Deformation behaviour in advanced heat resistant materials during slow strain rate testing at elevated temperature

In this study, slow strain rate tensile testing at elevated temperature is used to evaluate the influence of temperature and strain rate on deformation behaviour in two different austenitic alloys. One austenitic stainless steel （AISI 316L） and one nickel-base alloy （Alloy 617） have been investigated. Scanning electron microscopy related techniques as electron channelling contrast imaging and electron backscattering diffraction have been used to study the damage and fracture micromechanisms. For both alloys the dominante damage micromech- anisms are slip bands and planar slip interacting with grain bounderies or precipitates causing strain concentrations. The dominante fracture micromechanism when using a slow strain rate at elevated temperature, is microcracks at grain bounderies due to grain boundery embrittlement caused by precipitates. The decrease in strain rate seems to have a small influence on dynamic strain ageing at 650℃.

Local hardening and asymmetric twin growth by twin-twin interactions in a Mg alloy

In this study,the role of twin-twin interactions on the distributions of local defects(e.g.,dislocations)and stress fields in a magnesium alloy is investigated.A co-zone(1012)-(1012)tensile twin junction in a deformed Mg-3wt.%Y alloy is analyzed using transmission electron microscopy(TEM).The results show that the morphology of the impinging(1012)twin is asymmetric,and the non-interacting boundary of the recipient(1012)twin is irregular.Detailed analysis of TEM images reveals that type-II pyramidal[1213](1212)dislocations concentrate in the vicinity of the twin-twin junction site.The same<c+a>dislocations are also observed inside the interacting twin domains along with a few <a> dislocations.The<c+a>dislocations emanating from the impinging(1012)twin boundary have edge character and are extended with faults parallel to the basal plane.In contrast,the<c+a>dislocations connected to the recipient(1012)twin are predominantly screw orientation and compact.Elasto-viscoplastic fast Fourier transform based crystal plasticity calculations are performed to rationalize the observed twin morphology and local dislocation distribution.The model calculations suggest that the local stress fields generated at the junction site where the two twins meet are responsible for the experimentally observed concentration of<c+a>dislocations.The calculated stress fields are asymmetric with respect to the junction site,explaining the observed asymmetric morphology of the impinging twin.Overall,these findings show strong effects of twin-twin interactions on the distribution of dislocations as well as the evolution of the twinned microstructure and as such,can help advance understanding of twinning in Mg alloys and their effect on mechanical behavior.

In-situ SEM observation on fracture behavior of austempered silicon alloyed steel

Crack initiation, propagation and microfracture processes of austempered high silicon cast steel have been investigated by using an in-situ tensile stage installed inside a scanning electron microscope chamber. It is revealed that micro cracks always nucleate at the yielding near imperfections and the boundary of matrix-inclusions due to the stress concentration. There are four types of crack propagations in the matrix: crack propagates along the boundary of two clusters of bainitic ferrite; crack propagates along the boundary of ferrite-austenite in bainitic ferrite laths; crack propagates into bainitic ferrite laths; crack nucleates and propagates in the high carbon brittle plate shape martensite which is transformed from some blocky retained austenite due to plastic deformation. Based on the observation and analysis of microfracture processes, a schematic diagram of the crack nucleation and propagation process of high silicon cast steel is proposed.

材料组织和性能的计算机统计模拟与预测

本文简要介绍了极统计方法的原理在材料研究领域的应用.着重分析了其在纯净钢的质量控制和金属在自然环境条件下的腐蚀行为分析等方面的应用.

Experimental Approaches to Improve Thermal Stability of Organic Phase Change Properties

This study investigates the thermal behavior of Polyolefin containing Paraffin and Nano Hydrated aluminum silicate Al2Si2O5 (OH) 4 (Kaolin) particles to enhance store energy at ambient temperature. The hybrid Nano composite is based on polyolefin PE as a matrix, whereby paraffin wax and Kaolin were hot blended at varying concentrations. In addition Carbon Nanotube (CNTs) was added in different relative low concentrations to improve the thermal transition among the polymer matrix, since polymer domains are considered as isolator. The composite was prepared by melt mixing using a Brabender Plasrograph and a Two Role Mill. Thermal properties of the composite were determined using DSC and Melt flow Index. Because TES materials are subjected to melting and freezing during life time, multiple extrusion tests to simulate the degradation process of the composite were carried out. FTIR was applied to determine the degradation effect and investigate microstructure changes of the composite. The results obtained demonstrate that the blend shows a tendency to be thermally active at low temperatures. DSC tests evidenced a decrease in melt tempera-ture as a result of increasing Kaolin content and some changes in the latent heat of the compound.

Impact of Moisture Variation on Some Foundry Properties of Fori Silica Sand

Moulding Sand for metal casting is usually sourced from either natural deposit or synthetic mix of refractory sand grain binder and moisture. Each of the mix constituent is important in determining the characteristics of sand. The binding agent is responsible for bendability thereby determining the size of voids within the sand grain, while moisture level determines the plasticity of the foundry sand. Tests using American Foundry Society (AFS) Standard were followed in carrying out the experiment on Fori sand deposit to determine its suitability for foundry use. The sand was collected from the river bank of Fori, in Fori Community, Maiduguri, Borno State. The experimental test equipment includes: laboratory sand mixer, sand rammer, universal strength testing machine, permeability-meter, oven, mouldability machine, and as well as quick moisture teller. The chemical composition of the materials was carried out using atomic absorption spectrophotometer (AAS) model PG990AFG. The silica content in the material sample is about 78.65%, and with the traces of other elements, such as CaO (1.07%), Fe2O3 (0.76%), Al2O3 (15.81%), MgO (1.01%), TiO2 (2.21%), K2O (3.87%), and Na2O (1.16%), respectively. These percentages are within acceptable limits. The results of the physical properties revealed that the sand sample has clay content of 15.32% which is above the standard range of 10% - 12% recommended for natural moulding sands required for producing good quality castings. Other foundry properties of Forinatural moulding sand conducted include “moisture content” in the following ranges of percentages, 7.6%, 6.5%, 5.8%, 4.2% and 2.9% with the corresponding value of green compressive strength of (43.95, 53.47, 69.56, 68.21 and 61.16 KN/m2), dry compressive strength (93.50, 96.52, 105.50, 146.50 and 152.49 KN/m2), and permeability No. of 340, 390, 410, 430 and 440 respectively. It is clear from the test that, the lower the moisture content, the higher the dry compressive strength of the materials. The refractoriness value of the materials is 1400 ℃. The results of the physical and other foundry properties carried out show that Forisilica sand is suitable for casting non-ferrous alloys like bronze, brass and aluminium, and cast iron.

Evaluation of Cost Effectiveness of Onibode Fire-Clay for Production of High Quality Refractory Bricks

In establishing a plant or factory, the cost-benefit analysis is needed to determine the viability for such financial investment on the project. In this paper, the cost establishing a factory of 200,000 bricks per annum capacity, for the production of high quality refractory bricks from locally available raw materials in Nigeria was evaluated. The evaluation was conducted on the virgin refractory material (100%) as well as with varied percentages (10% - 40%) of alumina additions. The result of cost benefit analysis indicated that, the project was viable, with the highest cost at N1,203.66 per brick as against N1,800 (at 3 years projections) of the imported brick. The payback period for the capital investment was very short (within two years). The findings will aid investors in decision makings.

A new approach for determining GND and SSD densities based on indentation size effect:An application to additive-manufactured Hastelloy X

Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials.However,determining the densities of geometrically necessary dislocations(GNDs)and statistically stored dislocations(SSDs)is one of the classical problems in material research for several decades.Here,we proposed a new approach based on indentation size effect(ISE)and strengthening theories.This approach was performed on a laser powder bed fused(L-PBF)Hastelloy X(HX),and the results were verified by the Hough-based EBSD and modified Williamson-Hall(m-WH)methods.Furthermore,to better understand the new approach and essential mechanisms,an in-depth investigation of the microstructure was conducted.The distribution of dislocations shows a clear grain orientation-dependent:low density in large<101>preferentially orientated grains while high density in fine<001>orientated grains.The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs.Planar slip is the main operative deformation mechanism during indentation tests,and the slip step patterns depend mostly on grain orientations and stacking fault energy.This study provides quantitative results of GND and SSD density for L-PBF HX,which constructs a firm basis for future quantitative work on other metals with different crystal structures.

Interface-facilitated stable plasticity in ultra-fine layered FeAl/FeAl_(2) micro-pillar at high temperature

Fe-Al compounds possess a combination of high strength and corrosion resistance at high temperatures.However,increasing Al content to make them lighter results in embrittlement.Here,we investigate the high-temperature behavior of a novel,lightweight,ultra-fine-layered FeAl/FeAl_(2) material.We report a transition from unstable to stable plasticity at 450℃.Below 450℃,deformation is dominated by localized shear deformation within the soft FeAl layers,while above 450℃,it proceeds by co-deformation between Fe Al and the brittle FeAl_(2) layers.We show that co-deformation is associated with the temperature at which the interface converts from sliding to sourcing dislocations for Fe Al_(2).

Mn_(x)Cr_(0.3)Fe_(0.5)Co_(0.2)Ni_(0.5)Al_(0.3)high entropy alloys for magnetocaloric refrigeration near room temperature

High entropy alloys(HEAs)based on transition metals display rich magnetic characteristics,however attempts on their application in energy efficient technologies remain scarce.Here,we explore the magnetocaloric application for a series of Mn_(x)Cr_(0.3)Fe_(0.5)Co_(0.2)Ni_(0.5)Al_(0.3)(0.8<x<1.1)HEAs by integrated theoretical and experimental methods.Both theory and experiment indicate the designed HEAs have the Curie temperature close to room temperature and is tunable with Mn concentration.A non-monotonic evolution is observed for both the entropy change and the relative cooling power with changing Mn concentration.The underlying atomic mechanism is found to primarily emerge from the complex impact of Mn on magnetism.Advanced magnetocaloric properties can be achieved by tuning Mn concentration in combination with controlling structural phase stability for the designed HEAs.

Shear banding-inducedslip enables unprecedented strength-ductility combination of laminated metallic composites

Shear bands in metallic materials have been reported to be catastrophic because they normally lead to non-uniform plastic deformation. Ductility of laminated metallic composites deteriorates with increasing processing strain, particularly for those having hexagonal-close-packed(hcp) constituents due to inadequate slip systems and consequently prominent shear banding. Here, we propose a design strategy that counterintuitively tolerates the bands with localized strains, i.e. the shear banded laminar(SBL) structure, which promotes <c+a> dislocation activation in hcp metals and renders unprecedented strengthductility combination in hcp-metal-based composites fabricated by accumulative roll bonding(ARB). The SBL structure is characterized with one soft hcp metal constrained by adjacent hard metal in which dislocations have been accumulated near the bimetal interfaces. High-energy X-ray diffraction astonishingly reveals that more than 90% of dislocations are non-basal in Ti layers of the SBL Ti/Nb composite processed by eight ARB cycles. Moreover, <c+a> dislocations occupy a high fraction of ~30%, promoting further <c+a>cross slip. The unique stress field tailored by both shear banding and heterophase interface-mediated deformation accommodation triggers important <c+a> slip. This SBL design is of significance for developing hcp-based laminates and other heterostructured materials with high performances.