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• High-strength aluminum alloys via laser powder bed fusion are on demand for industrial applications. • Addition of Sc in aluminum alloys can achieve hitherto the highest range of strength with good laser powder bed fusion processability. • Sc addition plays a critical role in forming unique microstructure in strengthening Al alloys. Laser powder bed fusion (LPBF) is one of the typical additive manufacturing techniques that enables the fabrication of complex-shaped structures with great freedom of design. Due to the challenges encountered in LPBF processing of Al alloys, most studies have focused on AlSi alloys which are much easier to process but are not inherently designed for high-strength applications. In recent years, however, significant progress has been made to develop Sc -containing Al alloys that are designed specifically for LPBF production. These alloys display excellent processability and superior mechanical properties that open up a range of possible applications in industries that require high specific strength, good thermal stability, and increased functionality. As such, this paper reviews the available literature on how Sc additions influence the microstructure and properties of Al alloys when processed via LPBF and thus, aims to shed light on the considerations that have been made to achieve remarkable material consolidation alongside excellent mechanical properties, with the latter achieved through a high degree of Sc supersaturation and a great potential for nanoprecipitation. [Display omitted] [ABSTRACT FROM AUTHOR]

• Post heat treatment strategy has been designed for Inconel 718 superalloy fabricated by rolling-assisted laser-directed energy deposition. • Optimized microstructure containing fully recrystallized grains and a high number density of precipitates has been produced through designed post-heat treatment. • Tensile properties at both room temperature and 650 °C are significantly enhanced. Post-heat treatment is a necessary and important step for additive-manufactured products to relieve residual stress and to further improve mechanical performance. In this work, the heat treatment strategy for Inconel 718 superalloy fabricated by rolling-assisted laser-directed energy deposition (L-DED) has been designed and systematically investigated for the first time. The results show that the designed homogenization heat treatment at 1080 °C for 10 min can effectively dissolve most of the detrimental Lave phases existing in the rolling-assisted L-DED samples. Meanwhile, it results in a homogenous grain structure through static recrystallization, while maintaining a similar prior-refined grain size of ∼8 µm. On this basis, a high number density of γ″ and γ′ precipitates appear in the microstructure after applying a subsequent double-aging heat treatment. The optimized microstructure through such effective post-heat treatment designed in this work has led to a significant increase in material strength at both the room and elevated temperatures while maintaining good ductility. [ABSTRACT FROM AUTHOR]

• The origin of non-uniform plasticity of the Al-Mn-Sc based alloy produced by laser powder bed fusion has been clarified. • The non-uniform plasticity of the alloy originates from those microstructural areas containing fine and equiaxed grains. • The absence of work hardening in fine-grain areas is attributed to the lack of mobile dislocations and grain boundary particles. • Quantitative strength contributions from solutes, grain boundaries and secondary precipitates in bimodal grain microstructure have been identified. The Al-Mn-Sc-based alloys specific for additive manufacturing (AM) have been recently developed and can reach ultrahigh strength and adequate elongation. However, these alloys commonly exhibit non-uniform plasticity during tensile deformation, which is a critical issue hindering their wider application. In this work, the origin of this non-uniform plasticity of the alloys produced by laser powder bed fusion (LPBF) has been systematically investigated for the first time. The results show that the loss of uniform plasticity in the alloy originates from microstructural regions containing equiaxed fine-grains (FGs) (∼650 nm in size) at the bottom of the melt pools. In micro-tensile tests, the strength of these FG regions can reach a peak of ∼630 MPa. After this, an apparent yield drop occurs, followed by rapid strain softening. This FG behavior is associated with intermetallic particles along grain boundaries and a lack of uniform mobile dislocations during deformation. The columnar coarse-grain (CG) regions in the remaining melt pools show uniform plasticity and moderate work hardening. Furthermore, the quantitative calculations indicate that the solid solution strengthening in these two regions is similar. Nevertheless, secondary Al 3 Sc precipitates contribute to ∼260 MPa strength in the FG, compared to 310 MPa in the CG due to their different number density. In addition, grain boundary strengthening can reach 230 MPa in the FG region; nearly double the CG region value. [Display omitted]. [ABSTRACT FROM AUTHOR]

• As-built PBF-EB Hastelloy X has anisotropic microstructure and mechanical behavior. • Grain coarsening from solution heat treatment and HIP reduced the yield strength. • Crystallographic texture along 〈100〉 remained unaffected even after post-treatment. • Anisotropy in yield strength and ductility weakened following post-treatment. • Solid solution strengthening is the main contributor towards overall yield strength. Prior to the application of AM components for critical applications, it is necessary to have a better understanding of the effect of different post-fabrication treatments on the microstructure and mechanical properties of such parts. In this study, efforts were made to achieve an in-depth understanding of the effect of post-fabrication Solution Heat Treatment (SHT) and Hot Isostatic Pressing (HIP) on the microstructure and mechanical properties of Hastelloy X parts built by electron beam powder bed fusion (PBF-EB) process. The effects of SHT and HIP on porosity, microstructure, texture and mechanical properties have been investigated and compared with that of as-built PBF-EB Hastelloy X. Post-fabrication HIP treatment led to a significant reduction in the porosity content, whereas no notable difference in porosity was observed between SHT and as-built parts. There was no evidence of any recrystallization occurring following the post-fabrication treatments as elongated columnar grain structures observed within as-built part were found to be maintained even after SHT and HIP process alongside the strong 〈100〉 crystallographic texture. Emphasis was laid upon understanding the influence of SHT and HIP on mechanical properties through stress-strain curves and work-hardening behaviour. [Display omitted] [ABSTRACT FROM AUTHOR]

• The relative importance of β grain size and STA process parameters were analyzed using an orthogonal array method. • The relationship between specific microstructural features with strength, ductility and fracture toughness were established. • The contradictory β grain size effect on fracture toughness was explained. • A table was proposed to quasi-quantitatively unravel the competitive effect of microstructural features. The competitive effect of microstructural features including primary α (α p), secondary α (α s), grain boundary α (α GB) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes. The relative effect of β grain size and STA (solution treatment and ageing) processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method. These results were further explained via correlating microstructure with the fracture toughness and tensile properties. It was found that large numbers of fine α s precipitates and continuous α GB played greater roles than other features, resulting in a high strength and very low ductility (<2%) of STA process samples. The β grain size had a negative correlation with fracture toughness. In the samples prepared by BASCA (β anneal slow cooling and ageing) process, improved ductility and fracture toughness were obtained due to a lower density of α s precipitates, a basket-weave structure and zigzag morphology of α GB. For this heat treatment, an increase in prior β grain size had an observable positive effect on fracture toughness. The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained. It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties. This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties. A novel table was proposed to quasi-quantitatively unravel these competitive effects. [ABSTRACT FROM AUTHOR]

• Twinning transfer (TT) behavior depends on crystallographic alignment factor (m') of two twinning systems and misorientation angle (MA) of grain pairs. • The twin shape is dependent on the occurrence of TT. Twins always present a ruler shape when TT can occur and present a lenticular shape when TT is not able to occur. • A possible mechanism for the relationship between the twin shape and TT behavior is proposed based on the assumption of limited average concentrated stress in front of twin tips in different grain pairs. 1) For the grain pair with a low MA and a high m', such concentrated stress could be released effectively by inducing the outgoing twining in the neighboring grain, forming a ruler-shaped paired twins. 2) For the grain pair with a high MA and a small m', such concentrated stress could be reduced effectively by enlarging the distance between micro-twin tip and grain boundary along the [113] direction, forming a lenticular-shaped twin. The interaction between twins and grain boundaries (GB) has an important influence on the deformation and fracture behavior of materials. In the present work, {332}<113> twinning transfer (TT) behavior and its effect on the twin shape in a deformed β-type Ti-23.1Nb-2.0Zr-1.0O Ti alloy were investigated experimentally and with molecular dynamics simulation. The crystallographic alignment factor of the two twinning systems in the neighboring grains and the misorientation angle of the grain pairs were found to influence the occurrence of TT. This further determines the twin shape: twins present a ruler shape when TT occurs but are in a lenticular shape otherwise. Such different twin shapes are attributed to the local stress states related to TT occurring or not. Both ruler-shaped paired twinning and lenticular twinning would provide effective mechanisms to release or reduce the stress concentration in front of the twin tips in different grain pairs. [ABSTRACT FROM AUTHOR]

• Compression behavior of a Ti-23.1Nb-2.0Zr-1.0O (at.%) alloy has been explode in a temperature range of 100−200 °C and a strain rate range of 0.1-1000 s−1. • Multiple deformation modes, including dislocation slip, {332}〈113〉 twinning, {112}〈111〉 twinning, stress-induced α" martensitic (SIMα") and stress-induced ω phase (SIω) transformations were observed at high strain rates and low deformation temperatures. • Both twinning and SIω transformations prefer to be activated at low deformation temperatures and high strain rates. The compression behavior of a Ti-23.1Nb-2.0Zr-1.0O (at.%) alloy was investigated at strain rates from 0.1 s−1 to 1000 s−1 and temperatures from 100 °C to 200 °C on a Gleeble 3800 system and Split Hopkinson Pressure Bar (SHPB) compressive tester. Optical microscopy, electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to characterize the microstructure evolution during the deformation. Numerous deformation phenomena, including dislocation slip, twinning of both {332}〈113〉 and {112}〈111〉 modes, stress-induced α" martensite (SIMα") and stress-induced ω (SIω) transformations, were observed. The preferred activation of twinning and SIω transformations was observed in the sample compressed at lower temperatures and/or higher strain rates. The underlying mechanism is that twinning and stress induced phase transformations are attribute to higher stress concentrations at β grain boundaries and additional energy supplied by a higher strain rate, as well as high stacking fault energy because of higher temperature. [ABSTRACT FROM AUTHOR]

The microstructure and mechanical properties of 304 L stainless steel fabricated by selective laser melting are investigated in this study. With the optimized laser processing parameters, a relative density of 99.9% is achieved with fine austenite grains and nanoscale cellular subgrains in size of approximately 0.5 μm. The presence of δ-ferrite and σ phase precipitates is identified by the x-ray diffraction and transmission electron microscopy. Moreover, the microstructure is identified as an austenite matrix with about 4% δ ferrite and a trace amount of σ phase by using electron backscattered diffraction analysis. These small σ phase particles are mainly distributed along austenite grain boundaries. Furthermore, the presence of nanoscale cellular subgrains contributes to the good tensile strength and ductility of the selective laser melted 304 L, along with precipitate strengthening and strain hardening. Tensile property anisotropy is also identified in this 304 L, which is attributed to the microstructure difference on vertical and horizontal planes. [ABSTRACT FROM AUTHOR]

• The hardness of heat treated SLMed Ti-6Al-4V and lamellar width obey the Hall-Petch relationship. • The lamellae coarsening behaviour of SLMed Ti-6Al-4V can be interpreted by LSW theory. • The coarsening mechanism is identified from 700 to 950 °C for SLMed Ti-6Al-4V. Static coarsening mechanism of selective laser melted (SLMed) Ti–6Al–4V with a lamellar microstructure was established at temperatures from 700 °C to 950 °C. Microstructure evolution revealed that high heat treatment temperature facilitated martensite decomposition and promoted lamellae growth. At each temperature, the growth rate decreased with increasing holding time. The static coarsening behaviour of SLMed Ti–6Al–4V can be interpreted by Lifshitz, Slyozov, and Wagner (LSW) theory. The coarsening coefficient were 0.33, 0.33–0.4, 0.4–0.5 for 700–800 °C, 900 °C and 950 °C, respectively. This indicated the coarsening mechanism was bulk diffusion at 700–800 °C, and a combination of bulk diffusion and interface reaction at 900 °C and 950 °C conditions. [ABSTRACT FROM AUTHOR]