Microstructures,micro-segregation and solidification path of directionally solidified Ti-45Al-5Nb alloy

To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s^(-1) under the temperature gradients of 15-20 K·mm^(-1), and the primary phase is transformed into an α phase at relatively higher growth rates(V >20 μm·s^(-1)). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s^(-1) under a temperature gradient of 15 K·mm^(-1). The increase of temperature gradient to 20 K·mm^(-1) can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L+β L+β→αα+β_R β→ααα→γα+γα→α_2+γγ_R+(α_2+γ), in which β_R and γ_R mean the residual β and

Influence of Al addition on solidification path and hot tearing susceptibility of Mg-2Zn-(3+0.5 x)Y-x Al alloys

Hot tearing susceptibility(HTS)of Mg-2Zn-(3+0.5 x)Y-x Al(x=0,2 and 3 at%)alloys is predicted by using modified Clyne-Davies’model(CSC^(∗)).The solidification path,solidification characteristic temperatures and dendritic coherency solid fraction have been studied by double-thermocouple thermal analysis.The solidification contraction stress vs.temperature(and time)curves are measured by using a“T”type hot tearing permanent-mold.The results reveal that the CSC^(∗)prediction values are in good agreement with the experimental results.Moreover,Al_(2)Y phase acts as the heterogeneous nucleation core ofα-Mg and significantly influences the grain size.It has been observed that minimum grain size,optimal dendritic coherency and minimum HTS are exhibited by Mg-2Zn-(3+0.5 x)Y-x Al alloy(x=2).Furthermore,when Al content was increased to 3 at%,Al_(2)Y phase exhibited a peritectic reaction and transformed into a mixed structure of Al_(2)Y and Al+Al_(3)Y phases,which increased the HTS of the alloy due to reduced fine-grained Al_(2)Y content.

Thermo-Calc based multi-component micro-segregation model and solidification paths calculations

On the basis of a multi-length scale modeling, a mixture-averaged multi-component/multiphase micro- segregation model was proposed without pre-set function for the micro-scale solute profile. The model explains the effect of morphologies of solidifying phases and solid back diffusion （SBD） on segregation, and covers the two limiting solidification cases of Scheil and Lever-rule models. A commercial Thermo-Calc software package/database was linked to the algorithms via its TQ6-interface for instantaneous determination of the related thermodynamic data of the multi-component alloys. The influences of cooling rate and other parameters on the solidification path and micro-segregation behavior were numerically investigated by sample calculation of the ternary AI-Cu-Mg alloys. A parallel experimental investigation on AI-Cu-Si alloys solidified under different cooling conditions was conducted to validate the theoretical model. Reasonable agreements were gained between the predicted solidification paths and the measured results.

As-cast microstructures and solidification paths of the Nb–Si–Ti ternary alloys in Nb_5Si_3–Ti_5Si_3region

Abstract The as-cast microstructures and solidification paths of the Nb-Si-Ti ternary alloys in the NbsSi3-TisSi3 region were investigated. Since there exist some isomor- phous compounds in the NbsSi3-TisSi3 region, such as aNbsSi3 with B3Cr5 prototype, 13NbsSi3 with Si3W5 pro- totype, 7NbsSi3 with MnsSi3 prototype, and TisSi3 with MnsSi3 prototype, the primary solidification areas of these compounds were not typically indentified in previous experiments. In the present paper, the microstructure observation, the phase identification, and the composition measurement were performed using scanning electron microscopy （SEM）, X-ray diffraction （XRD）, and electron probe microanalysis （EPMA）, respectively. No ternary compound is found. There exist three primary solidification areas, 13Nbs_x（Ti）xSi3, ~Nbs_x（Ti）xSi3, and Tis-x（Nb）xSi3 in the NbsSi3-TisSi3 region. Together with the literaturereported experimental data and optimization results, the liquidus projection of the whole Nb-Si-Ti ternary system is constructed, and totally ten primary solidification areas-- diamond-Si, Nb1-x（Ti）xSi2, Ti1-x（Nb）xSi2, Ti1-x（Nb）xSi, Ti5-x（Nb）xSi4, βNb5-x（Ti）xSi3,αNb5-x（Ti）xSi3, Ti5-x （Nb）xSi3, （Nb,Ti）3Si, and BCC--and nine transitional invariant reactions-L ＋ Nb1-x（Ti）xSi2 → Ti1-x（Nb）x Si2 ＋ Si, L ＋ Nb1-x（Ti）xSi2 → Ti1-x（Nb）xSi2 ＋ Ti5- （Nb）xSi4, L ＋ Ti5-x（Nb）xSi4 → Ti1-x（Nb）xSi2 ＋ Ti1-x （Nb）xSi, L ＋ 13Nb5-x（Ti）5Si3→ Nb1-x（Ti）xSi2 ＋ Ti5-x （Nb）xSi4, L ＋ βNb5-x（Ti）xSi3→b5-x（Ti）xSi3 ＋Ti5-x （Nb）xSi4, L ＋ αNb5-x（Ti）αSi3 → Ti5-x（Nb）xSi3 ＋ Ti5-x（Nb）x Si4, L ＋ αNb5-x（Ti）xSi3 →βNb5-x（Ti）xSi3 ＋ Ti5-x（Nb）xSi3, L ＋ βNb5-xTb-xSi3 → Ti5-x（Nb）xSi3 ＋ （Nb,Ti）3Si, and L ＋ （Nb,Ti）3Si → Ti5-x（Nb）xSi3 ＋ BCC are confirmed.

Thermo-Calc and T-f_S-C_L coupling based method to determine solidification paths of alloys solidified under condition of Biot≤0.1

A binary continuum model for dendritic solidification transport phenomena and corresponding numerical algorithm for the strong nonlinear coupling of T-fS-CL were extended to multicomponent alloys solidified under condition of Biot≤0.1. Based on the extended model/algorithm, a method considering heat transfer was proposed to predict the solidification paths and microsegregation of alloys solidified under the same condition. The new algorithm and method were closely coupled with the commercial Thermo-Calc package via its TQ6-interface codes for instantaneous determination of the related thermodynamic data at each calculation time step. The sample simulation performed on an Al-2Si-3Mg alloy system indicates the availability and reliability of the model/algorithm and the proposed method for predicting solidification paths and microsegregation. Computional and experimental investigations on an Al-5.17Cu-2.63Si ternary alloy were conducted, and a reasonable agreement between the computation and experiment was obtained.

Solidification path calculations of Al-Zn-Mg alloys in Al-rich corner

The solidification paths of Al-Zn-Mg alloys in the Al-rich corner were investigated. The thermodynamic data for the calculation are obtained by direct coupling with the CALPHAD software Thermo-Calc via its TQ6-interface and the COST2 database. The influences of the initial compositions and the extent of solid back diffusion on the solidification path were numerically investigated by sample calculation of the ternary Al-Zn-Mg alloys. The calculation results of solidification paths of the selected alloys: Al-Zn-3 Mg(in wt.%), Al-5 Zn-10 Mg, Al-2.5Zn-15Mg, Al-10Zn-20.5 Mg, Al-8Zn-25 Mg, were: L→(L+α-Al), L→(L+α-Al)→(L+α-Al+TAU), L→(L+α-Al)→(L+α-Al+Al Mg_β), L→(L+α-Al)→(L+α-Al+TAU)→(L+α-Al+TAU+Al Mg_β), L→(L+α-Al)→(L+α-Al+Al Mg_β)→(L+α-Al+TAU+Al Mg_β), respectively. The results show that the initial compositions and the extent of solid back diffusion have a great influence on solidification path, and the amounts of eutectic phase increase with the decrease of the solid back diffusion coefficient. The equilibrium solute partition coefficients for Zn and Mg in alloys are also calculated and their influence on micro-segregation in the primary solidification of Al-5Zn-10 Mg alloy is analyzed.

Si-Assisted Solidification Path and Microstructure Control of 7075 Aluminum Alloy with Improved Mechanical Properties by Selective Laser Melting

The construction and application of traditional high-strength 7075 aluminum alloy(Al7075) through selective laser melting(SLM) are currently restricted by the serious hot cracking phenomenon. To address this critical issue, in this study, Si is employed to assist the SLM printing of high-strength Al7075. The laser energy density during SLM is optimized, and the eff ects of Si element on solidification path, relative density, microstructure and mechanical properties of Al7075 alloy are studied systematically. With the modified solidification path, laser energy density, and the dense microstructure with refined grain size and semi-continuous precipitates network at grain boundaries, which consists of fine Si, β-MgSi, Q-phase and θ-AlCu, the hot cracking phenomenon and mechanical properties are eff ectively improved. As a result, the tensile strength of the SLM-processed Si-modified Al7075 can reach 486 ± 3 MPa, with a high relative density of ~ 99.4%, a yield strength of 291 ± 8 MPa, fracture elongation of(6.4 ± 0.4)% and hardness of 162 ± 2(HV) at the laser energy density of 112.5 J/mm~3. The main strengthening mechanism with Si modification is demonstrated to be the synergetic enhancement of grain refinement, solution strengthening, load transfer, and dislocation strengthening. This work will inspire more new design of high-strength alloys through SLM.

Microstructure and microhardness of Ti-48Al alloy prepared by rapid solidification

To improve the microstructure and microhardness,Ti-48Al(at.%)alloy was rapidly solidified by melt spinning under different cooling rates.The microstructure and microhardness of rapidly solidified Ti-48Al alloy were systematically investigated.Results show that the average lamellar colony size of the alloy reduces from 60.6μm to 11μm as the cooling rate increases from 2.3×105 to 5.1×105 K·s-1,caused by the increase of nucleation rate at a higher cooling rate.At the high cooling rate of(4.3-5.1)×105 K·s-1,theαphase is the primary phase,and a few metastableαphases are reserved,which then transform intoα2 phase and subsequently lead to the formation ofα2 equiaxed grain.The lamellar spacing also decreases with the increase of cooling rate.The relationship between lamellar spacing(d)and cooling rate(v)is d=33.6v-1.34.The microhardness increases with the increase of cooling rate because the refined lamellar spacing and grain size can improve the microhardness.

Hot tearing behaviors and in-situ thermal analysis of Mg-7Zn-xCu-0.6Zr alloys

Thermal analysis was used to investigate the microstructural evolution of Mg-7 Zn-x Cu-0.6 Zr alloys during solidification. The effect of Cu content（0, 1, 2 and 3, mass fraction, %） on the hot tearing behavior of the Mg-7 Zn-x Cu-0.6 Zr alloys was investigated with a constrained rod casting（CRC） apparatus, equipped with a load sensor and a data acquisition system. The thermal analysis results of Mg-7 Zn-x Cu-0.6 Zr alloy revealed that the alloy consisted of two distinct phases： α-Mg and Mg Zn2. Three distinct peaks were observed in the alloys with Cu addition, which were identified as α-Mg, Mg Zn Cu and Mg Zn2. In addition, the reaction temperature of α-Mg decreased and the reaction temperatures of Mg Zn2 and Mg Zn Cu increased as the Cu content increased. The experimental results of hot tearing demonstrated that the addition of Cu significantly reduced the hot tearing susceptibility（HTS） of Mg-7 Zn-x Cu-0.6 Zr alloys due to the higher eutectic temperature and the shorter solidification temperature region.

Effects of Ru content on phase transformation and compression property of cast TiAl alloys

Ruthenium(Ru)is a promising element to heighten the comprehensive mechanical properties of TiAl alloys.In the present study,the phase transformation during the cooling of Ti-47Al-2Cr-2Nb-xRu(x=0,0.1,0.5,1.0,at.%)alloys was investigated.The results show that Ru tends to segregate in the interdendritic region during solidification,and therefore,refines the as-cast microstructure of the alloys.As aβ-stabilizer,Ru does not induce a novel solidification reaction but maintains the peritectic reaction of L+β→α.An excessive Ru content would cause the formation of(γ+τ1+B2)and(α+γ+τ1+B2)phase regions below 1,250°C.The precipitation ofτ1-containing mixture is attributed to the Ru-segregation,which inhibits the reaction ofα→γand facilitates the formation of B2 phase.The discontinuous coarsening ofγphase and blockτ1 phase formed alternately in a manner of analogous eutectoid decomposition within B2 phase.In addition,the effect of Ru content on compression property of the alloys was studied.The yield strength increases up to 427 MPa at 800°C with the addition of 1.0at.%Ru,which is mainly due to the solution strengthening effect of Ru.

Effects of Rapid Cooling Rate on Microstructure Formation and Microhardness of Binary Ti⁃44Al Alloy

In order to refine microstructure grains and improve mechanical properties of TiAl alloys,Ti44Al(at.%)alloy was rapidly solidified by melt spinning under different cooling rates.Microstructure and microhardness of the alloy before and after rapid solidification were investigated.XRD results show that the ratio ofα2 phase in binary alloy increased with the cooling rates,which is caused by moreαphases directly transforming toα2 phases.Grain morphology changed from long dendrite to the mixture of equiaxed and dendrite to equiaxed with the increase of cooling rates.The grain size was refined from 200-600μm of as⁃cast to 18μm of the alloy cooled at 4.9×10^5 K/s,which is caused by the undercooling induced from rapid solidification.Lamellar spacing was decreased from 4.5μm of as⁃cast to 1.1μm by rapid solidification.With the increase of cooling rate,the content ofα2 phase increased andγphase decreased gradually.Rapid solidification can reduce the segregation of elements.The microhardness was improved from 247 HV to 556 HV,which results from grain refinement strengthening,reduction of lamellar spacing,and more content ofα2 phase.