Powder-size driven facile microstructure control in powder-fusion metal additive manufacturing processes.

Microstructure control in metal additive manufacturing is highly desirable for superior and bespoke mechanical performance. Engineering the columnar-to-equiaxed transition during rapid solidification in the additive manufacturing process is crucial for its technological advancement. Here, we report a powder-size driven melt pool engineering approach, demonstrating facile and large-scale control in the grain morphology by triggering a counterintuitive response of powder size to the additively manufactured 316 L stainless steel microstructure. We obtain coarse-grained (>100 µm) or near-monocrystalline microstructure using fine powders and near-equiaxed, fine-grained (<10 µm) microstructure using coarse powders. This approach shows resourceful adaptability to directed energy deposition and powder-bed fusion with no added cost, where the particle-size dependent powder-flow preheating effects and powder-bed thermophysical properties drive the microstructural variations. This work presents a pathway for leveraging feedstock particle size distribution towards more controllable, cost-effective, and sustainable metal additive manufacturing.

A strong fracture-resistant high-entropy alloy with nano-bridged honeycomb microstructure intrinsically toughened by 3D-printing.

Strengthening materials via conventional "top-down" processes generally involves restricting dislocation movement by precipitation or grain refinement, which invariably restricts the movement of dislocations away from, or towards, a crack tip, thereby severely compromising their fracture resistance. In the present study, a high-entropy alloy Al0.5CrCoFeNi is produced by the laser powder-bed fusion process, a "bottom-up" additive manufacturing process similar to how nature builds structures, with the microstructure resembling a nano-bridged honeycomb structure consisting of a face-centered cubic (fcc) matrix and an interwoven hexagonal net of an ordered body-centered cubic B2 phase. While the B2 phase, combined with high-dislocation density and solid-solution strengthening, provides strength to the material, the nano-bridges of dislocations connecting the fcc cells, i.e., the channels between the B2 phase on the cell boundaries, provide highways for dislocation movement away from the crack tip. Consequently, the nature-inspired microstructure imparts the material with an excellent combination of strength and toughness.

Development of hyaluronic acid-silica composites via in situ precipitation for improved penetration efficiency in fast-dissolving microneedle systems.

Fast-dissolving microneedles (DMNs) hold significant promise for transdermal drug delivery, offering improved patient compliance, biocompatibility, and functional adaptability for various therapeutic purposes. However, the mechanical strength of the biodegradable polymers used in DMNs often proves insufficient for effective penetration into human skin, especially under high humidity conditions. While many composite strategies have been developed to reinforce polymer-based DMNs, simple mixing of the reinforcements with polymers often results in ineffective penetration due to inhomogeneous dispersion of the reinforcements and the formation of undesired micropores. In response to this challenge, this study aimed to enhance the mechanical performance of hyaluronic acid (HA)-based microneedles (MNs), one of the most commonly used DMN systems. We introduced in situ precipitation of silica nanoparticles (Si) into the HA matrix in conjunction with conventional micromolding. The precipitated silica nanoparticles were uniformly distributed, forming an interconnected network within the HA matrix. Experimental results demonstrated that the mechanical properties of the HA-Si composite MNs with up to 20 vol% Si significantly improved, leading to higher penetration efficiency compared to pure HA MNs, while maintaining structural integrity without any critical defects. The composite MNs also showed reduced degradation rates and preserved their drug delivery capabilities and biocompatibility. Thus, the developed HA-Si composite MNs present a promising solution for efficient transdermal drug delivery and address the mechanical limitations inherent in DMN systems. STATEMENT OF SIGNIFICANCE: HA-Si composite dissolving microneedle (DMN) systems were successfully fabricated through in situ precipitation and conventional micromolding processes. The precipitated silica nanoparticles formed an interconnected network within the HA matrix, ranging in size from 25 to 230 nm. The optimal silica content for HA-Si composite MN systems should be up to 20 % by volume to maintain structural integrity and mechanical properties. HA-Si composite MNs with up to 20 % Si showed improved penetration efficiency and reduced degradation rates compared to pure HA MNs, thereby expanding the operational window. The HA-Si composite MNs retained good drug delivery capabilities and biocompatibility.

Rational design and topochemical synthesis of polymorphs of a polymer.

Packing a polymer in different ways can give polymorphs of the polymer having different properties. ß-Turn forming peptides such as 2-aminoisobutyric acid (Aib)-rich peptides adopt several conformations by varying the dihedral angles. Aiming at this, a ß-turn-forming peptide monomer would give different polymorphs and these polymorphs upon topochemical polymerization would yield polymorphs of the polymer, we designed an Aib-rich monomer N3-(Aib)3-NHCH2-C[triple bond, length as m-dash]CH. This monomer crystallizes as two polymorphs and one hydrate. In all forms, the peptide adopts ß-turn conformations and arranges in a head-to-tail manner with their azide and alkyne units proximally placed in a ready-to-react alignment. On heating, both the polymorphs undergo topochemical azide-alkyne cycloaddition polymerization. Polymorph I polymerized in a single-crystal-to-single-crystal (SCSC) fashion and the single-crystal X-ray diffraction analysis of the polymer revealed its screw-sense reversing helical structure. Polymorph II maintains its crystallinity during polymerization but gradually becomes amorphous upon storage. The hydrate III undergoes a dehydrative transition to polymorph II. Nanoindentation studies revealed that different polymorphs of the monomer and the corresponding polymers exhibited different mechanical properties, in accordance with their crystal packing. This work demonstrates the promising future of the marriage of polymorphism and topochemistry for obtaining polymorphs of polymers.

Nanomechanical study of aqueous-processed h-BN reinforced PVA composites.

Hexagonal boron nitride (h-BN) as a filler has significantly improved the mechanical properties of various polymers composites. Among them, polyvinyl alcohol (PVA) is particularly important for its wide range of industrial applications and biocompatibility nature. However, preparing a homogenous composite of h-BN and PVA in water is troublesome as the aqueous processing of h-BN without any additives is challenging. In this context, a pre-processing technique is used to produce an additive-free aqueous dispersion of h-BN. The uniformly dispersed composites are then prepared with different concentrations of h-BN. Free-standing thin films are fabricated using the doctor blade technique, and nanoindentation is employed to understand their deformation behaviour at smaller length scale for better understanding of micro-mechanism involved. Reduced elastic modulus and hardness of 10 wt% h-BN/PVA composite film are enhanced by ∼93% and ∼159%, respectively, compared to pristine PVA. Frequency sweep dynamic mechanical analysis is performed between 1 and 50 Hz, and the elastic properties of composite materials are found to improve significantly upon addition of h-BN nanosheets. Besides, the impact of h-BN incorporation in stress relaxation behaviour and hardness depth profiling are also investigated. The observed improvement in mechanical properties of the composites may be attributed to the uniform distribution of the nanosheets and the strong interfacial interaction between h-BN and PVA, which ensures efficient mechanical stress transfer at the interface.

Extraction of mechanical properties of materials through deep learning from instrumented indentation.

Instrumented indentation has been developed and widely utilized as one of the most versatile and practical means of extracting mechanical properties of materials. This method is particularly desirable for those applications where it is difficult to experimentally determine the mechanical properties using stress-strain data obtained from coupon specimens. Such applications include material processing and manufacturing of small and large engineering components and structures involving the following: three-dimensional (3D) printing, thin-film and multilayered structures, and integrated manufacturing of materials for coupled mechanical and functional properties. Here, we utilize the latest developments in neural networks, including a multifidelity approach whereby deep-learning algorithms are trained to extract elastoplastic properties of metals and alloys from instrumented indentation results using multiple datasets for desired levels of improved accuracy. We have established algorithms for solving inverse problems by recourse to single, dual, and multiple indentation and demonstrate that these algorithms significantly outperform traditional brute force computations and function-fitting methods. Moreover, we present several multifidelity approaches specifically for solving the inverse indentation problem which 1) significantly reduce the number of high-fidelity datasets required to achieve a given level of accuracy, 2) utilize known physical and scaling laws to improve training efficiency and accuracy, and 3) integrate simulation and experimental data for training disparate datasets to learn and minimize systematic errors. The predictive capabilities and advantages of these multifidelity methods have been assessed by direct comparisons with experimental results for indentation for different commercial alloys, including two wrought aluminum alloys and several 3D printed titanium alloys.

Molecular Architectonics of Cyclic Dipeptide Amphiphiles and Their Application in Drug Delivery.

Assembly and co-assemblies of peptide amphiphiles through specific noncovalent forces expand the space of molecular architectonics-driven construction of diverse nanoarchitectures with potential biological applications. In this work, cyclic dipeptide amphiphiles (CDPAs) of cyclo(Gly-Asp) with varying lengths of alkyl chains (C8-C18) were synthesized, and their molecular organization was studied. The noncovalent interactions of the components, CDP and alkyl chain, drive the molecular self-assembly of CDPAs into well-defined and diverse nanoarchitectures such as nanotubes, nanospheres, nano/microsheets, and flowers. The co-assembly of CDPAs with biological molecules such as nucleosides was studied to ascertain their utility as potential drug delivery vehicles. Mechanical properties of these nanoarchitectures in nanoindentation study established them as robust in nature. A temperature-dependent NMR study confirmed the formation of stable co-assembly of CDPAs, primarily driven by the intermolecular hydrogen bonding interactions. Computational modeling of oligomers of CDPAs and their co-assembly with nucleosides/nucleotides reveal the molecular level interactions and driving force behind such assemblies. CDPAs exhibit good biocompatibility and cytocompatibility, as revealed by the cellular studies which substantiated their suitability for drug delivery applications. The co-assembly of CDPA with an anticancer drug 5-bromo-2'-deoxyuridine (BrdU) was studied as a drug delivery platform and cytotoxicity was successfully assessed in HeLa cells. Computational modeling of the oligomers of CDPAs and their co-assembly with the drug molecule was performed to understand the molecular level interactions and driving force behind the assemblies. Our findings reveal the design strategy to construct diverse structural architectures using CDP as the modular building unit and specific molecular interactions driven co-assembly for potential application as drug delivery carrier.

Guest Solvent-dependence of the Nanomechanical Response in Substituted Dihydropyrimidinone Crystals.

The nanomechanical responses of two crystalline phases of a dihydropyrimidine analogue (1) were similar irrespective of the presence (or absence) of the guest solvent. In contrast, the mechanical responses of two differently solvated forms of the second related (2) crystals were significantly different. These contrasting behaviors are rationalized in terms of intermolecular interactions and energy distributions.

Molecular Basis for the Mechanical Response of Sulfa Drug Crystals.

Comprehension of the nanomechanical response of crystalline materials requires the understanding of the elastic and plastic deformation mechanisms in terms of the underlying crystal structures. Nanoindentation data were combined with structural and computational inputs to derive a molecular-level understanding of the nanomechanical response in eight prototypical sulfa drug molecular crystals. The magnitude of the modulus, E, was strongly connected to the non-covalent bond features, that is, the bond strength, the relative orientation with the measured crystal facet and their disposition in the crystal lattice. Additional features derived from the current study are the following. Firstly, robust synthons well isolated by weak and dispersive interactions reduce the material stiffness; in contrast, the interweaving of interactions with diverse energetics fortifies the crystal packing. Secondly, mere observation of layered structures with orthogonal distribution of strong and weak interactions is a prerequisite, but inadequate, to attain higher plasticity. Thirdly, interlocked molecular arrangements prevent long-range sliding of molecular planes and, hence, lead to enhanced E values. In a broader perspective, the observations are remarkable in deriving a molecular basis of the mechanical properties of crystalline solids, which can be exploited through crystal engineering for the purposeful design of materials with specific properties.

Elastic flexibility tuning via interaction factor modulation in molecular crystals.

We report the design of a series of nonhalogenated and halogenated molecular crystals with specific structural features, which are essential for pronounced elasticity. These features involve (a) isotropic weak and dispersive interactions, and (b) corrugated molecular packing with interlocked structures. The effects of intermolecular interactions on the elastic properties of the crystals are ascertained using nano-scale mechanical characterization methods.

Intermediate Temperature Brittleness in Metallic Glasses.

All metallic glasses (MGs), irrespective of their compositions, become brittle in the intermediate temperature range of 0.6-0.7 Tg However, most materials are expected to carry higher strains during deformation with increasing temperature. This behavior of MGs is explained by describing the competition between shear banding and diffusive relaxation processes, and is reminiscent of the "intermediate temperature ductility minimum" observed in polycrystalline metals.

Reversible Polymorphism, Liquid Crystallinity, and Stimuli-Responsive Luminescence in a Bola-amphiphilic π-System: Structure-Property Correlations Through Nanoindentation and DFT Calculations.

We report the design, synthesis, detailed characterization, and analysis of a new multifunctional π-conjugated bola-amphiphilic chromophore: oligo-(p-phenyleneethynylene)dicarboxylic acid with dialkoxyoctadecyl side chains (OPE-C18-1). OPE-C18-1 shows two polymorphs at 123 K (OPE-C18-1') and 373 K (OPE-C18-1″), whose crystal structures were characterized via single crystal X-ray diffraction. OPE-C18-1 also exhibits thermotropic liquid crystalline property revealing a columnar phase. The inherent π-conjugation of OPE-C18-1 imparts luminescence to the system. Photoluminescence measurements on the mesophase also reveal similar luminescence as in the crystalline state. Additionally, OPE-C18-1 shows mechano-hypsochromic luminescence behavior. Density functional theory (DFT)-based calculations unravel the origins behind the simultaneous existence of all these properties. Nanoindentation experiments on the single crystal reveal its mechanical strength and accurately correlate the molecular arrangement with the liquid crystalline and mechanochromic luminescence behavior.

Cavitation-Induced Fracture Causes Nanocorrugations in Brittle Metallic Glasses.

Brittle metallic glasses exhibit a unique and intriguing fracture morphology of periodic nanocorrugations whose spacing and amplitude are of the order of tens of nanometers. We show through continuum simulations that they fail by spontaneous and simultaneous cavitation within multiple weak zones arising due to intrinsic atomic density fluctuations ahead of a notch tip. Dynamic crack growth would then occur along curved but narrowly confined shear bands that link the growing cavities. This mechanism involves little dissipation and also explains the formation of nanocorrugations.

Remarkable Improvement in the Mechanical Properties and CO2 Uptake of MOFs Brought About by Covalent Linking to Graphene.

Metal-organic frameworks (MOFs) are exceptional as gas adsorbents but their mechanical properties are poor. We present a successful strategy to improve the mechanical properties along with gas adsorption characteristics, wherein graphene (Gr) is covalently bonded with M/DOBDC (M=Mg(2+) , Ni(2+) , or Co(2+) , DOBDC=2,5-dioxido-1,4-benzene dicarboxylate) MOFs. The surface area of the graphene-MOF composites increases up to 200-300 m(2) g(-1) whereas the CO2 uptake increases by ca. 3-5 wt % at 0.15 atm and by 6-10 wt % at 1 atm. What is significant is that the composites exhibit improved mechanical properties. In the case of Mg/DOBDC, a three-fold increase in both the elastic modulus and hardness with 5 wt % graphene reinforcement is observed. Improvement in both the mechanical properties and gas adsorption characteristics of porous MOFs on linking them to graphene is a novel observation and suggests a new avenue for the design and synthesis of porous materials.

A first-principles study of pressure-induced phase transformation in a rare-earth formate framework.

Among the panoply of exciting properties that metal-organic frameworks (MOFs) exhibit, fully reversible pressure-induced phase transformations (PIPTs) are particularly interesting as they intrinsically relate to the flexibility of MOFs. Recently, a number of MOFs have been reported to exhibit this feature, which is attributed to bond rearrangement with applied pressure. However, the experimental assessment of whether a given MOF exhibits PIPT or not requires sophisticated instruments as well as detailed structural investigations. Can we capture such low pressure transformations through simulations is the question we seek to answer in this paper. For this, we have performed first-principles calculations based on the density functional theory, on a MOF, [tmenH2][Y(HCOO)4]2 (tmenH2(2+) = N,N,N',N'-tetramethylethylenediammonium). The estimated lattice constants for both the parent and product phases of the PIPT agree well with the earlier experimental results available for the same MOF with erbium. Importantly, the results confirm the observed PIPT, and thus provide theoretical corroborative evidence for the experimental findings. Our calculations offer insights into the energetics involved and reveal that the less dense phase is energetically more stable than the denser phase. From detailed analyses of the two phases, we correlate the changes in bonding and electronic structure across the PIPT with elastic and electronic conduction behavior that can be verified experimentally, to develop a deeper understanding of the PIPT in MOFs.

Bioinspired Reductionistic Peptide Engineering for Exceptional Mechanical Properties.

A simple solution-processing and self-assembly approach that exploits the synergistic interactions between multiple hydrogen bonded networks and aromatic interactions was utilized to synthesize molecular crystals of cyclic dipeptides (CDPs), whose molecular weights (~0.2 kDa) are nearly three orders of magnitude smaller than that of natural structural proteins (50-300 kDa). Mechanical properties of these materials, measured using the nanoindentation technique, indicate that the stiffness and strength are comparable and sometimes better than those of natural fibres. The measured mechanical responses were rationalized by recourse to the crystallographic structural analysis and intermolecular interactions in the self-assembled single crystals. With this work we highlight the significance of developing small molecule based bioinspired design strategies to emulate biomechanical properties. A particular advantage of the successfully demonstrated reductionistic strategy of the present work is its amenability for realistic industrial scale manufacturing of designer biomaterials with desired mechanical properties.

Structure-mechanical property correlations in mechanochromic luminescent crystals of boron difluoride dibenzoylmethane derivatives.

The structure and mechanical properties of crystalline materials of three boron difluoride dibenzoylmethane (BF2dbm) derivatives were investigated to examine the correlation, if any, among mechanochromic luminescence (ML) behaviour, solid-state structure, and the mechanical behaviour of single crystals. Qualitative mechanical deformation tests show that the crystals of BF2dbm( (t) Bu)2 can be bent permanently, whereas those of BF2dbm(OMe)2 exhibit an inhomogeneous shearing mode of deformation, and finally BF2dbmOMe crystals are brittle. Quantitative mechanical analysis by nano-indentation on the major facets of the crystals shows that BF2dbm( (t) Bu)2 is soft and compliant with low values of elastic modulus, E, and hardness, H, confirming its superior suceptibility for plastic deformation, which is attributed to the presence of a multitude of slip systems in the crystal structure. In contrast, both BF2dbm(OMe)2 and BF2dbmOMe are considerably stiffer and harder with comparable E and H, which are rationalized through analysis of the structural attributes such as the intermolecular interactions, slip systems and their relative orientation with respect to the indentation direction. As expected from the qualitative mechanical behaviour, prominent ML was observed in BF2dbm( (t) Bu)2, whereas BF2dbm(OMe)2 exhibits only a moderate ML and BF2dbmOMe shows no detectable ML, all examined under identical conditions. These results confirm that the extent of ML in crystalline organic solid-state fluorophore materials can be correlated positively with the extent of plasticity (low recovery). In turn, they offer opportunities to design new and improved efficient ML materials using crystal engineering principles.

Crystal chemistry and photomechanical behavior of 3,4-dimethoxycinnamic acid: correlation between maximum yield in the solid-state topochemical reaction and cooperative molecular motion.

A new monoclinic polymorph, form II (P21/c, Z = 4), has been isolated for 3,4-dimethoxycinnamic acid (DMCA). Its solid-state 2 + 2 photoreaction to the corresponding α-truxillic acid is different from that of the first polymorph, the triclinic form I ([Formula: see text], Z = 4) that was reported in 1984. The crystal structures of the two forms are rather different. The two polymorphs also exhibit different photomechanical properties. Form I exhibits photosalient behavior but this effect is absent in form II. These properties can be explained on the basis of the crystal packing in the two forms. The nanoindentation technique is used to shed further insights into these structure-property relationships. A faster photoreaction in form I and a higher yield in form II are rationalized on the basis of the mechanical properties of the individual crystal forms. It is suggested that both Schmidt-type and Kaupp-type topochemistry are applicable for the solid-state trans-cinnamic acid photodimerization reaction. Form I of DMCA is more plastic and seems to react under Kaupp-type conditions with maximum molecular movements. Form II is more brittle, and its interlocked structure seems to favor Schmidt-type topochemistry with minimum molecular movement.

Hardness Alternation in α,ω-Alkanedicarboxylic Acids.

The variation of hardness as a function of the number of carbon atoms in α,ω-alkanedicarboxylic acids, C(N)H(2N-2)O4 (4≤N≤9), was examined by recourse to nanoindentation on the major faces of single crystals. Hardness exhibits odd-even alternation, with the odd acids being softer and the even ones harder; the differences decrease with increasing chain length. These variations are similar to those seen for other mechanical, physical, and thermal properties of these diacids. The softness of odd acids is rationalized due to strained molecular conformations in them, which facilitate easier plastic deformation. Relationships between structural features, such as interplanar spacing, interlayer separation distance, molecular chain length, and signatures of the nanoindentation responses, namely, discrete displacement bursts, were also examined. Shear sliding of molecular layers past each other during indentation is key to the mechanism for plastic deformation in these organic crystals.

Designing elastic organic crystals: highly flexible polyhalogenated N-benzylideneanilines.

The intermolecular interactions and structural features in crystals of seven halogenated N-benzylideneanilines (Schiff bases), all of which exhibit remarkable flexibility, were examined to identify the common packing features that are the raison d'être for the observed elasticity. The following two features, in part related, were identified as essential to obtain elastic organic crystals: 1) A multitude of weak and dispersive interactions, including halogen bonds, which may act as structural buffers for deformation through easy rupture and reformation during bending; and 2) corrugated packing patterns that would get interlocked and, in the process, prevent long-range sliding of molecular planes.