Resolving aging dynamics of a 3D colloidal glass

Physical aging is an inherent property of glassy matter, but understanding its microscopic mechanism remains a challenge particularly at the particle level. In this work, we use a confocal microscope to in-situ trace the particle trajectories in a 3D colloidal glass for 73000 s, aiming at resolving the aging dynamics. By calculating the mean square displacement of particle motions, we find that the glass aging with time can be divided into three stages: β relaxation, α relaxation and free diffusion. The system's mean square displacement at each aging state is quantitatively resolved into three contributions of particle dynamics modes: vibration within the nearest-neighbor cages, hopping between cages and cooperative rearrangement. We further calculate the particle's free volume and find that the β-to-α transition is accompanied by the temporary increase of the system-averaged free volume due to pronounced hops of particles. Nevertheless, the temporal autocorrelation of the free volume spatial distribution still obeys a monotonically stretched exponential decay with an exponent of 0.76, which is related to the sub-diffusion dynamics of cooperative rearrangements and hops mixed in α relaxation. According to the resolved vibrational displacements,we calculate the vibrational density of states of this 3D glass, and the characteristic boson peak is reproduced at low frequencies.Our findings shed insight into the particle-level aging dynamics of a real glass under purely thermal activation.

通过非均匀结构显著提高预折叠管的能量吸收能力

在薄壁管中引入预折叠图案已被证明可以有效增强其能量吸收能力.然而,仅仅靠预折叠图案设计所提高的能量吸收能力已经达到天花板,难以进一步提高。本研究为解决这一问题,提供了一种全新的技术路线.我们首先设计了一种结构均匀的预折叠薄壁管作为参考结构.然后,我们对参考结构的壁厚进行非均匀化设计。在保持薄壁管总质量不变的情况下,加厚了塑性变形相对较大的局部区域的壁厚,同时变薄了塑性变形相对较小的局部区域的壁厚。这种非均匀设计方式使薄壁管的整体变形更加协调,不仅显著提高了结构的比吸能(SEA),而且降低了峰值力(PCF),实现了SEA和PCF的更优组合.

Laser additive manufacturing of laminated bulk metallic glass composite with desired strength-ductility combination

Introducing ductile crystalline dendrites into a glassy matrix to produce bulk metallic glass composites(BMGCs)is an effective way to improve the poor ductility of bulk metallic glasses(BMGs).However,the presence of soft crystalline phases tends to decrease the strength and causes the strength-ductility tradeoff.Here,relying on the flexible laser additive manufacturing(LAM)technique that allows the composition tailoring of each layer,we successfully fabricate a lamellated Zr-based BMGC constructed by the alternating superimposition of soft and hard layers.The lamellated BMGC shows an exceptional combination of yield strength(∼1.2 GPa)and ductility(∼5%).Such enhanced strength-ductility synergy is attributed to the asynchronous deformation at two scales,i.e.,inter-laminar and intra-laminar,and the unique dual-scale Ta particles that uniformly distribute on the amorphous matrix.The lamellated structure design motif,enabled by the flexible LAM technology,provides a new window for the development of high-performance BMGCs.It is also applicable to the synergistic enhancement of strength and plasticity of other brittle metallic materials.

The relationship between thermo-mechanical history,microstructure and mechanical properties in additively manufactured CoCrFeMnNi high entropy alloy

Here,a single-track CoCrFeMnNi high entropy alloy(HEA)was successfully fabricated by laser melting deposition(LMD).Combining the experimental observations and numerical simulation,the microstructure and mechanical properties of the as-deposited parts were systematically studied from the perspective of thermo-mechanical history experienced during the LMD process.The strengthening mechanisms of the LMDed CoCrFeMnNi HEA parts were clarified.The frictional stress strengthening,grain boundary strengthening and dislocation strengthening contributed the whole yield strength of the parts.Dislocation strengthening dominated the strengthening mechanism.It was expected that the establishment of the relationship between thermo-mechanical history,microstructure and mechanical properties of the LMDed CoCrFeMnNi HEA could shed more insights into achieving HEA parts with the desired microstructure and high performance.

Beneficial effects of deep cryogenic treatment on mechanical properties of additively manufactured high entropy alloy:cyclic vs single cryogenic cooling

Additively manufactured(AM)metallic materials commonly possess substantial tensile surface residual stress,which is detrimental to the load-bearing service behavior.Recently,we demonstrated that deep cryogenic treatment(DCT)is an effective method for improving the tensile properties of CoCrFeMnNi high-entropy alloy(HEA)samples fabricated by laser melting deposition(LMD),by introducing high compressive residual stress and deformation microstructures without destroying the AM shape.However,carrying out the DCT in a single-step mode does not improve the residual stress gradients inherent from the LMD process,which are undesirable as the mechanical properties will not be homogeneous within the sample.In this work,we show that carrying out the DCT in a cyclic mode with repeated cryogenic cooling and reheating can significantly homogenize the residual stress in LMD-fabricated Co Cr Fe Mn Ni HEA,and improve tensile strength and ductility,compared with single-step DCT of the same cryogenic soaking duration.Under cyclic DCT,the thermal stress is re-elevated to a high value at each cryogenic cooling step,leading to the formation of denser and more intersecting reinforcing crystalline defects and hcp phase transformation,compared to single-step DCT of the same total cryogenic soaking duration in which the thermal stress relaxes towards a low value over time.The enhancement of defect formation in the cyclic mode of DCT also leads to more uniform residual stress distribution in the sample after the DCT.The results here provide important insights on optimizing DCT processes for post-fabrication improvement of mechanical properties of AM metallic net shapes.

Enhanced elastocaloric effect and refrigeration properties in a Si-doped Ni-Mn-In shape memory alloy

We demonstrate giant elastocaloric effect and outstanding refrigeration capacity in a <0 0 1>A textured Ni_(50)Mn_(35)In_(13)Si_(2) alloy with large transformation entropy change △S_(tr) and low-hysteresis △T_(hys). On unloading from a relatively low compressive stress of 300 MPa, giant adiabatic temperature variation △T_(ad) up to –17.7 K was realized. Moreover, large stress-induced entropy change △S_(σ) of 25.9 J kg^(–1)K^(–1) andgiant refrigeration capacity RC_(σ) of 1330 J kg^(–1) were achieved under the compressive stress of 300 MPa.Simultaneously achieving giant △T_(ad) and outstanding refrigeration capacity indicates that this alloy ispromising to be the candidate material for elastocaloric refrigeration.

Accelerated exploration of TRIP metallic glass composite by laser additive manufacturing

Introducing transformation-induced plasticity(TRIP)effect into bulk metallic glass composites(BMGCs)is an effective route to improve their ductility and strain-hardening ability.Since the morphology and structure of the crystalline austenite phases responsible for the TRIP phenomenon are strongly dependent on the alloy composition and cooling rate during freezing,distinguishing the optimal cases from a vast variety of candidates is the primary task of exploring TRIP BMGCs.However,without a suitable theoretical guidance,the exploration of BMGCs is usually performed via the traditional trial-and-error route,making the BMGC development extremely time consuming and labor intensive.Here,we present a novel high-throughput strategy to accelerate the exploration process of TRIP BMGCs.The efficiency of this strategy was demonstrated on a well-studied Cu-Zr-Al alloy system.A screening library,comprised by121 cylindrical samples with different conditions,was rapidly prepared by laser additive manufacturing(LAM).The phases of the library were efficiently identified by micro-area X-ray diffraction(M-XRD)to screen the optimal compositions and cooling rates that precipitate only B2-Cu Zr phase.The distribution uniformity of the B2-Cu Zr phase was further evaluated based on digital image processing technology to screen the candidates of better ductility.The high-throughput results are in good agreement with the previous casting investigations of discrete samples,confirming the validity of the present high-throughput strategy.

Laser additive manufacturing of ductile Fe-based bulk metallic glass composite

Laser additive manufacturing(LAM)is a promising technology for processing bulk metallic glass(BMG)with freeform geometries or unlimited size.However,the inherently brittle Fe-based BMGs produced by LAM have always been plagued by the micro-cracking induced by the large thermal stresses during the LAM process.To solve this dilemma,316L stainless steel(SS)with excellent toughness and similar elemental composition was carefully selected as the second phase to form Fe-based BMG composites(BMGCs).The obtained Fe-based BMGCs are equipped with a heterogeneous structure,i.e.,the 316L SS phase is wrapped by the metallic glass and forms a unique"fishbone"structure with a micron-scale.Excitedly,the special structure nicely improves a plastic strain of the Fe-based BMGC with a strength of 2355 MPa to~17%,achieving a record-breaking achievement among Fe-based amorphous with critical dimensions over 1mm.