanical performance can suffer as a result of this strategy. This study proposes and demonstrates a method to enable the interlacing of multiple layer thicknesses within one part, allowing for finer layers within regions where they are specifically required, whilst maintaining overall component integrity for specific load cases. Thicker layers are used within regions with lower property requirements in order to optimise an overall part for improved production rate. The design of interfaces between two disparate layer thickness regions could also be tailored for control of material properties and such will be investigated in an independent study. Ti6Al4V LPBF samples are fabricated, characterised by way of tensile testing, porosity analysis and microstructural analysis. The study demonstrate parts can be additively built using multiple layer thickness regions with consistent ultimate tensile strength (1110–1135 MPa) and varying penalties to ductility, depending on layer thickness and interface design (elongation to failure reductions up to 40% in the most extreme case)

Failure modes in dual layer thickness Laser Powder Bed Fusion components using a novel post-mortem reconstruction technique

To exploit the design freedoms of Powder Bed Fusion, parameters can be varied within sub-volumes of components to achieve the optimal part for both service conditions and manufacturing productivity. This involves prioritising mechanical strength in areas of structural significance and high volumetric build rates in areas of low structural significance. In theory, a component with similar mechanical behaviour to that seen in standard Laser Powder Bed Fusion parts can be built in significantly less time and at a reduced cost. In practice however, the boundary between such regions is yet to be understood and discretising components into sub-volumes can induce interfacial defects. In this study, an in-depth analysis of interfaces between disparate layer thickness volumes in single components has been explored, to gain information vital to solving interface quality issues so that LPBF design freedoms can be fully exploited. A novel 3D reconstruction technique has been demonstrated to characterise transient plastic behaviour of interfacial pores post-fracture. This technique enables post-mortem evaluation of additively manufactured parts and tracking of pore deformation during subsequent mechanical testing. X-ray Computed Tomography (XCT) identified interfacial pores up to 170 µm Feret diameter, with a voxel resolution of 6 µm. Micro tensile testing with in-situ microscopy exhibited a real-time mechanical response, observing evidence that these interfacial defects lead to fracture at interface locations. The 3D reconstruction technique found that pores constricted 10.0 – 14.1% in the x direction and 10.3 – 14.6% in the y direction after fracture – normal to the loading direction. These findings contribute towards improving Additively Manufactured biomedical implants and airframe components with reduced time and cost